Security Features Matrix

By Default

Available

Unimplemented

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

Configuration

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, 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

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.

Subsystems

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 avaiable 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 privelage operation is done operating system checks for bit in effective set of the processes rather than effective uid.

2. permitted (P) Indicates which capabilities process can use. Process migh have capabilities set in permitted set but not in the effective set, that would mean that particualar 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 privelages

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

RELEASE	KERNEL	CAPABILITY
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 1 127.0.0.1
 ping: icmp open socket: Operation not permitted

 # ./ping -q -c 1 127.0.0.1
 PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
 --- 127.0.0.1 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 1 127.0.0.1
 PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
 --- 127.0.0.1 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 1 127.0.0.1
 PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
 --- 127.0.0.1 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 are handled via the kernel LSM hooks.

SELinux user confinement

Support for SELinux to confine users access on a system. See this page for more information.

SELinux XACE

Support for SELinux X Access Control Extension (XACE). See this page for more information.

SELinux sandbox

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

PR_SET_SECCOMP

Setting SECCOMP for a process is meant to confine it to a small subsystem of system calls. The systemd init daemon supports the seccomp filter mechanism found in the 3.5 kernel. The end result is that processes can be easily configured to be run in a sandboxed environment. See this article 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.

per-user temp folder

Similar to the previous security feature,, per-user /tmp protect users and the system from mistakes in /tmp handling, by having a private /tmp for each users. This is transparent for them, and each directory is separated. This feature is implemented using pam_namespace.so.

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.

eCryptfs

eCryptfs 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. See eCryptfs homepage 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 Randomisation (ASLR)

ASLR is implemented by the kernel and the ELF loader by randomising 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. 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.

Exec ASLR

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.

VDSO ASLR

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

Full RELRO

- 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.

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 possbile 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_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/System.map, /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. See Fedora + UEFI Secure Boot article for more details.

Tamper Resistant Logs

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

Overflow checking in new operator

GCC performs overflow checking in operator new[], and all C++ code is compiled with this hardening feature. See Array allocation in C++ article for more information.

Additional Documentation

• Coordination with Ubuntu: https://wiki.ubuntu.com/Security/Features
• Coordination with Debian: http://wiki.debian.org/Hardening
• Gentoo's Hardening project: http://www.gentoo.org/proj/en/hardened/hardened-toolchain.xml