Re: [PATCH v6 18/41] mm: Introduce VM_SHADOW_STACK for shadow stack memory

[David Hildenbrand]

On 18.02.23 22:14, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
>
> The x86 Control-flow Enforcement Technology (CET) feature includes a new
> type of memory called shadow stack. This shadow stack memory has some
> unusual properties, which requires some core mm changes to function
> properly.
>
> A shadow stack PTE must be read-only and have _PAGE_DIRTY set. However,
> read-only and Dirty PTEs also exist for copy-on-write (COW) pages. These
> two cases are handled differently for page faults. Introduce
> VM_SHADOW_STACK to track shadow stack VMAs.

I suggest simplifying and abstracting that description.

"New hardware extensions implement support for shadow stack memory, such 
as x86 Control-flow Enforcement Technology (CET). Let's add a new VM 
flag to identify these areas, for example, to be used to properly 
indicate shadow stack PTEs to the hardware."

> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
> Tested-by: John Allen <john.allen@xxxxxxx>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
>
> ---
> v6:
>   - Add comment about VM_SHADOW_STACK not being allowed with VM_SHARED
>     (David Hildenbrand)

Might want to add some more meat to the patch description why that is 
the case.

> v3:
>   - Drop arch specific change in arch_vma_name(). The memory can show as
>     anonymous (Kirill)
>   - Change CONFIG_ARCH_HAS_SHADOW_STACK to CONFIG_X86_USER_SHADOW_STACK
>     in show_smap_vma_flags() (Boris)
> ---
>   Documentation/filesystems/proc.rst | 1 +
>   fs/proc/task_mmu.c                 | 3 +++
>   include/linux/mm.h                 | 8 ++++++++
>   3 files changed, 12 insertions(+)
>
> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> index e224b6d5b642..115843e8cce3 100644
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -564,6 +564,7 @@ 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
>       ==    =======================================
>   
>   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 af1c49ae11b1..9e2cefe47749 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)
>   #ifdef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
>   		[ilog2(VM_UFFD_MINOR)]	= "ui",
>   #endif /* CONFIG_HAVE_ARCH_USERFAULTFD_MINOR */
> +#ifdef CONFIG_X86_USER_SHADOW_STACK
> +		[ilog2(VM_SHADOW_STACK)] = "ss",
> +#endif
>   	};
>   	size_t i;
>   
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index e6f1789c8e69..76e0a09aeffe 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -315,11 +315,13 @@ extern unsigned int kobjsize(const void *objp);
>   #define VM_HIGH_ARCH_BIT_2	34	/* bit only usable on 64-bit architectures */
>   #define VM_HIGH_ARCH_BIT_3	35	/* bit only usable on 64-bit architectures */
>   #define VM_HIGH_ARCH_BIT_4	36	/* bit only usable on 64-bit architectures */
> +#define VM_HIGH_ARCH_BIT_5	37	/* bit only usable on 64-bit architectures */
>   #define VM_HIGH_ARCH_0	BIT(VM_HIGH_ARCH_BIT_0)
>   #define VM_HIGH_ARCH_1	BIT(VM_HIGH_ARCH_BIT_1)
>   #define VM_HIGH_ARCH_2	BIT(VM_HIGH_ARCH_BIT_2)
>   #define VM_HIGH_ARCH_3	BIT(VM_HIGH_ARCH_BIT_3)
>   #define VM_HIGH_ARCH_4	BIT(VM_HIGH_ARCH_BIT_4)
> +#define VM_HIGH_ARCH_5	BIT(VM_HIGH_ARCH_BIT_5)
>   #endif /* CONFIG_ARCH_USES_HIGH_VMA_FLAGS */
>   
>   #ifdef CONFIG_ARCH_HAS_PKEYS
> @@ -335,6 +337,12 @@ extern unsigned int kobjsize(const void *objp);
>   #endif
>   #endif /* CONFIG_ARCH_HAS_PKEYS */
>   
> +#ifdef CONFIG_X86_USER_SHADOW_STACK


Should we abstract this to CONFIG_ARCH_USER_SHADOW_STACK, seeing that 
other architectures might similarly need it?

--
Thanks,

David / dhildenb