When serving a page fault, maybe_mkwrite() makes a PTE writable if its vma
has VM_WRITE.

A shadow stack vma has VM_SHADOW_STACK.  Its PTEs have _PAGE_DIRTY, but not
_PAGE_WRITE.  In fork(), _PAGE_DIRTY is cleared to cause copy-on-write,
and in the page fault handler, _PAGE_DIRTY is restored and the shadow stack
page is writable again.

Introduce an x86 version of maybe_mkwrite(), which sets proper PTE bits
according to VM flags.

Apply the same changes to maybe_pmd_mkwrite().

Signed-off-by: Yu-cheng Yu <redacted>
Cc: Kees Cook <redacted>
Cc: Kirill A. Shutemov <redacted>
---
v24:
- Instead of doing arch_maybe_mkwrite(), overwrite maybe*_mkwrite() with x86
  versions.
- Change VM_SHSTK to VM_SHADOW_STACK.

 arch/x86/include/asm/pgtable.h |  8 ++++++++
 arch/x86/mm/pgtable.c          | 20 ++++++++++++++++++++
 include/linux/mm.h             |  2 ++
 mm/huge_memory.c               |  2 ++
 4 files changed, 32 insertions(+)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 46d9394b884f..51cdf14488b7 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h

@@ -1686,6 +1686,14 @@ static inline bool arch_faults_on_old_pte(void)
 	return false;
 }
 
+#define maybe_mkwrite maybe_mkwrite
+extern pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma);
+
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+#define maybe_pmd_mkwrite maybe_pmd_mkwrite
+extern pmd_t maybe_pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma);
+#endif /* CONFIG_TRANSPARENT_HUGEPAGE */