On 2/5/20 10:19 AM, 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.
How about starting with a definition:
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.
> In access_error(), verify a SHSTK page fault is within a SHSTK memory area.
> It is always an error otherwise.
How about: 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.
> For a valid SHSTK access, set FAULT_FLAG_WRITE to effect copy-on-write.
It seems rather odd to want copy-on-write behavior for read faults.
Could you elaborate on why, please?
> 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.
> + */
Nit: there was an access that caused the fault. We can be a bit more
broad in the implications from the comment if we say "access" instead of
"fault".
> + 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)))
Is there an analogous check for !X86_PF_SHSTK faults to VM_SHSTK VMAs?
> @@ -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)
It would be great if you could also include the *why*. *Why* do read
faults need copy-on-write semantics?
