linux-kernel - Re: [PATCH v3 08/29] mm: Define VM_SHADOW

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Date: Thu, 4 Apr 2024 20:58:06 +0200
From: David Hildenbrand <david@...hat.com>
To: Deepak Gupta <debug@...osinc.com>, paul.walmsley@...ive.com,
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Subject: Re: [PATCH v3 08/29] mm: Define VM_SHADOW_STACK for RISC-V

On 04.04.24 01:34, Deepak Gupta wrote:
> VM_SHADOW_STACK is defined by x86 as vm flag to mark a shadow stack vma.
> 
> x86 uses VM_HIGH_ARCH_5 bit but that limits shadow stack vma to 64bit only.
> arm64 follows same path (see links)
> 
> To keep things simple, RISC-V follows the same.
> This patch adds `ss` for shadow stack in process maps.
> 
> Links:
> https://lore.kernel.org/lkml/20231009-arm64-gcs-v6-12-78e55deaa4dd@kernel.org/#r
> 
> Signed-off-by: Deepak Gupta <debug@...osinc.com>
> ---
>   fs/proc/task_mmu.c |  3 +++
>   include/linux/mm.h | 11 ++++++++++-
>   2 files changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 3f78ebbb795f..d9d63eb74f0d 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -702,6 +702,9 @@ 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_RISCV_USER_CFI
> +		[ilog2(VM_SHADOW_STACK)] = "ss",
>   #endif
>   	};
>   	size_t i;
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index f5a97dec5169..64109f6c70f5 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -352,7 +352,16 @@ extern unsigned int kobjsize(const void *objp);
>    * for more details on the guard size.
>    */
>   # define VM_SHADOW_STACK	VM_HIGH_ARCH_5
> -#else
> +#endif
> +
> +#ifdef CONFIG_RISCV_USER_CFI
> +/*
> + * RISC-V is going along with using VM_HIGH_ARCH_5 bit position for shadow stack
> + */

Wow, really?! I could never have guesses that from the code :P

Just drop that comment. Are them semantics the same as for the x86 variant documented?
("VM_SHADOW_STACK should not be set with VM_SHARED because of lack of")


I assume so. So it might now make sense to merge both paths

#if defined(CONFIG_X86_USER_SHADOW_STACK) || defined(CONFIG_RISCV_USER_CFI)


or even introduce some ARCH_HAS_SHADOW_STACK so we can remove these
arch-specific thingies here.

-- 
Cheers,

David / dhildenb