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?
