Re: [PATCH v3 09/29] mm: abstract shadow stack vma behind `vma_is_shadow_stack`

David Hildenbrand
2024-04-04T19:02:34Z

On 04.04.24 01:34, Deepak Gupta wrote:
> VM_SHADOW_STACK (alias to VM_HIGH_ARCH_5) to encode shadow stack VMA.
>
> This patch changes checks of VM_SHADOW_STACK flag in generic code to call
> to a function `vma_is_shadow_stack` which will return true if its a
> shadow stack vma and default stub (when support doesnt exist) returns false.
>
> Signed-off-by: Deepak Gupta <debug@rivosinc.com>
> Suggested-by: Mike Rapoport <rppt@kernel.org>
> ---
> include/linux/mm.h | 13 ++++++++++++-
> mm/gup.c | 5 +++--
> mm/internal.h | 2 +-
> 3 files changed, 16 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 64109f6c70f5..9952937be659 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -363,8 +363,19 @@ extern unsigned int kobjsize(const void *objp);
>
> #ifndef VM_SHADOW_STACK
> # define VM_SHADOW_STACK VM_NONE
> +
> +static inline bool vma_is_shadow_stack(vm_flags_t vm_flags)
> +{
> + return false;
> +}
> +#else
> +static inline bool vma_is_shadow_stack(vm_flags_t vm_flags)
> +{
> + return (vm_flags & VM_SHADOW_STACK);
> +}
> #endif

You can simply do outside the ifdef

static inline bool vma_is_shadow_stack(vm_flags_t vm_flags)
{
return !!(vm_flags & VM_SHADOW_STACK);
}

This will work even when VM_SHADOW_STACK is defined to be VM_NONE.

>
> +

unrelated code change

> #if defined(CONFIG_X86)
> # define VM_PAT VM_ARCH_1 /* PAT reserves whole VMA at once (x86) */
> #elif defined(CONFIG_PPC)
> @@ -3473,7 +3484,7 @@ static inline unsigned long stack_guard_start_gap(struct vm_area_struct *vma)
> return stack_guard_gap;
>
> /* See reasoning around the VM_SHADOW_STACK definition */
> - if (vma->vm_flags & VM_SHADOW_STACK)
> + if (vma->vm_flags && vma_is_shadow_stack(vma->vm_flags))

Pretty sure:

if (vma_is_shadow_stack(vma->vm_flags))

> return PAGE_SIZE;
>
> return 0;
> diff --git a/mm/gup.c b/mm/gup.c
> index df83182ec72d..a7a02eb0a6b3 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1053,7 +1053,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
> !writable_file_mapping_allowed(vma, gup_flags))
> return -EFAULT;
>
> - if (!(vm_flags & VM_WRITE) || (vm_flags & VM_SHADOW_STACK)) {
> + if (!(vm_flags & VM_WRITE) || vma_is_shadow_stack(vm_flags)) {
> if (!(gup_flags & FOLL_FORCE))
> return -EFAULT;
> /* hugetlb does not support FOLL_FORCE|FOLL_WRITE. */
> @@ -1071,7 +1071,8 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
> if (!is_cow_mapping(vm_flags))
> return -EFAULT;
> }
> - } else if (!(vm_flags & VM_READ)) {
> + } else if (!(vm_flags & VM_READ) && !vma_is_shadow_stack(vm_flags)) {
> + /* reads allowed if its shadow stack vma */
> if (!(gup_flags & FOLL_FORCE))
> return -EFAULT;
> /*

Unless I am missing something, this is not a simple cleanup. It should
go into a separate patch with a clearly documented reason for that change.

--
Cheers,

David / dhildenb