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Date:   Sat, 18 Mar 2023 17:15:22 -0700
From:   Rick Edgecombe <>
To:, "H . Peter Anvin" <>,
        Thomas Gleixner <>,
        Ingo Molnar <>,,,,,,
        Arnd Bergmann <>,
        Andy Lutomirski <>,
        Balbir Singh <>,
        Borislav Petkov <>,
        Cyrill Gorcunov <>,
        Dave Hansen <>,
        Eugene Syromiatnikov <>,
        Florian Weimer <>,
        "H . J . Lu" <>, Jann Horn <>,
        Jonathan Corbet <>,
        Kees Cook <>,
        Mike Kravetz <>,
        Nadav Amit <>,
        Oleg Nesterov <>, Pavel Machek <>,
        Peter Zijlstra <>,
        Randy Dunlap <>,
        Weijiang Yang <>,
        "Kirill A . Shutemov" <>,
        John Allen <>,,,,,,,,,,,
Subject: [PATCH v8 27/40] x86/mm: Warn if create Write=0,Dirty=1 with raw prot

When user shadow stack is in use, Write=0,Dirty=1 is treated by the CPU as
shadow stack memory. So for shadow stack memory this bit combination is
valid, but when Dirty=1,Write=1 (conventionally writable) memory is being
write protected, the kernel has been taught to transition the Dirty=1
bit to SavedDirty=1, to avoid inadvertently creating shadow stack
memory. It does this inside pte_wrprotect() because it knows the PTE is
not intended to be a writable shadow stack entry, it is supposed to be
write protected.

However, when a PTE is created by a raw prot using mk_pte(), mk_pte()
can't know whether to adjust Dirty=1 to SavedDirty=1. It can't
distinguish between the caller intending to create a shadow stack PTE or
needing the SavedDirty shift.

The kernel has been updated to not do this, and so Write=0,Dirty=1
memory should only be created by the pte_mkfoo() helpers. Add a warning
to make sure no new mk_pte() start doing this, like, for example,
set_memory_rox() did.

Signed-off-by: Rick Edgecombe <>
Acked-by: Mike Rapoport (IBM) <>
Tested-by: Pengfei Xu <>
Tested-by: John Allen <>
Tested-by: Kees Cook <>
 - Update commit log verbiage (Boris)

 - New patch (Note, this has already been a useful warning, it caught the
   newly added set_memory_rox() doing this)
 arch/x86/include/asm/pgtable.h | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index e5b3dce0d9fe..7142f99d3fbb 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -1032,7 +1032,15 @@ static inline unsigned long pmd_page_vaddr(pmd_t pmd)
  * (Currently stuck as a macro because of indirect forward reference
  * to linux/mm.h:page_to_nid())
-#define mk_pte(page, pgprot)   pfn_pte(page_to_pfn(page), (pgprot))
+#define mk_pte(page, pgprot)						 \
+({									 \
+	pgprot_t __pgprot = pgprot;					 \
+									 \
+	WARN_ON_ONCE(cpu_feature_enabled(X86_FEATURE_USER_SHSTK) &&	 \
+		    (pgprot_val(__pgprot) & (_PAGE_DIRTY | _PAGE_RW)) == \
+		    _PAGE_DIRTY);					 \
+	pfn_pte(page_to_pfn(page), __pgprot);				 \
 static inline int pmd_bad(pmd_t pmd)

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