Re: [PATCHv6 07/16] x86/vsyscall: Reorganize the #PF emulation code

Sohil Mehta
2025-06-20T23:19:00Z

On 6/20/2025 4:08 PM, Andrew Cooper wrote:
>> But, the resulting code is wonky. It needs to do something more like this:
>>
>> if ((error_code & (X86_PF_WRITE | X86_PF_USER)) != X86_PF_USER)
>> return false;
>>
>> if (error_code & X86_PF_INSTR))
>> return __emulate_vsyscall(regs, address);
>
> To do this, LASS needs a proper interlink against NX || SMEP.
>
> If neither NX nor SMEP are active, the CPU does not report X86_PF_INSTR,
> meaning that fetches are reported as plain reads.
>
> This leads to some fun corner cases in SMAP and now LASS too for virt.

Maybe I am missing something, but LASS works pre-paging so it wouldn't
generate a PF, right?

We have a new vsyscall emulation for #GP (patch 9) that wouldn't follow
this path.