Re: [PATCH] x86/shstk: Move arch detail comment out of core mm

[Mike Rapoport]

Hi Dave, Rick,

It seems it didn't get into the current tip.

On Thu, Jul 06, 2023 at 04:32:48PM -0700, Rick Edgecombe wrote:
> The comment around VM_SHADOW_STACK in mm.h refers to a lot of x86
> specific details that don't belong in a cross arch file. Remove these
> out of core mm, and just leave the non-arch details.
> 
> Since the comment includes some useful details that would be good to
> retain in the source somewhere, put the arch specifics parts in
> arch/x86/shstk.c near alloc_shstk(), where memory of this type is
> allocated. Include a reference to the existence of the x86 details near
> the VM_SHADOW_STACK definition mm.h.
> 
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> ---
>  arch/x86/kernel/shstk.c | 25 +++++++++++++++++++++++++
>  include/linux/mm.h      | 32 ++++++--------------------------
>  2 files changed, 31 insertions(+), 26 deletions(-)
> 
> diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
> index b26810c7cd1c..47f5204b0fa9 100644
> --- a/arch/x86/kernel/shstk.c
> +++ b/arch/x86/kernel/shstk.c
> @@ -72,6 +72,31 @@ static int create_rstor_token(unsigned long ssp, unsigned long *token_addr)
>  	return 0;
>  }
>  
> +/*
> + * VM_SHADOW_STACK will have 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.
> + */
>  static unsigned long alloc_shstk(unsigned long addr, unsigned long size,
>  				 unsigned long token_offset, bool set_res_tok)
>  {
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 535c58d3b2e4..b647cf2e94ea 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -343,33 +343,13 @@ extern unsigned int kobjsize(const void *objp);
>  
>  #ifdef CONFIG_X86_USER_SHADOW_STACK
>  /*
> - * 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:
> + * VM_SHADOW_STACK should not be set with VM_SHARED because of lack of
> + * support core mm.
>   *
> - * 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.
> + * These VMAs will get a single end guard page. This helps userspace protect
> + * itself from attacks. A single page is enough for current shadow stack archs
> + * (x86). See the comments near alloc_shstk() in arch/x86/kernel/shstk.c
> + * for more details on the guard size.
>   */
>  # define VM_SHADOW_STACK	VM_HIGH_ARCH_5
>  #else
> -- 
> 2.34.1
> 

-- 
Sincerely yours,
Mike.