Re: [PATCH v5 11/37] mm: Define VM_SHADOW_STACK for arm64 when we support GCS

David Hildenbrand
2023-08-22T15:22:06Z

On 22.08.23 15:56, Mark Brown wrote:
> Use VM_HIGH_ARCH_5 for guarded control stack pages.
>
> Signed-off-by: Mark Brown <broonie@kernel.org>
> ---
> Documentation/filesystems/proc.rst | 2 +-
> fs/proc/task_mmu.c | 3 +++
> include/linux/mm.h | 12 +++++++++++-
> 3 files changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> index 6ccb57089a06..086a0408a4d7 100644
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -566,7 +566,7 @@ encoded manner. The codes are the following:
> mt arm64 MTE allocation tags are enabled
> um userfaultfd missing tracking
> uw userfaultfd wr-protect tracking
> - ss shadow stack page
> + ss shadow/guarded control stack page
> == =======================================
>
> Note that there is no guarantee that every flag and associated mnemonic will
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index cfab855fe7e9..e8c50848bb16 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -711,6 +711,9 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
> #endif /* CONFIG_HAVE_ARCH_USERFAULTFD_MINOR */
> #ifdef CONFIG_X86_USER_SHADOW_STACK
> [ilog2(VM_SHADOW_STACK)] = "ss",
> +#endif
> +#ifdef CONFIG_ARM64_GCS
> + [ilog2(VM_SHADOW_STACK)] = "ss",
> #endif

See my comment below.

> };
> size_t i;
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 43fe625b85aa..3f939ae212e5 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -372,7 +372,17 @@ extern unsigned int kobjsize(const void *objp);
> * having a PAGE_SIZE guard gap.
> */
> # define VM_SHADOW_STACK VM_HIGH_ARCH_5
> -#else
> +#endif
> +
> +#if defined(CONFIG_ARM64_GCS)
> +/*
> + * arm64's Guarded Control Stack implements similar functionality and
> + * has similar constraints to shadow stacks.
> + */
> +# define VM_SHADOW_STACK VM_HIGH_ARCH_5
> +#endif

Shouldn't that all just merged with the previous define(s)?

Also, I wonder if we now want to have CONFIG_HAVE_ARCH_SHADOW_STACK or
similar.

--
Cheers,

David / dhildenb