[PATCH v2 11/39] x86/mm: Update pte_modify for _PAGE_COW

[Rick Edgecombe]

From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>

The Write=0,Dirty=1 PTE has been used to indicate copy-on-write pages.
However, newer x86 processors also regard a Write=0,Dirty=1 PTE as a
shadow stack page. In order to separate the two, the software-defined
_PAGE_DIRTY is changed to _PAGE_COW for the copy-on-write case, and
pte_*() are updated to do this.

pte_modify() takes a "raw" pgprot_t which was not necessarily created
with any of the existing PTE bit helpers. That means that it can return a
pte_t with Write=0,Dirty=1, a shadow stack PTE, when it did not intend to
create one.

However pte_modify() changes a PTE to 'newprot', but it doesn't use the
pte_*(). Modify it to also move _PAGE_DIRTY to _PAGE_COW. Apply the same
changes to pmd_modify().

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>

---

v2:
 - Update commit log with text and suggestions from (Dave Hansen)
 - Drop fixup_dirty_pte() in favor of clearing the HW dirty bit along
   with the _PAGE_CHG_MASK masking, then calling pte_mkdirty() (Dave
   Hansen)

 arch/x86/include/asm/pgtable.h | 41 +++++++++++++++++++++++++++++-----
 1 file changed, 35 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index ad201dae7316..2f2963429f48 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -790,26 +790,55 @@ static inline u64 flip_protnone_guard(u64 oldval, u64 val, u64 mask);
 
 static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
 {
+	pteval_t _page_chg_mask_no_dirty = _PAGE_CHG_MASK & ~_PAGE_DIRTY;
 	pteval_t val = pte_val(pte), oldval = val;
+	pte_t pte_result;
 
 	/*
 	 * Chop off the NX bit (if present), and add the NX portion of
 	 * the newprot (if present):
 	 */
-	val &= _PAGE_CHG_MASK;
-	val |= check_pgprot(newprot) & ~_PAGE_CHG_MASK;
+	val &= _page_chg_mask_no_dirty;
+	val |= check_pgprot(newprot) & ~_page_chg_mask_no_dirty;
 	val = flip_protnone_guard(oldval, val, PTE_PFN_MASK);
-	return __pte(val);
+
+	pte_result = __pte(val);
+
+	/*
+	 * Dirty bit is not preserved above so it can be done
+	 * in a special way for the shadow stack case, where it
+	 * needs to set _PAGE_COW. pte_mkdirty() will do this in
+	 * the case of shadow stack.
+	 */
+	if (pte_dirty(pte))
+		pte_result = pte_mkdirty(pte_result);
+
+	return pte_result;
 }
 
 static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot)
 {
+	pteval_t _hpage_chg_mask_no_dirty = _HPAGE_CHG_MASK & ~_PAGE_DIRTY;
 	pmdval_t val = pmd_val(pmd), oldval = val;
+	pmd_t pmd_result;
 
-	val &= _HPAGE_CHG_MASK;
-	val |= check_pgprot(newprot) & ~_HPAGE_CHG_MASK;
+	val &= _hpage_chg_mask_no_dirty;
+	val |= check_pgprot(newprot) & ~_hpage_chg_mask_no_dirty;
 	val = flip_protnone_guard(oldval, val, PHYSICAL_PMD_PAGE_MASK);
-	return __pmd(val);
+
+
+	pmd_result = __pmd(val);
+
+	/*
+	 * Dirty bit is not preserved above so it can be done
+	 * specially for the shadow stack case. It needs to move
+	 * the HW dirty bit to the software COW bit. Set in the
+	 * result if it was set in the original value.
+	 */
+	if (pmd_dirty(pmd))
+		pmd_result = pmd_mkdirty(pmd_result);
+
+	return pmd_result;
 }
 
 /*
-- 
2.17.1