Security Features Matrix

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By Default
Security Features RHEL 3 RHEL 4 RHEL 5 RHEL 6 RHEL 7 Fedora 19 Fedora 20 Fedora 21 Rawhide
Configurable Firewall iptables iptables iptables iptables iptables firewalld firewalld firewalld firewalld
Signed updates yum yum yum yum yum yum / dnf yum / dnf yum / dnf yum / dnf
SELinux N Y Y Y Y Y Y Y Y
SELinux targeted policy N Y Y Y Y Y Y Y Y
SELinux Executable Memory Protection N N Y Y Y Y Y Y Y
Password hashing md5crypt md5crypt md5crypt sha512crypt sha512crypt sha512crypt sha512crypt sha512crypt sha512crypt
Filesystem Capabilities -- kernel kernel kernel kernel kernel kernel kernel kernel
SELinux user confinement N N Y Y Y Y Y Y Y
SELinux sandbox N N N Y Y Y Y Y Y
PR_SET_SECCOMP -- -- -- -- kernel kernel kernel kernel kernel
SELinux Deny Ptrace N N N N Y Y Y Y Y
SELinux restricted module loading N N  ?  ? Y Y Y Y Y
User namespaces N N N N N N N N N
/tmp namespace for systemd N N N N Y Y Y Y Y
Polyinstantiate /tmp, /var/tmp and user home folders N N N Y Y Y Y Y Y
Encrypted LVM  ?  ? Y Standard Installer Standard Installer Standard Installer Standard Installer Standard Installer Standard Installer
eCryptfs N N Y Y Y Optional Package Optional Package Optional Package Optional Package
Non-Executable Memory (NX) Y (since 9/2004) Y Y Y Y Y Y Y Y
Built as PIE package list (since 9/2004) package list package list package list package list package list package list package list package list
Pointer Obfuscation N N Y Y Y glibc glibc glibc glibc
Heap Protector N glibc glibc glibc glibc glibc glibc glibc glibc
Built with Fortify Source N Y Y Y Y Y Y Y Y
Stack Protector N N Y Y Y Y Y Y Y
Stack ASLR Y (since 9/2004) kernel kernel kernel kernel kernel kernel kernel kernel
Libs/mmap ASLR kernel (since 9/2004) kernel kernel kernel kernel kernel kernel kernel kernel
Exec ASLR (since 9/2004) Y y Y Y Y Y Y Y
brk ASLR N N  ? Y Y Y Y Y Y
VDSO ASLR no vDSO kernel kernel kernel kernel kernel kernel kernel kernel
Built with RELRO -- -- -- -- gcc patch gcc patch gcc patch gcc patch gcc patch
Built with BIND_NOW N  ? package list package list package list package list package list package list package list
/proc/$pid/maps protection -- -- -- -- kernel & sysctl kernel & sysctl kernel & sysctl kernel & sysctl kernel & sysctl
Symlink restrictions N N N N Y kernel kernel kernel kernel
Hardlink restrictions N N N N Y kernel kernel kernel kernel
ptrace scope N N N N N N N N N
0-address protection Y (since 11/2009) Y (since 9/2009) Y (since 5/2008) Y Y Y Y Y Y
Block module loading Y Y Y Y Y Y Y Y Y
/dev/mem protection N Y Y Y Y Y Y Y Y
/dev/kmem disabled N Y Y Y Y Y Y Y Y
Module RO/NX -- -- -- -- kernel kernel kernel kernel kernel
Kernel Address Display Restriction -- -- -- -- kernel kernel kernel kernel kernel
Blacklist Rare Protocols Y Y Y Y Y Y Y Y Y
Write-protect kernel .rodata sections N N N Y Y Y Y Y Y
Kernel Stack Protector N N N Y Y Y Y Y Y
sVirt labelling N N N Y Y Y Y Y Y
SYN cookies  ? kernel kernel kernel kernel kernel kernel kernel kernel
Syscall Filtering N N N  ? Y Y Y Y Y
Secure Boot Support N N N N Y Y Y Y Y
Tamper Resistant Logs N N N N Y Y Y Y Y
Overflow checking in new operator N N N N Y Y Y Y Y
Built with Format Security -- -- -- -- -- -- -- Y Y
Crypto Policy -- -- -- -- -- -- -- Y Y
Built with Stack Protector Strong -- -- -- -- -- -- Y Y Y




Configurable Firewall

firewalld provides a dynamically managed firewall with support for network/firewall zones to define the trust level of network. The former firewall model with system-config-firewall/lokkit was static and every change required a complete firewall restart. The firewall daemon on the other hand manages the firewall dynamically and applies changes without restarting the whole firewall. See FirewallD and system-config-firewall for more information.

Signed updates

Each stable RPM package that is published by Fedora Project is signed with a GPG signature. By default, DNF, YUM and the graphical update tools will verify these signatures and refuse to install any packages that are not signed or have bad signatures. You should always verify the signature of a package before you install it. These signatures ensure that the packages you install are what was produced by the Fedora Project and have not been altered (accidentally or maliciously) by any mirror or website that is providing the packages. See this page for more information. [MOVE] We use a number of GPG keys to sign our software packages. The necessary public keys are included in the relevant products and are used to automatically verify software updates. See this page for more information.


SELinux is an inode-based MAC. See this page and this page for more information.

SELinux targeted policy

SELinux enabled with targeted policy by default. See discussion of policies page and this page for more information.

SELinux Executable Memory Protection

SELinux restricts certain memory protection operation if the appropriate boolean values enable these checks. See this page for more information.

Password hashing

The system password used for logging into Fedora is stored in /etc/shadow. Very old style password hashes were based on DES and visible in /etc/passwd. Modern Linux has long since moved to /etc/shadow, and for some time now has used salted MD5-based hashes for password verification (crypt id 1). Since MD5 is considered "broken" for some uses and as computational power available to perform brute-forcing of MD5 increases, modern Fedora versions have proactively moved to using salted SHA-512 based password hashes (crypt id 6), which are orders of magnitude more difficult to brute-force. See the crypt(3) manpage for additional details.


Filesystem Capabilities

The need for setuid applications can be reduced via the application of filesystem capabilities using the xattrs available to most modern filesystems. This reduces the possible misuse of vulnerable setuid applications. The kernel provides the support and the user-space tools are available in the standard repositories.

Capabilities are defined in /usr/include/linux/capability.h

Linux Capability Version 1 "_LINUX_CAPABILITY_U32S_1" defined as 1 indicates kernel has 32 or less capabilities

Linux Capability Version 2 constant "_LINUX_CAPABILITY_U32S_2" defined as 2 indicated kernel has more than 32 capabilities,

Linux Capability Version 3

"_LINUX_CAPABILITY_U32S_2" is deprecated by "_LINUX_CAPABILITY_U32S_3"

32 bit integer is in /proc/sys/kernel/cap_last_cap which defines the current capability sets Every linux process has sets of bitmaps

  typedef struct __user_cap_data_struct {
        __u32 effective;
        __u32 permitted;
        __u32 inheritable;
  } *cap_user_data_t;

each capability is implemented as a bit in each of these bitmaps which is either set or unset.

1. effective (E) Effective capability set indicates which capabilities are effective. When some privileged operation is done, operating system checks for the bit in effective set of the processes rather than effective uid.

2. permitted (P) Indicates which capabilities process can use. Process might have capabilities set in permitted set but not in the effective set, that would mean that particular capability is disabled for the process Process can set capability in effective set only if it is available in permitted set.

This combinations of effective and permitted bits allow to enable , disable and drop privileges

3. inheritable (I) Inheritable capability set indicates which capabilities are inheritable by the process which is going to be executed by the current process.

If P1 has X capabilities , then the process P1' which is ran or forked by P1 for example using exec(), how many capabilities out of X can be inherited by P1' is decided by inheritable capabilities set.

The need for setuid applications can be reduced via the application of filesystem capabilities using the xattrs available to most modern filesystems. This reduces the possible misuse of vulnerable setuid applications. The kernel provides the support and the user-space tools are available in the standard repositories.

Programmes have been vulnerable to set-UID, there is no need for having root privileges every time for a process to run, it is logical to provide to minimum set of privileges to programme that can enable the programme to run effectively. With the normal set-UID approach programmes would run more than the privileges required, increasing the risk of Privilege Escalation. Enabling Capabilities to programme has been started since kernel 2.6.24 known as file capability implemented in fs/exec.c in Kernel itself.

Common capabilities are implemented in security/commoncap.c

Implementation in Red Hat Enterprise Linux

RHEL 2 2.4.9-e.X N
RHEL 3 2.4.21-X N
RHEL 4 2.6.9-X Y
RHEL 5 2.6.18-X Y
RHEL 6 2.6.32-X Y

Modifying Filesystem Capabilities

There is no specific system call provided by the linux to modify filesystem capabilities. But as its implemented as inode getxattr() , fsetxattr system calls can be used.

Here "$" means normal user and "#" means root user. Let's take 'ping' as working example to show how capabilities work.

 $ mkdir CapabilityTest
 $ cd CapabilityTest
 $ cp `which ping` .
 $ ./ping -q -c 1
 ping: icmp open socket: Operation not permitted
 # ./ping -q -c 1
 PING ( 56(84) bytes of data.
 --- ping statistics ---
 1 packets transmitted, 1 received, 0% packet loss, time 0ms
 rtt min/avg/max/mdev = 0.213/0.213/0.213/0.000 ms
 # setcap cap_net_raw=ep ./ping
 # getcap ./ping
 ./ping = cap_net_raw+ep
 $ ./ping -q -c 1
 PING ( 56(84) bytes of data.
 --- ping statistics ---
 1 packets transmitted, 1 received, 0% packet loss, time 0ms
 rtt min/avg/max/mdev = 0.170/0.170/0.170/0.000 ms

from administrators perspective effective bit has to be disabled , so logical way of doing this will be

 # setcap cap_net_raw=p ./ping
 # getcap ./ping
 ./ping = cap_net_raw+p
 $ ./ping -q -c 1
 PING ( 56(84) bytes of data.
 --- ping statistics ---
 1 packets transmitted, 1 received, 0% packet loss, time 0ms
 rtt min/avg/max/mdev = 0.170/0.170/0.170/0.000 ms

from that it can be concluded that, ping requires more privileges then a normal user for specially crafted network packets, so while running with 'root' user it works as 'root' has all effective capabilities. In the Linux Kernel there is a check which sees if application is capable, which means to run it should have effective capability for CAP_NET_RAW.

Using set-UID root makes 'ping' over privileged, if buffer overflow is detected then attacker could do local privilege escalation giving back shell.

Mandatory Access Control (MAC)

Mandatory Access Controls specifies which subject can access specific data. Mandatory Access Controls are handled via the kernel LSM(Linux Security Modules) hooks. MAC is based on the security labels. Data on the system has clearance and classification data stored with security labels, which can be accessed by specific subjects or objects.When some subject tries to access the data on the system then the rules defined by the policy are checked to take access control decision.Security Levels are classified like Unclassified -> Confidential -> Secret -> Top Secret.If user has clearance to access the requested object then user will be allowed otherwise user will be denied access. It is a system wide policy which states that who is allowed to access, an individual user cannot alter the access. MAC model is mostly used in environment where confidentiality is important like in Government organizations like military, an example of widely used of MAC is SELinux.Security-Enhanced Linux (SELinux) employs MAC rules to facilitate fine-grained security.

see MAC

SELinux user confinement

Support for SELinux to confine users access on a system. Each Linux user is mapped to an SELinux user via SELinux policy, allowing Linux users to inherit the restrictions placed on SELinux users, for example (depending on the user), not being able to: run the X Window System; use networking; run setuid applications (unless SELinux policy permits it); or run the su and sudo commands

# semanage login -l

Login Name           SELinux User         MLS/MCS Range        Service

__default__          unconfined_u         s0-s0:c0.c1023       *
root                 unconfined_u         s0-s0:c0.c1023       *
system_u             system_u             s0-s0:c0.c1023       *

All the linux users are mapped to __default__ which maps to unconfined_u user. SELinux users that are available are guest_u, xguest_u, user_u, staff_u.

# ls /etc/selinux/targeted/contexts/users
guest_u  root  staff_u  sysadm_u  unconfined_u  user_u  xguest_u

# ls /etc/selinux/mls/contexts/users
guest_u  root  staff_u  unconfined_u  user_u  xguest_u

* sysadm_u is not present in MLS Policy

As listed

User Domain X Window System su and sudo Execute in home directory and /tmp/ Networking
guest_u guest_t no no no optional no
xguest_u xguest_t yes no optional only Firefox
user_u user_t yes no optional yes
staff_u staff_t yes only sudo optional yes

Users are defined in /etc/selinux/<target or mls>/contexts/users.

See Confined and Unconfined Users for more information.


Support for SELinux X Access Control Extension (XACE). XACE (X Access Control Extension) provides a wrapper to do security checks at places where untrusted clients should be restricted. XACE provides control over X server objects including colormaps, windows, pixmaps, cursors, fonts which are assigned unique ID numbers stored. ID numbers can store client ID numbers so that resources can be allocated to the clients. clients access resources by the their ID numbers when making protocol requests. Developer can place XACE hooks in the code at the places where clients should be restricted. XACE hooks when present in the code triggers different types of hooks, for e.g while authenticating XACE_AUTH_AVAIL hook can be placed there, if code present in the application tries to access any device like system bell, cdrom etc. XACE_DEVICE_ACCESS hook can be used similarly there are more hooks present in XACE, to use #include<Xext/xace.h> is the header to be included which includes everything with constants and function declarations, if only structure definitions are needed use #include<Xext/xacestr.h>

List of Hook Identifiers:


with each identifier there is a callback function attached

For complete information about XACE and security hooks provided by it :

XACE security hooks can be used like for e.g in case of DEVICE ACCESS:

 static int check_something(DeviceIntPtr dev, ClientPtr client, ....<some_other_args>) {

	int res;

	/* DixManageAccess : Global device configuration is being performed.
         * on ChangeKeyboardMapping, XiChangeDeviceControl, XkbSetControls

        res = XaceHook(XACE_DEVICE_ACCESS, client, dev, DixManageAccess);
	if (res != Success) {
		client->errorValue = dev->id;
		return res;

SELinux sandbox

Support for SELinux to test untrusted content via a sandbox. See this page and this page for more information.


SECCOMP(SECure COMPuting) which is meant to confine it to small subsystem of system calls, is available since Linux 2.6.23. PR_SET_SECCOMP set the secure computing mode for the the calling thread this limits the system calls for using this in code #include<linux/seccomp.h> and #include<sys/prctl.h>. The systemd init daemon supports the seccomp filter mechanism in 3.5 kernel. The result is that process can be easily configured to be run in a sandboxed environment.

 int main() {

  /* int prctl(int option, unsigned long arg2, unsigned long arg3, 
   * unsigned long arg4, unsigned long arg5);
   * option is PR_SET_SECCOMP, rest args are set according to option passed into
   * prctl function.


See this article and SECCOMP for more information.

SELinux Deny Ptrace

A boolean variable to allow SELinux to turn off all processes ability to ptrace other process. See this page and this for more information.

SELinux restricted module loading

Support for SELinux to restrict the loading of kernel modules by unprivileged processes in confined domains was implemented in this commit.

User namespaces

User namespaces allow per-namespace mappings of user and group IDs. This means that a process' user and group IDs inside a user namespace can be different from its IDs outside of the namespace. Most notably, a process can have a nonzero user ID outside a namespace while at the same time having a user ID of zero inside the namespace; in other words, the process is unprivileged for operations outside the user namespace but has root privileges inside the namespace. See this page and this page for more information. See this bug to track this feature.

/tmp namespace for systemd

Run some services started by systemd with a private /tmp directory. This would mitigate the chance of a service making a mistake with how it handles its /tmp data allowing a user on the system to get a privilege escalation, since users would not have access to the services /tmp directory.

See this page for more information.

Polyinstantiate /tmp, /var/tmp and user home folders

To protect the world writable shared folders like /tmp and /var/tmp PAM (Pluggable Authentication Modules) can help by creating namespace for users on the system. Security of a system works at different layers, Polyinstantiating these world writable folders add an extra layer to protect from further intrusion into the system. Polyinstanting means that a new instance of /tmp or /var/tmp directory is created for each user. This feature is implemented using To enable this feature :

uncomment the respective lines in /etc/security/namespace.conf

#/tmp     /tmp-inst/            level      root,adm
#/var/tmp /var/tmp/tmp-inst/    level      root,adm
# Remove the line below if required to polyinstantiate HOME directory of the user
#$HOME    $HOME/$USER.inst/     level


 session    required 

to /etc/pam.d/login. File /etc/security/namespace.conf specifies which directories will be polyinstantiated. It also specifies how they will be polyinstantiated , what will the names of the directories which will be polyinstantiated and also for users where Polyinstantiation would not be performed.

create the directories and set selinux context and bool value to polyinstantiate

~]# mkdir /tmp-inst /var/tmp-inst
~]# chmod 000 /tmp-inst
~]# chmod 000 /var/tmp-inst
~]# chcon -R -t tmp_t /tmp-inst
~]# chcon -R -t tmp_t /var/tmp-inst
~]# setsebool polyinstantiation_enabled 1
  • ~$ man 8 pam_namespace
  • ~$ man 5 namespace.conf

As per reference

Polyinstantiation of world-writeable directories prevents the following types of attacks:

  • Race-condition attacks with symbolic links
  • Exposing a file name considered secret information or useful to an attacker
  • Attacks by one user on another user
  • Attacks by a user on a daemon
  • Attacks by a non-root daemon on a user

However, polyinstantiation does NOT prevent these types of attacks:

  • Attacks by a root daemon on a user
  • Attacks by root (account or escalated privilege) on any user

see Polyinstantiation of directories in an SE Linux system Improve security with polyinstantiation

Filesystem encryption

Encrypted LVM

Modern Fedora versions include the ability to install Fedora onto an encrypted LVM, which allows all partitions in the logical volume, including swap, to be encrypted. LVM uses LUKS encryption (Linux Unified Key Setup). Except the boot partition All Other partitions can be encrypted. As the Linux Kernel modules reside on root partition so they are also protected if Encryption is applied. With the use of LVM Encryption user can just encrypt Physical Volume where other partitions reside making encryption and decryption much faster. LVM is created under big encrypted blockdevice which hides the LVM until blockdevice is unecrypted. Once the blockdevice is unencrypted it reads the volume structure and mounts all the detected partitions at boot time.


eCryptfs (Enterprise cryptographic Filesystem) is a cryptographic stacked Linux filesystem. eCryptfs stores cryptographic metadata in the header of each file written, so that encrypted files can be copied between hosts; the file will be decrypted with the proper key in the Linux kernel keyring. It has been there since Kernel 2.6.19. It works at filesystem-level, so this type of encryption can be applied to specific folders/directories as needed after creation of Filesystem.

See eCryptfs homepage and eCryptfs Article for more details.

Userspace Hardening

Many security features are available through the default compiler flags used to build packages and through the kernel in Fedora.

Non-Executable Memory (NX)

Modern processors support a feature called NX which allows a system to control the execution of various portions of memory. Data memory is flagged as non-executable and program memory is flagged as non-writeable. This helps prevent certain types of buffer overflow exploits from working as expected. Most modern CPUs protect against executing non-executable memory regions (heap, stack, etc). Since not all processors support the NX feature, attempts have been made to support this feature via segment limits. A segment limit will prevent certain portions of memory from being executed. This provides very similar functionality to NX technology. After booting, you can see what NX protection is in effect:

  • Hardware-based (via PAE mode):
    • [ 0.000000] NX (Execute Disable) protection: active
  • Partial Emulation (via segment limits):
    • [ 0.000000] Using x86 segment limits to approximate NX protection

For more information, see Security Features page.

Built as PIE

All programs built as Position Independent Executables (PIE) with "-fPIE -pie" can take advantage of the exec ASLR. This protects against "return-to-text" and generally frustrates memory corruption attacks. This requires centralized changes to the compiler options when building the entire archive. PIE has a large (5-10%) performance penalty on architectures with small numbers of general registers (e.g. x86), so it should only be used for a select number of security-critical packages. PIE on x86_64 does not have the same penalties, and will eventually be made the default, but more testing is required. See this paper and this FESCo ticket for more information.

Pointer Obfuscation

Some pointers stored in glibc are obfuscated via PTR_MANGLE/PTR_UNMANGLE macros internally in glibc, preventing libc function pointers from being overwritten during runtime.

Heap Protector

The GNU C Library heap protector (both automatic via ptmalloc and manual) provides corrupted-list/unlink/double-free/overflow protections to the glibc heap memory manager (first introduced in glibc 2.3.4). This stops the ability to perform arbitrary code execution via heap memory overflows that try to corrupt the control structures of the malloc heap memory areas.This protection has evolved over time, adding more and more protections as additional corner-cases were researched. As it currently stands, glibc 2.10 and later appears to successfully resist even these hard-to-hit conditions. See this page for more details.

Built with Fortify Source

Programs built with "-D_FORTIFY_SOURCE=2" (and -O1 or higher), enable several compile-time and run-time protections in glibc:

  • expand unbounded calls to "sprintf", "strcpy" into their "n" length-limited cousins when the size of a destination buffer is known (protects against memory overflows).
  • stop format string "%n" attacks when the format string is in a writable memory segment.
  • require checking various important function return codes and arguments (e.g. system, write, open).
  • require explicit file mask when creating new files.

-D_FORTIFY_SOURCE=2 also protects C++ code. See this page for more information.

Stack Protector

gcc's -fstack-protector provides a randomized stack canary that protects against stack overflows, and reduces the chances of arbitrary code execution via controlling return address destinations. Enabled at compile-time. The routines used for stack checking are actually part of glibc, but gcc is patched to enable linking against those routines by default. See this page for more information.

Address Space Layout Randomization (ASLR)

ASLR is implemented by the kernel and the ELF loader by randomizing the location of memory allocations (stack, heap, shared libraries, etc). This makes memory addresses harder to predict when an attacker is attempting a memory-corruption exploit. ASLR is controlled system-wide by the value of /proc/sys/kernel/randomize_va_space.

  • 0 - No randomization, everything would get loaded at same address
  • 1 - Partial randomization, shared libraries , stack, mmap(), VDSO and heap are randomized
  • 2 - Full Randomization, in addition to Partial Randomization it randomizes Memory managed through brk().

ASLR on 32 bit systems is less effective as compared to 64 bit systems. It depends upon the amount of entropy available.


void* __get_eip() {
 * This function returns the return address of the currentfunction,or of
 * one of its callers. The level argument is number of frames to scan up
 * the call stack. A value of 0 yields the return address of the current
 * function, a value of 1 yields the return address of the caller of the
 * current function, and so forth. When inlining the expected behavior is
 * that the function returns the address of the function that is returned
 * to. To work around this behavior use the noinline function attribute. 

  return __builtin_return_address(0)-0x5;

int main(int argc, char **argv) {
  printf("EBP located at: %p
  return 0;
~]$ cat /proc/sys/kernel/randomize_va_space

~]$ gcc get_eip.c -o get_eip

~]$ ldd ./get_eip =>  (0x00007fff9a330000) => /lib64/ (0x0000003e9da00000)
        /lib64/ (0x0000003e9d600000)
~]$ ldd ./get_eip =>  (0x00007fffe77b1000) => /lib64/ (0x0000003e9da00000)
        /lib64/ (0x0000003e9d600000)

~]$ ./get_eip
EBP located at: 0x400552

~]$ ./get_eip
EBP located at: 0x400552

As from the test it can be seen that even if FULL Randomization is enabled, .text section remains static, to make ASLR effective all segments must be randomized, leaving some segment non randomized neutralizes protection provided by the ASLR, attacker can use this non randomized area to identify gadgets and can build exploit. So even if ASLR is forced not all the segments are randomized for all executable. Code segement and Text segment dont get randomized until compiled with PIE (Position Independent Executable).

See this article and this article for more information. ASLR is now enabled for all packages by default in Rawhide.

Stack ASLR

Each execution of a program results in a different stack memory space layout. This makes it harder to locate in memory where to attack or deliver an executable attack payload. This feature has been available in the mainline kernel since 2.6.15.

Libs/mmap ASLR

Each execution of a program results in a different mmap memory space layout. This causes the dynamically loaded libraries to get loaded into different locations each time. This makes it harder to locate in memory where to jump to for "return to libc" to similar attacks. This was available in the mainline kernel since 2.6.15.


Each execution of a program that has been built with "-fPIE -pie" will get loaded into a different memory location. This makes it harder to locate in memory where to attack or jump to when performing memory-corruption-based attacks. This was available in the mainline kernel since 2.6.25.

brk ASLR

Similar to exec ASLR, brk ASLR adjusts the memory locations relative between the exec memory area and the brk memory area (for small mallocs). The randomization of brk offset from exec memory was added in 2.6.22.


Each execution of a program results in a random vdso location. This has existed in the mainline kernel since 2.6.18 (x86, PPC) and 2.6.22 (x86_64). People needing ancient pre-libc6 static high vdso mappings can use "vdso=2" on the kernel boot command line to gain COMPAT_VDSO again. See this article for more information.

Built with RELRO

RELRO stands for RELocation Read-Only, it is a mitigation technique to harden data sections of an ELF/process. It is used to move commonly exploited structures in ELF binary to a read-only location.It Hardens ELF programs against loader memory area overwrites by having the loader mark any areas of the relocation table as read-only for any symbols resolved at load-time ("read-only relocations"). This reduces the area of possible GOT-overwrite-style memory corruption attacks, specially the GOT is made read-only after relocation by the dynamic linker.

RELRO can be classified into:

Partial RELRO

  • Compilation: gcc -Wl,-z,relro
  • ELF sections are reordered, so that ELF internal data sections (.got, .dtors, etc) precede the program's data sections (.data and .bss)
  • non-PLT GOT is read-only
  • GOT is writable


  • compilation: gcc -Wl,-z,relro,-z,now
  • Supports all the features of partial RELRO
  • In addition , GOT is also remapped as read-only

Only Full RELRO can protect from exploiting technique of overwriting GOT entry to get control over program execution flow.

So the question is what are GOT and PLT?

GOT (Global Offset Table) redirects position independent address calculations to an absolute location and is located in .got section of an ELF executable or shared object. It has the final location of a function calls symbol, used with dynamically linked code. By default GOT is created dynamically while program is running. The first time function is called GOT contains pointer back to PLT (Procedure Linkage Table), where linker is called to find actual location of the function. The location found is written to GOT, Second time whenever the function is called GOT already knows location of the function known as lazy binding.

PLT (Procedure Linker Table) works with GOT to reference and relocate functions. PLT reference will cause a jmp into the GOT and find the location of the called function. On the first call there wont be no entry in GOT, so PLT will hand over the request to the rtld for resolving the function's absolute location, after this GOT will be updated for future references.

Few Constraints about PLT and GOT

1. PLT will always contain code that is called by program directly,so it will be allocated at a known offset from the .text segment.

2. GOT contains data used by different parts of the program directly,so it will be at a static address in the memory.

3. As GOT is "lazy binded",so it needs to be writable

In case of a bss or data overflow bug both partial and full RELRO can protect the ELF internal data sections from being overwritten. With full RELRO a working mitigation technique to successfully prevent the modification of GOT entries is available.Only one reason why full RELRO is not widely used is that the startup of processes is slowed down as the linker has to perform all relocations at startup time.

In short, RELRO hardens ELF programs against loader memory area overwrites by having the loader mark any areas of the relocation table as read-only for any symbols resolved at load-time ("read-only relocations"). This reduces the area of possible GOT-overwrite-style memory corruption attacks.

Built with BIND_NOW

Marks ELF programs to resolve all dynamic symbols at start-up (instead of on-demand, also known as "immediate binding") so that the GOT can be made entirely read-only (when combined with RELRO above).

/proc/$pid/maps protection

With ASLR, a process's memory space layout suddenly becomes valuable to attackers. The "maps" file is made read-only except to the process itself or the owner of the process. Went into mainline kernel with sysctl toggle in 2.6.22. The toggle was made non-optional in 2.6.27, forcing the privacy to be enabled regardless of sysctl settings (this is a good thing).

Symlink restrictions

A long-standing class of security issues is the symlink-based ToCToU race, most commonly seen in world-writable directories like /tmp/. The common method of exploitation of this flaw is crossing privilege boundaries when following a given symlink (i.e. a root user follows a symlink belonging to another user).

In modern Fedora version, symlinks in world-writable sticky directories (e.g. /tmp) cannot be followed if the follower and directory owner do not match the symlink owner. The behavior is controllable through the /proc/sys/kernel/yama/protected_sticky_symlinks sysctl.

Hardlink restrictions

Hardlinks can be abused in a similar fashion to symlinks above, but they are not limited to world-writable directories. If /etc/ and /home/ are on the same partition, a regular user can create a hardlink to /etc/shadow in their home directory. While it retains the original owner and permissions, it is possible for privileged programs that are otherwise symlink-safe to mistakenly access the file through its hardlink. Additionally, a very minor untraceable quota-bypassing local denial of service is possible by an attacker exhausting disk space by filling a world-writable directory with hardlinks.

In modern Fedora versions, hardlinks cannot be created to files that the user would be unable to read and write originally, or are otherwise sensitive.

ptrace scope

A troubling weakness of the Linux process interfaces is that a single user is able to examine the memory and running state of any of their processes. For example, if one application was compromised, it would be possible for an attacker to attach to other running processes (e.g. SSH sessions, GPG agent, etc) to extract additional credentials and continue to immediately expand the scope of their attack without resorting to user-assisted phishing or trojans. It is provided by YAMA , can be enabled by CONFIG_SECURITY_YAMA in the kernel.

Independent of this configuration, processes that know they store secrets in memory may already use prctl(PR_SET_DUMPABLE,0); to prevent ptrace and other memory-snooping attacks.

Kernel Hardening

The kernel itself has protections enabled to make it more difficult to become compromised.

0-address protection

Since the kernel and userspace share virtual memory addresses, the "NULL" memory space needs to be protected so that userspace mmap'd memory cannot start at address 0, stopping "NULL dereference" kernel attacks. This is possible with 2.6.22 kernels, and was implemented with the "mmap_min_addr" sysctl setting. See this article for more information.

Block module loading

It is possible to remove CAP_SYS_MODULES from the system-wide capability bounding set , which would stop any new kernel modules from being loaded. This was another layer of protection to stop kernel rootkits from being installed. This feature to block module loading can be enabled setting 1 in /proc/sys/kernel/modules_disabled.

/dev/mem protection

Some applications (Xorg) need direct access to the physical memory from user-space. The special file /dev/mem exists to provide this access. In the past, it was possible to view and change kernel memory from this file if an attacker had root access. See this page and this page for details.

/dev/kmem disabled

There is no modern user of /dev/kmem any more beyond attackers using it to load kernel rootkits. CONFIG_DEVKMEM is set to n.

Module RO/NX

This feature extends CONFIG_DEBUG_RODATA to include similar restrictions for loaded modules in the kernel. This can help resist future kernel exploits that depend on various memory regions in loaded modules. Enabled via the CONFIG_DEBUG_SET_MODULE_RONX option.

Kernel Address Display Restriction

When attackers try to develop run anywhere exploits for kernel vulnerabilities, they frequently need to know the location of internal kernel structures. By treating kernel addresses as sensitive information, those locations are not visible to regular local users. /proc/sys/kernel/kptr_restrict is set to 1 to block the reporting of known kernel address leaks. Additionally, various files and directories were made readable only by the root user: /boot/vmlinuz, /boot/, /sys/kernel/debug/, /proc/slabinfo.

Blacklist Rare Protocols

Normally the kernel allows all network protocols to be autoloaded on demand. Many of these protocols are old, rare, or generally of little use to the average Fedora user and may contain undiscovered exploitable vulnerabilities. These include: ax25, netrom, x25, rose, decnet, econet, rds, and af_802154. If any of the protocols are needed, they can speficially loaded via modprobe, or the /etc/modprobe.d/blacklist-rare-network.conf file can be updated to remove the blacklist entry. A FESCo proposal to do this for Fedora is in progress.

Write-protect kernel .rodata sections

Enabled write-protection for kernel read-only data structures by default. See this commit for details. This makes sure that certain kernel data sections are marked to block modification. This helps protect against some classes of kernel rootkits. Enabled via the CONFIG_DEBUG_RODATA option.

Kernel Stack Protector

Similar to the stack protector used for ELF programs in userspace, the kernel can protect its internal stacks as well. This feature is enabled via the CONFIG_CC_STACKPROTECTOR option.

See commits 1, 2 and 3 for more details.

sVirt labelling

Support for sVirt labelling to provide security over guest instances. See this page for more information.

SYN cookies

When a system is overwhelmed by new network connections, SYN cookie use is activated, which helps mitigate a SYN-flood attack. This feature can be controlled by /proc/sys/net/ipv4/tcp_syncookies file.

Syscall Filtering

Programs can filter out the availability of kernel syscalls by using the seccomp_filter interface. This is done in containers or sandboxes that want to further limit the exposure to kernel interfaces when potentially running untrusted software.

Secure Boot Support

"Secure Boot" describes a UEFI feature by which malware is prevented from inserting itself into the boot process before the operating system loads. Secure Boot is an optional feature which can be enabled and disabled on will of user.

For more indepth information about Secure boot see [1]


UEFI_Secure_Boot_in_Modern_Computer_Security_Solutions_2013.pdf article for more details.

Tamper Resistant Logs

When system get attacked attackers might tamper logs on the system being attacked, this can be prevented by using FSS ( Forward Secure Sealing ) which is implemented in systemd journal. Binary logs maintained by systemd are sealed at certain time intervals. Sealing is an cryptographic operation on the logs so that any tempering on the logs can be detected, though an attacker can completely remove entire logs but this will get noticed by administrator too. FSS is based on "Forward Secure Pseudo Random Generators" (FSPRG)

# journalctl --setup-keys

there are two keys generated with this

1. Sealing key : It is stored on the system and after certain time intervals new sealing key is generated with the use of FSPRG and its a non-reversible process old key is deleted after this.

2. Verification Key : Verification key should be stored at safe place, could be phone device or any place else which can be trusted. This key can be used to generate sealing key at any point of given time. Attacker can only access current sealing key ,so changing the log files using current sealing key would result in verification failure as it wont verify by the sealing key generated from Verification key.

FSS will seal logs after every 15 min by default, which can be changed by using "--interval=60s" to seal logs after every minute. Default time 15min is too much of time for attacker to work, so it should be changed accordingly by system administrators to harden such tasks for attackers.

# journalctl --setup-keys --interval=60s

Deleting of Old Sealing keys is handled by two file attributes FS_SECRM_FL and FS_NOCOW_FL, which may or may not be supported by filesystem in use.

See Forward Secure Sealing (FSS) article for more information.

Overflow checking in new operator

GCC performs overflow checking in operator new[]. new operator is used to dynamically allocate memory.It throws bad_alloc exception, header to include for using it is <new> new() or new[]() without declaration of exception cannot signal memory exhaustion.If there is an option to choose between calloc/malloc/new for allocation of the memory, new should be used. If new[] is used to allocate memory then delete[] should be used to free the allocated memory. Using delete without [] will cause memory leak. Use try-catch block with new, as it throws exception and does not return value, though it can be forced to return a value by using nothrow.

 using namespace std;
 /* this should return a value */
 alpha* pt = new (nothrow) alpha[200];

 or it will throw bad_alloc exception which can be handled by the following code
 class bad_alloc : public exception {
 /* error to be thrown to be implemented here */
 struct alpha_t{};

 extern const alpha_t alpha;  // indicator for allocation to prevent exceptions

 /* should throw exception */
 int* ptr = new int[100000];

 /* to avoid exception correct usage would be */
 int* ptr = new(alpha) int[100000];

See Array allocation in C++ article for more information.

Built with Format Security

Enable "-Werror=format-security" compilation flag for all packages in Fedora. Once this flag is enabled, GCC will refuse to compile code that could be vulnerable to a string format security flaw. see Format Security for more information

Crypto Policy

Unify the crypto policies used by different applications and libraries. That is allow setting a consistent security level for crypto on all applications in a Fedora system. The implementation approach will be to initially modify SSL libraries to respect the policy and gradually adding more libraries and applications. see Crypto Policy for more information

Built with Stack Protector Strong

see Stack Protector Strong for more information

Additional Documentation