Re: [PATCH v15 0/6] KVM: arm64: Provide guest support for GCS

[Marc Zyngier <maz@kernel.org>]

On Wed, 20 Aug 2025 15:14:40 +0100,
Mark Brown <broonie@kernel.org> 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 managing GCS for KVM guests, it also
> includes a fix for S1PIE which has also been sent separately as this
> feature is a dependency for GCS.  It is based on:
> 
>    https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-next/gcs
>

Is this cover letter accurate? I don't see any PIE-related patch, and
you indicate this being rebased on 6.17-rc1...

	M.

-- 
Jazz isn't dead. It just smells funny.