On 1/25/2021 10:27 AM, Borislav Petkov wrote:

On Tue, Dec 29, 2020 at 01:30:38PM -0800, Yu-cheng Yu wrote:

[...]

quoted

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 666c25ab9564..1c84f1ba32b9 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h

@@ -1226,6 +1226,32 @@ static inline pte_t ptep_get_and_clear_full(struct mm_struct *mm,
  static inline void ptep_set_wrprotect(struct mm_struct *mm,
  				      unsigned long addr, pte_t *ptep)
  {
+	/*
+	 * Some processors can start a write, but end up seeing a read-only
+	 * PTE by the time they get to the Dirty bit.  In this case, they
+	 * will set the Dirty bit, leaving a read-only, Dirty PTE which
+	 * looks like a shadow stack PTE.
+	 *
+	 * However, this behavior has been improved

Improved how?

Processors supporting Shadow Stack will not set a read-only PTE's dirty 
bit.  I will revise the comments.

quoted

and will not occur on
+	 * processors supporting Shadow Stack.  Without this guarantee, a

Which guarantee? That it won't happen on CPUs which support SHSTK?

Yes.

quoted

+	 * transition to a non-present PTE and flush the TLB would be

s/flush the TLB/TLB flush/

quoted

+	 * needed.
+	 *
+	 * When changing a writable PTE to read-only and if the PTE has
+	 * _PAGE_DIRTY set, move that bit to _PAGE_COW so that the PTE is
+	 * not a shadow stack PTE.
+	 */

This sentence doesn't belong here as it refers to what pte_wrprotect()
does. You could expand the comment in pte_wrprotect() with this here as
it is better.

I will move this paragraph to pte_wrprotect().

quoted

+	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
+		pte_t old_pte, new_pte;
+
+		do {
+			old_pte = READ_ONCE(*ptep);
+			new_pte = pte_wrprotect(old_pte);

Maybe I'm missing something but those two can happen outside of the
loop, no? Or is *ptep somehow changing concurrently while the loop is
doing the CMPXCHG and you need to recreate it each time?

IOW, you can generate upfront and do the empty loop...

*ptep can change concurrently.

quoted

+
+		} while (!try_cmpxchg(&ptep->pte, &old_pte.pte, new_pte.pte));
+
+		return;
+	}
  	clear_bit(_PAGE_BIT_RW, (unsigned long *)&ptep->pte);
  }
  

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