Re: [PATCH v10 00/40] arm64/gcs: Provide support for GCS in userspace

Anders Roxell
2024-08-02T16:03:55Z

On 2024-08-01 13:06, Mark Brown wrote:
> The arm64 Guarded Control Stack (GCS) feature provides support for
> hardware protected stacks of return addresses, intended to provide
> hardening against return oriented programming (ROP) attacks and to make
> it easier to gather call stacks for applications such as profiling.
>
> When GCS is active a secondary stack called the Guarded Control Stack is
> maintained, protected with a memory attribute which means that it can
> only be written with specific GCS operations. The current GCS pointer
> can not be directly written to by userspace. When a BL is executed the
> value stored in LR is also pushed onto the GCS, and when a RET is
> executed the top of the GCS is popped and compared to LR with a fault
> being raised if the values do not match. GCS operations may only be
> performed on GCS pages, a data abort is generated if they are not.
>
> The combination of hardware enforcement and lack of extra instructions
> in the function entry and exit paths should result in something which
> has less overhead and is more difficult to attack than a purely software
> implementation like clang's shadow stacks.
>
> This series implements support for use of GCS by userspace, along with
> support for use of GCS within KVM guests. It does not enable use of GCS
> by either EL1 or EL2, this will be implemented separately. Executables
> are started without GCS and must use a prctl() to enable it, it is
> expected that this will be done very early in application execution by
> the dynamic linker or other startup code. For dynamic linking this will
> be done by checking that everything in the executable is marked as GCS
> compatible.
>
> x86 has an equivalent feature called shadow stacks, this series depends
> on the x86 patches for generic memory management support for the new
> guarded/shadow stack page type and shares APIs as much as possible. As
> there has been extensive discussion with the wider community around the
> ABI for shadow stacks I have as far as practical kept implementation
> decisions close to those for x86, anticipating that review would lead to
> similar conclusions in the absence of strong reasoning for divergence.
>
> The main divergence I am concious of is that x86 allows shadow stack to
> be enabled and disabled repeatedly, freeing the shadow stack for the
> thread whenever disabled, while this implementation keeps the GCS
> allocated after disable but refuses to reenable it. This is to avoid
> races with things actively walking the GCS during a disable, we do
> anticipate that some systems will wish to disable GCS at runtime but are
> not aware of any demand for subsequently reenabling it.
>
> x86 uses an arch_prctl() to manage enable and disable, since only x86
> and S/390 use arch_prctl() a generic prctl() was proposed[1] as part of a
> patch set for the equivalent RISC-V Zicfiss feature which I initially
> adopted fairly directly but following review feedback has been revised
> quite a bit.
>
> We currently maintain the x86 pattern of implicitly allocating a shadow
> stack for threads started with shadow stack enabled, there has been some
> discussion of removing this support and requiring the use of clone3()
> with explicit allocation of shadow stacks instead. I have no strong
> feelings either way, implicit allocation is not really consistent with
> anything else we do and creates the potential for errors around thread
> exit but on the other hand it is existing ABI on x86 and minimises the
> changes needed in userspace code.
>
> glibc and bionic changes using this ABI have been implemented and
> tested. Headless Android systems have been validated and Ross Burton
> has used this code has been used to bring up a Yocto system with GCS
> enabed as standard, a test implementation of V8 support has also been
> done.
>
> There is an open issue with support for CRIU, on x86 this required the
> ability to set the GCS mode via ptrace. This series supports
> configuring mode bits other than enable/disable via ptrace but it needs
> to be confirmed if this is sufficient.
>
> The series depends on support for shadow stacks in clone3(), that series
> includes the addition of ARCH_HAS_USER_SHADOW_STACK.
>
> https://lore.kernel.org/r/20240731-clone3-shadow-stack-v7-0-a9532eebfb1d@kernel.org
>

Verified this patchset on a FVP.

Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>

Cheers,
Anders