On Fri, Nov 04, 2022 at 03:35:42PM -0700, Rick Edgecombe wrote: > @@ -1331,6 +1345,18 @@ void do_user_addr_fault(struct pt_regs *regs, > > perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address); > > + /* > + * To service shadow stack read faults, unlike normal read faults, the > + * fault handler needs to create a type of memory that will also be > + * writable (with instructions that generate shadow stack writes). > + * In the case of COW memory, the COW needs to take place even with > + * a shadow stack read. Otherwise the shared page will be left (shadow > + * stack) writable in userspace. So to trigger the appropriate behavior > + * by setting FAULT_FLAG_WRITE for shadow stack accesses, even if the > + * access was a shadow stack read. > + */ Clear as mud... So SS pages are 'Write=0,Dirty=1', which, per construction, lack a RW bit. And these pages are writable (WRUSS). pte_wrprotect() seems to do: _PAGE_DIRTY->_PAGE_COW (which is really weird in this situation), resulting in: 'Write=0,Dirty=0,Cow=1'. That's regular RO memory and won't raise read-faults. But I'm thinking RET will trip #PF here when it tries to read the SS because the SSP is not a proper shadow stack page? And in that case you want to tickle pte_mkwrite() to undo the pte_wrprotect() above? So while the #PF is a 'read' fault due to RET not actually writing to the shadow stack, you want to force a write fault so it will re-instate the SS page. Did I get that right? > + 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) > -- > 2.17.1 >
