Re: [PATCH v9 16/42] mm: Add guard pages around a shadow stack.

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jun 12, 2023 at 05:10:42PM -0700, Rick Edgecombe wrote:
> +++ b/include/linux/mm.h
> @@ -342,7 +342,36 @@ extern unsigned int kobjsize(const void *objp);
>  #endif /* CONFIG_ARCH_HAS_PKEYS */
>  
>  #ifdef CONFIG_X86_USER_SHADOW_STACK
> -# define VM_SHADOW_STACK	VM_HIGH_ARCH_5 /* Should not be set with VM_SHARED */
> +/*
> + * This flag should not be set with VM_SHARED because of lack of support
> + * core mm. It will also get a guard page. This helps userspace protect
> + * itself from attacks. The reasoning is as follows:
> + *
> + * The shadow stack pointer(SSP) is moved by CALL, RET, and INCSSPQ. The
> + * INCSSP instruction can increment the shadow stack pointer. It is the
> + * shadow stack analog of an instruction like:
> + *
> + *   addq $0x80, %rsp
> + *
> + * However, there is one important difference between an ADD on %rsp
> + * and INCSSP. In addition to modifying SSP, INCSSP also reads from the
> + * memory of the first and last elements that were "popped". It can be
> + * thought of as acting like this:
> + *
> + * READ_ONCE(ssp);       // read+discard top element on stack
> + * ssp += nr_to_pop * 8; // move the shadow stack
> + * READ_ONCE(ssp-8);     // read+discard last popped stack element
> + *
> + * The maximum distance INCSSP can move the SSP is 2040 bytes, before
> + * it would read the memory. Therefore a single page gap will be enough
> + * to prevent any operation from shifting the SSP to an adjacent stack,
> + * since it would have to land in the gap at least once, causing a
> + * fault.
> + *
> + * Prevent using INCSSP to move the SSP between shadow stacks by
> + * having a PAGE_SIZE guard gap.
> + */
> +# define VM_SHADOW_STACK	VM_HIGH_ARCH_5
>  #else
>  # define VM_SHADOW_STACK	VM_NONE
>  #endif

This is a lot of very x86-specific language in a generic header file.
I'm sure there's a better place for all this text.