Re: [PATCH v3 15/37] x86/mm: Check Shadow Stack page fault errors
From: Peter Zijlstra <peterz@infradead.org>
Date: 2022-11-15 19:34:27
Also in:
linux-arch, linux-doc, linux-mm, lkml
On Fri, Nov 04, 2022 at 03:35:42PM -0700, Rick Edgecombe wrote:
quoted hunk ↗ jump to hunk
@@ -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