Re: [PATCH v23 14/28] x86/mm: Shadow Stack page fault error checking

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Mar 16, 2021 at 08:10:40AM -0700, Yu-cheng Yu wrote:
> Shadow stack accesses are those that are performed by the CPU where it
> expects to encounter a shadow stack mapping.  These accesses are performed
> implicitly by CALL/RET at the site of the shadow stack pointer.  These
> accesses are made explicitly by shadow stack management instructions like
> WRUSSQ.
> 
> Shadow stacks accesses to shadow-stack mapping can see faults in normal,
> valid operation just like regular accesses to regular mappings.  Shadow
> stacks need some of the same features like delayed allocation, swap and
> copy-on-write.
> 
> Shadow stack accesses can also result in errors, such as when a shadow
> stack overflows, or if a shadow stack access occurs to a non-shadow-stack
> mapping.
> 
> In handling a shadow stack page fault, verify it occurs within a shadow
> stack mapping.  It is always an error otherwise.  For valid shadow stack
> accesses, set FAULT_FLAG_WRITE to effect copy-on-write.  Because clearing
> _PAGE_DIRTY (vs. _PAGE_RW) is used to trigger the fault, shadow stack read
> fault and shadow stack write fault are not differentiated and both are
> handled as a write access.
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> ---
>  arch/x86/include/asm/trap_pf.h |  2 ++
>  arch/x86/mm/fault.c            | 19 +++++++++++++++++++
>  2 files changed, 21 insertions(+)
> 
> diff --git a/arch/x86/include/asm/trap_pf.h b/arch/x86/include/asm/trap_pf.h
> index 10b1de500ab1..afa524325e55 100644
> --- a/arch/x86/include/asm/trap_pf.h
> +++ b/arch/x86/include/asm/trap_pf.h
> @@ -11,6 +11,7 @@
>   *   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
>   *   bit 15 ==				1: SGX MMU page-fault
>   */
>  enum x86_pf_error_code {
> @@ -20,6 +21,7 @@ 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,
>  	X86_PF_SGX	=		1 << 15,
>  };
>  
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index a73347e2cdfc..4316732a18c6 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1100,6 +1100,17 @@ access_error(unsigned long error_code, struct vm_area_struct *vma)
>  				       (error_code & X86_PF_INSTR), foreign))
>  		return 1;
>  
> +	/*
> +	 * Verify a shadow stack access is within a shadow stack VMA.
> +	 * It is always an error otherwise.  Normal data access to a
> +	 * shadow stack area is checked in the case followed.
> +	 */
> +	if (error_code & X86_PF_SHSTK) {
> +		if (!(vma->vm_flags & VM_SHSTK))
> +			return 1;
> +		return 0;

Any reason to return 0 here? I would rather keep the single return 0 in
the function, after all checks are done.

> +	}
> +
>  	if (error_code & X86_PF_WRITE) {
>  		/* write, present and write, not present: */
>  		if (unlikely(!(vma->vm_flags & VM_WRITE)))
> @@ -1293,6 +1304,14 @@ void do_user_addr_fault(struct pt_regs *regs,
>  
>  	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
>  
> +	/*
> +	 * Clearing _PAGE_DIRTY is used to detect shadow stack access.
> +	 * This method cannot distinguish shadow stack read vs. write.
> +	 * For valid shadow stack accesses, set FAULT_FLAG_WRITE to effect
> +	 * copy-on-write.
> +	 */
> +	if (error_code & X86_PF_SHSTK)
> +		flags |= FAULT_FLAG_WRITE;
>  	if (error_code & X86_PF_WRITE)
>  		flags |= FAULT_FLAG_WRITE;
>  	if (error_code & X86_PF_INSTR)
> -- 
> 2.21.0
> 

-- 
 Kirill A. Shutemov