linux-kernel - Re: [RFC PATCH v1 09/28] mm: abstract shadow stack vma behind `arch_is_shadow

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Date: Thu, 25 Jan 2024 09:18:07 +0100
From: David Hildenbrand <david@...hat.com>
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Subject: Re: [RFC PATCH v1 09/28] mm: abstract shadow stack vma behind
 `arch_is_shadow_stack`

On 25.01.24 07:21, debug@...osinc.com wrote:
> From: Deepak Gupta <debug@...osinc.com>
> 
> x86 has used VM_SHADOW_STACK (alias to VM_HIGH_ARCH_5) to encode shadow
> stack VMA. VM_SHADOW_STACK is thus not possible on 32bit. Some arches may
> need a way to encode shadow stack on 32bit and 64bit both and they may
> encode this information differently in VMAs.
> 
> This patch changes checks of VM_SHADOW_STACK flag in generic code to call
> to a function `arch_is_shadow_stack` which will return true if arch
> supports shadow stack and vma is shadow stack else stub returns false.
> 
> There was a suggestion to name it as `vma_is_shadow_stack`. I preferred to
> keep `arch` prefix in there because it's each arch specific.
> 
> Signed-off-by: Deepak Gupta <debug@...osinc.com>
> ---
>   include/linux/mm.h | 18 +++++++++++++++++-
>   mm/gup.c           |  5 +++--
>   mm/internal.h      |  2 +-
>   3 files changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index dfe0e8118669..15c70fc677a3 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -352,6 +352,10 @@ extern unsigned int kobjsize(const void *objp);
>    * for more details on the guard size.
>    */
>   # define VM_SHADOW_STACK	VM_HIGH_ARCH_5
> +static inline bool arch_is_shadow_stack(vm_flags_t vm_flags)
> +{
> +	return (vm_flags & VM_SHADOW_STACK);
> +}
>   #endif
>   
>   #ifdef CONFIG_RISCV_USER_CFI
> @@ -362,10 +366,22 @@ extern unsigned int kobjsize(const void *objp);
>    * with VM_SHARED.
>    */
>   #define VM_SHADOW_STACK	VM_WRITE
> +
> +static inline bool arch_is_shadow_stack(vm_flags_t vm_flags)
> +{
> +	return ((vm_flags & (VM_WRITE | VM_READ | VM_EXEC)) == VM_WRITE);
> +}
> +

Please no such hacks just to work around the 32bit vmflags limitation.

-- 
Cheers,

David / dhildenb