Thread (67 messages) 67 messages, 5 authors, 2021-02-08

Re: [PATCH v19 08/25] x86/mm: Introduce _PAGE_COW

From: Yu, Yu-cheng <hidden>
Date: 2021-02-08 23:04:49
Also in: linux-arch, linux-doc, linux-mm, lkml
Subsystem: the rest, x86 architecture (32-bit and 64-bit) · Maintainers: Linus Torvalds, Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen

On 2/5/2021 10:41 AM, Yu, Yu-cheng wrote:
On 2/4/2021 12:27 PM, Dave Hansen wrote:
quoted
On 2/4/21 12:19 PM, Kees Cook wrote:
quoted
quoted
(e) A page where the processor observed a Write=1 PTE, started a 
write, set
     Dirty=1, but then observed a Write=0 PTE.  That's possible 
today, but
     will not happen on processors that support shadow stack.
What happens for "e" with/without CET? It sounds like direct writes to
such pages will be (correctly) rejected by the MMU?
A page fault would be generated regardless of CET support.

If CET were not around, the fault would be reported as a present, write
fault.

If this happened and CET were around (which shouldn't happen in
practice, it means we have a hardware issue) a page fault exception is
generated. 
Thanks for the clarification.  With or without CET, direct write to 
Write=0, Dirty=1 PTE triggers page fault.
quoted
Yu-cheng, I'm not sure there's enough debugging around to
tell us if this happens.  Would we even notice?
That potential hardware issue is, on a CET-capable system, a processor 
writes to a Write=1, Dirty=0 page, and then observes the PTE is Write=0, 
Dirty=1.  Let me think about it...
One way to detect the potential issue is adding a check when a 
non-shadow stack page's PTE goes from RW=0 to RW=1, like the following...
diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 29aa6f07e3c9..241b94a0fa77 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -405,6 +405,8 @@ static inline pte_t pte_mkyoung(pte_t pte)
  static inline pte_t pte_mkwrite(pte_t pte)
  {
  	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
+		WARN_ONCE((pte_flags(pte) & (_PAGE_RW | _PAGE_DIRTY)) ==
+			  _PAGE_DIRTY, "Found transient shadow stack PTE\n");
  		if (pte_flags(pte) & _PAGE_COW) {
  			pte = pte_clear_flags(pte, _PAGE_COW);
  			pte = pte_set_flags(pte, _PAGE_DIRTY);
I run all my routine stress tests with the changes, and do not see any 
warning triggered.  If this change is desirable, we can probably add 
#ifdef CONFIG_DEBUG_VM around it and make it a separate patch.

--
Yu-cheng
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