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 <rick.p.edgecombe@xxxxxxxxx>
Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
Tested-by: John Allen <john.allen@xxxxxxx>
Tested-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
v9:
- Always do the check since 32 bit now supports SavedDirty
---
arch/x86/include/asm/pgtable.h | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 89cfa93d0ad6..5383f7282f89 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -1032,7 +1032,14 @@ 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((pgprot_val(__pgprot) & (_PAGE_DIRTY | _PAGE_RW)) == \
+ _PAGE_DIRTY); \
+ pfn_pte(page_to_pfn(page), __pgprot); \
+})
static inline int pmd_bad(pmd_t pmd)
{
--
2.34.1
