On Thu, Sep 29, 2022 at 03:29:07PM -0700, Rick Edgecombe wrote:

+/*
+ * Normally the Dirty bit is used to denote COW memory on x86. But
+ * in the case of X86_FEATURE_SHSTK, the software COW bit is used,
+ * since the Dirty=1,Write=0 will result in the memory being treated
+ * as shaodw stack by the HW. So when creating COW memory, a software
+ * bit is used _PAGE_BIT_COW. The following functions pte_mkcow() and
+ * pte_clear_cow() take a PTE marked conventially COW (Dirty=1) and
+ * transition it to the shadow stack compatible version of COW (Cow=1).
+ */
+
+static inline pte_t pte_mkcow(pte_t pte)
+{
+	if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
+		return pte;
+
+	pte = pte_clear_flags(pte, _PAGE_DIRTY);
+	return pte_set_flags(pte, _PAGE_COW);
+}
+
+static inline pte_t pte_clear_cow(pte_t pte)
+{
+	/*
+	 * _PAGE_COW is unnecessary on !X86_FEATURE_SHSTK kernels.
+	 * See the _PAGE_COW definition for more details.
+	 */
+	if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
+		return pte;
+
+	/*
+	 * PTE is getting copied-on-write, so it will be dirtied
+	 * if writable, or made shadow stack if shadow stack and
+	 * being copied on access. Set they dirty bit for both
+	 * cases.
+	 */
+	pte = pte_set_flags(pte, _PAGE_DIRTY);
+	return pte_clear_flags(pte, _PAGE_COW);
+}

These X86_FEATURE_SHSTK checks make me uneasy. Maybe use the _PAGE_COW
logic for all machines with 64-bit entries. It will get you much more
coverage and more universal rules.

Hm. What about ptep/pmdp_set_wrprotect()? They clear _PAGE_RW blindly.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov