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

Yu, Yu-cheng
2021-03-22T17:28:52Z

On 3/22/2021 3:38 AM, Kirill A. Shutemov wrote:
> On Tue, Mar 16, 2021 at 08:10:40AM -0700, Yu-cheng Yu wrote:
[...]
>>
>> 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.
>

For shadow stack fault, X86_PF_SHSTK and X86_PF_WRITE both can be set.
So for shadow stack fault, return from here and don't go into the normal
write fault case.

Thanks,
Yu-cheng

>> + }
>> +
>> 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
>>
>