Re: [PATCH 15/35] x86/mm: Check Shadow Stack page fault errors

[Dave Hansen]

On 1/30/22 13:18, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> 
> 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.

The passive voice is killing me.  Here's a rewrite:

	The CPU performs "shadow stack accesses" when it expects to
	encounter shadow stack mappings.  These accesses can be
	implicit (via CALL/RET instructions) or explicit (instructions
	like WRUSSQ).

Since we defined what a shadow stack access *is*, shouldn't we also
connect it to X86_PF_SHSTK?

> Shadow stacks accesses to shadow-stack mapping can see faults in normal,

					   ^ mappings

> 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.

... and use faults to implement those features.

> 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.

Those two paragraphs tell a pretty good story.  Nice.

> 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.

This paragraph is a rehash of what the code does.  It can go.

*But*, with or without this paragraph, the reader is left with all
background and no discussion of why this patch exists.

Even just this would be fine:

	Handle valid and invalid shadow-stack accesses in the page fault
	handler.


> 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 d0074c6ed31a..6769134986ec 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1107,6 +1107,17 @@ 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.
> +	 */

That comment needs some help.  Maybe:

	Shadow stack accesses (PF_SHSTK=1) are only permitted to
	shadow stack VMAs.  All other accesses result in an error.

I don't think we need to talk about the other cases being handled below.

> +	if (error_code & X86_PF_SHSTK) {
> +		if (!(vma->vm_flags & VM_SHADOW_STACK))
> +			return 1;
> +		return 0;
> +	}
> +
>  	if (error_code & X86_PF_WRITE) {
>  		/* write, present and write, not present: */
>  		if (unlikely(!(vma->vm_flags & VM_WRITE)))
> @@ -1300,6 +1311,14 @@ 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.
> +	 */

Too much detail.  This is also rather unconnected to the code I can see:

> +	if (error_code & X86_PF_SHSTK)
> +		flags |= FAULT_FLAG_WRITE;

Also, the use of "effect" here is arguably wrong.  It's odd at best.
I'd use some alternative wording.

Let's stick to the facts:
 1. Shadow stack pages architecturally can't be read-only
 2. Don't bother with read faults, consider everything a write

BTW, what happens if we don't do this?  What breaks?