Re: [PATCH v5 10/39] x86/mm: Introduce _PAGE_COW

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On 19.01.23 22:22, Rick Edgecombe wrote:
Some OSes have a greater dependence on software available bits in PTEs than
Linux. That left the hardware architects looking for a way to represent a
new memory type (shadow stack) within the existing bits. They chose to
repurpose a lightly-used state: Write=0,Dirty=1. So in order to support
shadow stack memory, Linux should avoid creating memory with this PTE bit
combination unless it intends for it to be shadow stack.

The reason it's lightly used is that Dirty=1 is normally set by HW
_before_ a write. A write with a Write=0 PTE would typically only generate
a fault, not set Dirty=1. Hardware can (rarely) both set Dirty=1 *and*
generate the fault, resulting in a Write=0,Dirty=1 PTE. Hardware which
supports shadow stacks will no longer exhibit this oddity.

So that leaves Write=0,Dirty=1 PTEs created in software. To achieve this,
in places where Linux normally creates Write=0,Dirty=1, it can use the
software-defined _PAGE_COW in place of the hardware _PAGE_DIRTY. In other
words, whenever Linux needs to create Write=0,Dirty=1, it instead creates
Write=0,Cow=1 except for shadow stack, which is Write=0,Dirty=1.
Further differentiated by VMA flags, these PTE bit combinations would be
set as follows for various types of memory:

  - A modified, copy-on-write (COW) page. Previously when a typical
    anonymous writable mapping was made COW via fork(), the kernel would
    mark it Write=0,Dirty=1. Now it will instead use the Cow bit. This
    happens in copy_present_pte().
  - A R/O page that has been COW'ed. The user page is in a R/O VMA,
    and get_user_pages(FOLL_FORCE) needs a writable copy. The page fault
    handler creates a copy of the page and sets the new copy's PTE as
    Write=0 and Cow=1.
  - A shared shadow stack PTE. When a shadow stack page is being shared
    among processes (this happens at fork()), its PTE is made Dirty=0, so
    the next shadow stack access causes a fault, and the page is
    duplicated and Dirty=1 is set again. This is the COW equivalent for
    shadow stack pages, even though it's copy-on-access rather than

  - A shadow stack PTE.
  - A Cow PTE created when a processor without shadow stack support set

There are six bits left available to software in the 64-bit PTE after
consuming a bit for _PAGE_COW. No space is consumed in 32-bit kernels
because shadow stacks are not enabled there.

Implement only the infrastructure for _PAGE_COW. Changes to start
creating _PAGE_COW PTEs will follow once other pieces are in place.

Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
Tested-by: John Allen <john.allen@xxxxxxx>
Co-developed-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>

  - Fix log, comments and whitespace (Boris)
  - Remove capitalization on shadow stack (Boris)

  - Teach pte_flags_need_flush() about _PAGE_COW bit
  - Break apart patch for better bisectability

  - Add comment around _PAGE_TABLE in response to comment
    from (Andrew Cooper)
  - Check for PSE in pmd_shstk (Andrew Cooper)
  - Get to the point quicker in commit log (Andrew Cooper)
  - Clarify and reorder commit log for why the PTE bit examples have
    multiple entries. Apply same changes for comment. (peterz)
  - Fix comment that implied dirty bit for COW was a specific x86 thing
  - Fix swapping of Write/Dirty (PeterZ)

  - Update commit log with comments (Dave Hansen)
  - Add comments in code to explain pte modification code better (Dave)
  - Clarify info on the meaning of various Write,Cow,Dirty combinations

  arch/x86/include/asm/pgtable.h       | 78 ++++++++++++++++++++++++++++
  arch/x86/include/asm/pgtable_types.h | 59 +++++++++++++++++++--
  arch/x86/include/asm/tlbflush.h      |  3 +-
  3 files changed, 134 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index b39f16c0d507..6d2f612c04b5 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -301,6 +301,44 @@ static inline pte_t pte_clear_flags(pte_t pte, pteval_t clear)
  	return native_make_pte(v & ~clear);
+ * Normally COW memory can result in Dirty=1,Write=0 PTEs. But in the case
+ * of X86_FEATURE_USER_SHSTK, the software COW bit is used, since the
+ * Dirty=1,Write=0 will result in the memory being treated as shadow 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 conventionally 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_USER_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_USER_SHSTK kernels, since
+	 * the HW dirty bit can be used without creating shadow stack memory.
+	 * See the _PAGE_COW definition for more details.
+	 */
+	if (!cpu_feature_enabled(X86_FEATURE_USER_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 the dirty bit for both
+	 * cases.
+	 */
+	pte = pte_set_flags(pte, _PAGE_DIRTY);
+	return pte_clear_flags(pte, _PAGE_COW);
  static inline int pte_uffd_wp(pte_t pte)
@@ -413,6 +451,26 @@ static inline pmd_t pmd_clear_flags(pmd_t pmd, pmdval_t clear)
  	return native_make_pmd(v & ~clear);
+/* See comments above pte_mkcow() */
+static inline pmd_t pmd_mkcow(pmd_t pmd)
+	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
+		return pmd;
+	pmd = pmd_clear_flags(pmd, _PAGE_DIRTY);
+	return pmd_set_flags(pmd, _PAGE_COW);
+/* See comments above pte_mkcow() */
+static inline pmd_t pmd_clear_cow(pmd_t pmd)
+	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
+		return pmd;
+	pmd = pmd_set_flags(pmd, _PAGE_DIRTY);
+	return pmd_clear_flags(pmd, _PAGE_COW);
  static inline int pmd_uffd_wp(pmd_t pmd)
@@ -484,6 +542,26 @@ static inline pud_t pud_clear_flags(pud_t pud, pudval_t clear)
  	return native_make_pud(v & ~clear);
+/* See comments above pte_mkcow() */
+static inline pud_t pud_mkcow(pud_t pud)
+	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
+		return pud;
+	pud = pud_clear_flags(pud, _PAGE_DIRTY);
+	return pud_set_flags(pud, _PAGE_COW);
+/* See comments above pte_mkcow() */
+static inline pud_t pud_clear_cow(pud_t pud)
+	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
+		return pud;
+	pud = pud_set_flags(pud, _PAGE_DIRTY);
+	return pud_clear_flags(pud, _PAGE_COW);
  static inline pud_t pud_mkold(pud_t pud)
  	return pud_clear_flags(pud, _PAGE_ACCESSED);
diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
index 0646ad00178b..5c3f942865d9 100644
--- a/arch/x86/include/asm/pgtable_types.h
+++ b/arch/x86/include/asm/pgtable_types.h
@@ -21,7 +21,8 @@
  #define _PAGE_BIT_SOFTW2	10	/* " */
  #define _PAGE_BIT_SOFTW3	11	/* " */
  #define _PAGE_BIT_PAT_LARGE	12	/* On 2MB or 1GB pages */
-#define _PAGE_BIT_SOFTW4	58	/* available for programmer */
+#define _PAGE_BIT_SOFTW4	57	/* available for programmer */
+#define _PAGE_BIT_SOFTW5	58	/* available for programmer */
  #define _PAGE_BIT_PKEY_BIT0	59	/* Protection Keys, bit 1/4 */
  #define _PAGE_BIT_PKEY_BIT1	60	/* Protection Keys, bit 2/4 */
  #define _PAGE_BIT_PKEY_BIT2	61	/* Protection Keys, bit 3/4 */
@@ -34,6 +35,15 @@
  #define _PAGE_BIT_SOFT_DIRTY	_PAGE_BIT_SOFTW3 /* software dirty tracking */
+ * Indicates a copy-on-write page.
+ */
+#define _PAGE_BIT_COW		_PAGE_BIT_SOFTW5 /* copy-on-write */
+#define _PAGE_BIT_COW		0
  /* If _PAGE_BIT_PRESENT is clear, we use these: */
  /* - if the user mapped it with PROT_NONE; pte_present gives true */
@@ -117,6 +127,40 @@
  #define _PAGE_SOFTW4	(_AT(pteval_t, 0))
+ * The hardware requires shadow stack to be read-only and Dirty.
+ * _PAGE_COW is a software-only bit used to separate copy-on-write PTEs
+ * from shadow stack PTEs:

Is that really required?

For anon pages, we have PG_anon_exclusive, that can tell you whether the page is "certainly exclusive" (now cow necessary) vs. "maybe shared" (cow maybe necessary).

Why isn't that sufficient to make the same decisions here?


David / dhildenb

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