On 21/06/2025 12:18 am, Sohil Mehta wrote: > 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? Oh right, yes. This is a preexisting bug in vsyscall #PF handling. It simply became obvious with Dave's suggested rearrangement. ~Andrew
