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 <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/20230613001108.3040476-19-rick.p.edgecombe%40intel.com

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 9255b5b7a9d95cb91418d6cf216925e269e85886..61b52445e4d61d85615416f712ffda63906a441c 100644 (file)
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -1033,7 +1033,14 @@ 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)
 {