Mitigations(7) - SerenityOS man pages

Name

Mitigations - Security mitigations implemented by SerenityOS

Description

The SerenityOS developers have put substantial effort into integrating various mitigation technologies into the system in order to enhance its security. The goal of this document is to collect and describe the mitigations in one centralized place.

List of Mitigations

SMEP (Supervisor Mode Execution Protection)

Supervisor Mode Execution Protection is an Intel CPU feature which prevents execution of userspace code with kernel privileges.

It was enabled in the following commit:

commit 8602fa5b49aa4e2b039764a14698f0baa3ad0532
Author: Andreas Kling <awesomekling@gmail.com>
Date:   Wed Jan 1 01:56:58 2020 +0100

Kernel: Enable x86 SMEP (Supervisor Mode Execution Protection)

SMAP (Supervisor Mode Access Prevention)

Supervisor Mode Access Prevention complements SMEP by also guarding read/write access to userspace memory while executing in kernel mode.

It was enabled in the following commit:

commit 9eef39d68a99c5e29099ae4eb4a56934b35eecde
Author: Andreas Kling <awesomekling@gmail.com>
Date:   Sun Jan 5 18:00:15 2020 +0100

Kernel: Start implementing x86 SMAP support

Pledge

pledge is a mitigation which originated from OpenBSD. It allows a program to voluntarily restrict its access to system calls and kernel facilities.

It was first added in the following commit, and the majority of programs were enlightened later:

commit 41c504a33becea8aa9b437cd3c0dc312b2bf1fe9
Author: Andreas Kling <awesomekling@gmail.com>
Date:   Sat Jan 11 20:45:51 2020 +0100

Kernel: Add pledge() syscall :^)

Unveil

unveil is a mitigation originating from OpenBSD. It allows a program to voluntarily restrict its access to the filesystem.

It was first added in the following commit, and the majority of programs were enlightened later:

commit 0569123ad7cb9c54df724c2bb85933ea3cf97134
Author: Andreas Kling <kling@serenityos.org>
Date:   Mon Jan 20 22:12:04 2020 +0100

Kernel: Add a basic implementation of unveil()

syscall call-from verification

syscall call-from verification is a mitigation which originated from OpenBSD. In short the kernel checks that all syscalls originate from the address of the system's libc. This makes attacks on OpenBSD more difficult as they random-relink their libc on boot, which makes finding syscall stubs in libc difficult for attackers. On serenity it is mostly just an inconvenience, as there currently is no libc random-relinking.

It was first enabled in the following commit:

commit 823186031d9250217f9a51829d34a96b74113334
Author Andreas Kling <kling@serenityos.org>
Date:  Tue Feb 2 19:56:11 2021 +0100

Kernel: Add a way to specify which memory regions can make syscalls

Post-init read-only memory

Post-init read-only memory is a mitigation which originated from the Linux Kernel. It tracks data that is initialized during kernel boot and never changed again. Post kernel initialization, the memory is marked read-only to protect it from potentially being modified by exploits.

It was first enabled in the following commit and other kernel data structures were enlightened later:

commit d8013c60bb52756788e747183572067d6e3f204a
Author: Andreas Kling <kling@serenityos.org>
Date:   Sun Feb 14 17:35:07 2021 +0100

Kernel: Add mechanism to make some memory read-only after init finishes

KUBSAN (Kernel Undefined Behavior Sanitizer)

UndefinedBehaviorSANitizer is a dynamic analysis tool, implemented in GCC, which instruments generated code to flag undefined behavior at runtime. It can find various issues, including integer overflows, out-of-bounds array accesses, type corruption, and more.

It was first enabled in the following commit:

commit d44be968938ecf95023351a358c43c4957638d87
Author: Andreas Kling <kling@serenityos.org>
Date:   Fri Feb 5 19:44:26 2021 +0100

Kernel: KUBSAN! (Kernel Undefined Behavior SANitizer) :^)

Kernel unmap-after-init

Unmap-after-init allows the kernel to remove functions which contain potentially dangerous ROP gadgets from kernel memory after we've booted up and they are no longer needed. Notably the write_cr4(..) function used to control processor features like the SMEP/SMAP bits in the CR4 register, and the write_cr0(..) function used to control processor features like write protection, etc.

With this mitigation it is now more difficult to craft a kernel exploit to do something like disabling SMEP / SMAP.

It was first enabled in the following commit:

commit 6136faa4ebf6a878606f33bc03c5e62de9d5e662
Author: Andreas Kling <kling@serenityos.org>
Date:   Fri Feb 19 18:21:54 2021 +0100

Kernel: Add .unmap_after_init section for code we don't need after init

Relocation Read-Only (RELRO)

RELRO is a mitigation in the linker and loader that hardens the data sections of an ELF binary.

When enabled, it segregates function pointers resolved by the dynamic loader into a separate section of the runtime executable memory, and allows the loader to make that memory read-only before passing control to the main executable.

This prevents attackers from overwriting the Global Offset Table (GOT).

It was first enabled for executables in the following commit:

commit fa4c249425a65076ca04a3cb0c173d49472796fb
Author: Andreas Kling <kling@serenityos.org>
Date:   Thu Feb 18 18:43:20 2021 +0100

LibELF+Userland: Enable RELRO for all userland executables :^)

Shared libraries were enabled in a follow-up commit:

commit 713b3b36be4f659e58e253b6c830509898dbd2fa
Author: Andreas Kling <kling@serenityos.org>
Date:   Thu Feb 18 22:49:58 2021 +0100

DynamicLoader+Userland: Enable RELRO for shared libraries as well :^)

-fstack-clash-protection

The GCC compiler option -fstack-clash-protection is a mitigation which helps prevent stack clash style attacks by generating code that probes the stack in page-sized increments to ensure a fault is provoked. This prevents attackers from using a large stack allocation to "jump over" the stack guard page into adjacent memory.

It was first enabled in the following commit:

commit 7142562310e631156d1f64aff22f068ae2c48a5e
Author: Andreas Kling <kling@serenityos.org>
Date:   Fri Feb 19 09:11:02 2021 +0100

Everywhere: Build with -fstack-clash-protection

-fstack-protector / -fstack-protector-strong

The GCC compiler provides a few variants of the -fstack-protector option mitigation. This family of flags enables buffer overflow protection to mitigate stack-smashing attacks.

The compiler implements the mitigation by storing a canary value randomized on program startup into the preamble of all functions. Code is then generated to validate that stack canary on function return and crash if the value has been changed (and hence a stack corruption has been detected.)

-fstack-protector was first enabled in the following commit:

commit 842716a0b5eceb8db31416cd643720c1037032b2
Author: Andreas Kling <awesomekling@gmail.com>
Date:   Fri Dec 20 20:51:50 2019 +0100

Kernel+LibC: Build with basic -fstack-protector support

It was later re-enabled and refined to -fstack-protector-strong in the following commits:

commit fd08c93ef57f71360d74b035214c71d7f7bfc5b8
Author: Brian Gianforcaro <b.gianfo@gmail.com>
Date:   Sat Jan 2 04:27:35 2021 -0800

LibC: Randomize the stack check cookie value on initialization

commit 79328b2aba6192caf28f560881e56ff23fcb7294
Author: Brian Gianforcaro <b.gianfo@gmail.com>
Date:   Sat Jan 2 03:02:42 2021 -0800

Kernel: Enable -fstack-protector-strong (again)

commit 06da50afc71a5ab2bc63de54c66930a2dbe379cd
Author: Brian Gianforcaro <b.gianfo@gmail.com>
Date:   Fri Jan 1 15:27:42 2021 -0800

Build + LibC: Enable -fstack-protector-strong in user space

Protected Kernel Process Data

The kernel applies a exploit mitigation technique where vulnerable data related to the state of a process is separated out into it's own region in memory which is always remmaped as read-only after it's initialized or updated. This means that an attacker needs more than an arbitrary kernel write primitive to be able to elevate a process to root for example.

It was first enabled in the following commit:

commit cbcf891040e9921ff628fdda668c9738f358a178
Author: Andreas Kling <kling@serenityos.org>
Date:   Wed Mar 10 19:59:46 2021 +0100

Kernel: Move select Process members into protected memory

-fzero-call-used-regs

GCC-11 added a new option -fzero-call-used-regs which causes the compiler to zero function arguments before return of a function. The goal being to reduce the possible attack surface by disarming ROP gadgets that might be potentially useful to attackers, and reducing the risk of information leaks via stale register data.

It was first enabled when compiling the Kernel in the following commit:

commit 204d5ff8f86547a8b100cf26a958aaabf49211f2
Author: Brian Gianforcaro <bgianf@serenityos.org>
Date:   Fri Jul 23 00:42:54 2021 -0700

Kernel: Reduce useful ROP gadgets by zeroing used function registers

See also