Re: [RFC PATCH v9 13/27] x86/mm: Shadow Stack page fault error checking

[Kees Cook]

On Wed, Feb 05, 2020 at 10:19:21AM -0800, Yu-cheng Yu wrote:
> If a page fault is triggered by a Shadow Stack (SHSTK) access
> (e.g. CALL/RET) or SHSTK management instructions (e.g. WRUSSQ), then bit[6]
> of the page fault error code is set.
> 
> In access_error(), verify a SHSTK page fault is within a SHSTK memory area.
> It is always an error otherwise.
> 
> For a valid SHSTK access, set FAULT_FLAG_WRITE to effect copy-on-write.
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>  arch/x86/include/asm/traps.h |  2 ++
>  arch/x86/mm/fault.c          | 18 ++++++++++++++++++
>  2 files changed, 20 insertions(+)
> 
> diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
> index 7ac26bbd0bef..8023d177fcd8 100644
> --- a/arch/x86/include/asm/traps.h
> +++ b/arch/x86/include/asm/traps.h
> @@ -169,6 +169,7 @@ enum {
>   *   bit 3 ==				1: use of reserved bit detected
>   *   bit 4 ==				1: fault was an instruction fetch
>   *   bit 5 ==				1: protection keys block access
> + *   bit 6 ==				1: shadow stack access fault
>   */
>  enum x86_pf_error_code {
>  	X86_PF_PROT	=		1 << 0,
> @@ -177,5 +178,6 @@ enum x86_pf_error_code {
>  	X86_PF_RSVD	=		1 << 3,
>  	X86_PF_INSTR	=		1 << 4,
>  	X86_PF_PK	=		1 << 5,
> +	X86_PF_SHSTK	=		1 << 6,
>  };
>  #endif /* _ASM_X86_TRAPS_H */
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 304d31d8cbbc..9c1243302663 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1187,6 +1187,17 @@ access_error(unsigned long error_code, struct vm_area_struct *vma)
>  				       (error_code & X86_PF_INSTR), foreign))
>  		return 1;
>  
> +	/*
> +	 * Verify X86_PF_SHSTK is within a Shadow Stack VMA.
> +	 * It is always an error if there is a Shadow Stack
> +	 * fault outside a Shadow Stack VMA.
> +	 */
> +	if (error_code & X86_PF_SHSTK) {
> +		if (!(vma->vm_flags & VM_SHSTK))
> +			return 1;
> +		return 0;
> +	}
> +
>  	if (error_code & X86_PF_WRITE) {
>  		/* write, present and write, not present: */
>  		if (unlikely(!(vma->vm_flags & VM_WRITE)))
> @@ -1344,6 +1355,13 @@ void do_user_addr_fault(struct pt_regs *regs,
>  
>  	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
>  
> +	/*
> +	 * If the fault is caused by a Shadow Stack access,
> +	 * i.e. CALL/RET/SAVEPREVSSP/RSTORSSP, then set
> +	 * FAULT_FLAG_WRITE to effect copy-on-write.
> +	 */
> +	if (hw_error_code & X86_PF_SHSTK)
> +		flags |= FAULT_FLAG_WRITE;
>  	if (hw_error_code & X86_PF_WRITE)
>  		flags |= FAULT_FLAG_WRITE;
>  	if (hw_error_code & X86_PF_INSTR)
> -- 
> 2.21.0
> 

-- 
Kees Cook