[PATCH v20 08/25] x86/mm: Introduce _PAGE_COW

From: Yu-cheng Yu
Date: Wed Feb 10 2021 - 13:03:22 EST

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There is essentially no room left in the x86 hardware PTEs on some OSes
(not 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.

The reason it's lightly used is that Dirty=1 is normally set by hardware
and cannot normally be set by hardware on a Write=0 PTE. Software must
normally be involved to create one of these PTEs, so software can simply
opt to not create them.

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. This
clearly separates shadow stack from other data, and results in the
following:

(a) A modified, copy-on-write (COW) page: (Write=0, Cow=1)
(b) A R/O page that has been COW'ed: (Write=0, Cow=1)
The user page is in a R/O VMA, and get_user_pages() 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.
(c) A shadow stack PTE: (Write=0, Dirty=1)
(d) A shared shadow stack PTE: (Write=0, Cow=1)
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 copy-on-write.
(e) A page where the processor observed a Write=1 PTE, started a write, set
Dirty=1, but then observed a Write=0 PTE. That's possible today, but
will not happen on processors that support shadow stack.

Define _PAGE_COW and update pte_*() helpers and apply the same changes to
pmd and pud.

After this, there are six free bits left in the 64-bit PTE, and no more
free bits in the 32-bit PTE (except for PAE) and Shadow Stack is not
implemented for the 32-bit kernel.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
---
arch/x86/include/asm/pgtable.h | 136 ++++++++++++++++++++++++---
arch/x86/include/asm/pgtable_types.h | 42 ++++++++-
2 files changed, 165 insertions(+), 13 deletions(-)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index a02c67291cfc..2309fa9d412e 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -121,9 +121,12 @@ extern pmdval_t early_pmd_flags;
* The following only work if pte_present() is true.
* Undefined behaviour if not..
*/
-static inline int pte_dirty(pte_t pte)
+static inline bool pte_dirty(pte_t pte)
{
- return pte_flags(pte) & _PAGE_DIRTY;
+ /*
+ * A dirty PTE has Dirty=1 or Cow=1.
+ */
+ return pte_flags(pte) & _PAGE_DIRTY_BITS;
}

@@ -160,9 +163,12 @@ static inline int pte_young(pte_t pte)
return pte_flags(pte) & _PAGE_ACCESSED;
}

-static inline int pmd_dirty(pmd_t pmd)
+static inline bool pmd_dirty(pmd_t pmd)
{
- return pmd_flags(pmd) & _PAGE_DIRTY;
+ /*
+ * A dirty PMD has Dirty=1 or Cow=1.
+ */
+ return pmd_flags(pmd) & _PAGE_DIRTY_BITS;
}

static inline int pmd_young(pmd_t pmd)
@@ -170,9 +176,12 @@ static inline int pmd_young(pmd_t pmd)
return pmd_flags(pmd) & _PAGE_ACCESSED;
}

-static inline int pud_dirty(pud_t pud)
+static inline bool pud_dirty(pud_t pud)
{
- return pud_flags(pud) & _PAGE_DIRTY;
+ /*
+ * A dirty PUD has Dirty=1 or Cow=1.
+ */
+ return pud_flags(pud) & _PAGE_DIRTY_BITS;
}

static inline int pud_young(pud_t pud)
@@ -182,7 +191,15 @@ static inline int pud_young(pud_t pud)

static inline int pte_write(pte_t pte)
{
- return pte_flags(pte) & _PAGE_RW;
+ /*
+ * When shadow stack is enabled, a directly writable PTE has RW=1.
+ * A shadow stack PTE is logically writable and has RW=0, Dirty=1.
+ * See comments for _PAGE_COW.
+ */
+ if (cpu_feature_enabled(X86_FEATURE_SHSTK))
+ return pte_flags(pte) & (_PAGE_RW | _PAGE_DIRTY);
+ else
+ return pte_flags(pte) & _PAGE_RW;
}

static inline int pte_huge(pte_t pte)
@@ -333,7 +350,7 @@ static inline pte_t pte_clear_uffd_wp(pte_t pte)

static inline pte_t pte_mkclean(pte_t pte)
{
- return pte_clear_flags(pte, _PAGE_DIRTY);
+ return pte_clear_flags(pte, _PAGE_DIRTY_BITS);
}

static inline pte_t pte_mkold(pte_t pte)
@@ -343,6 +360,18 @@ static inline pte_t pte_mkold(pte_t pte)

static inline pte_t pte_wrprotect(pte_t pte)
{
+ /*
+ * Blindly clearing _PAGE_RW might accidentally create
+ * a shadow stack PTE (RW=0, Dirty=1). Move the hardware
+ * dirty value to the software bit.
+ */
+ if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
+ if (pte_flags(pte) & _PAGE_DIRTY) {
+ pte = pte_clear_flags(pte, _PAGE_DIRTY);
+ pte = pte_set_flags(pte, _PAGE_COW);
+ }
+ }
+
return pte_clear_flags(pte, _PAGE_RW);
}

@@ -353,6 +382,18 @@ static inline pte_t pte_mkexec(pte_t pte)

static inline pte_t pte_mkdirty(pte_t pte)
{
+ pteval_t dirty = _PAGE_DIRTY;
+
+ /* Avoid creating (HW)Dirty=1, Write=0 PTEs */
+ if (cpu_feature_enabled(X86_FEATURE_SHSTK) && !pte_write(pte))
+ dirty = _PAGE_COW;
+
+ return pte_set_flags(pte, dirty | _PAGE_SOFT_DIRTY);
+}
+
+static inline pte_t pte_mkwrite_shstk(pte_t pte)
+{
+ pte = pte_clear_flags(pte, _PAGE_COW);
return pte_set_flags(pte, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
}

@@ -363,6 +404,13 @@ static inline pte_t pte_mkyoung(pte_t pte)

static inline pte_t pte_mkwrite(pte_t pte)
{
+ if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
+ if (pte_flags(pte) & _PAGE_COW) {
+ pte = pte_clear_flags(pte, _PAGE_COW);
+ pte = pte_set_flags(pte, _PAGE_DIRTY);
+ }
+ }
+
return pte_set_flags(pte, _PAGE_RW);
}

@@ -434,16 +482,40 @@ static inline pmd_t pmd_mkold(pmd_t pmd)

static inline pmd_t pmd_mkclean(pmd_t pmd)
{
- return pmd_clear_flags(pmd, _PAGE_DIRTY);
+ return pmd_clear_flags(pmd, _PAGE_DIRTY_BITS);
}

static inline pmd_t pmd_wrprotect(pmd_t pmd)
{
+ /*
+ * Blindly clearing _PAGE_RW might accidentally create
+ * a shadow stack PMD (RW=0, Dirty=1). Move the hardware
+ * dirty value to the software bit.
+ */
+ if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
+ if (pmd_flags(pmd) & _PAGE_DIRTY) {
+ pmd = pmd_clear_flags(pmd, _PAGE_DIRTY);
+ pmd = pmd_set_flags(pmd, _PAGE_COW);
+ }
+ }
+
return pmd_clear_flags(pmd, _PAGE_RW);
}

static inline pmd_t pmd_mkdirty(pmd_t pmd)
{
+ pmdval_t dirty = _PAGE_DIRTY;
+
+ /* Avoid creating (HW)Dirty=1, Write=0 PMDs */
+ if (cpu_feature_enabled(X86_FEATURE_SHSTK) && !(pmd_flags(pmd) & _PAGE_RW))
+ dirty = _PAGE_COW;
+
+ return pmd_set_flags(pmd, dirty | _PAGE_SOFT_DIRTY);
+}
+
+static inline pmd_t pmd_mkwrite_shstk(pmd_t pmd)
+{
+ pmd = pmd_clear_flags(pmd, _PAGE_COW);
return pmd_set_flags(pmd, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
}

@@ -464,6 +536,13 @@ static inline pmd_t pmd_mkyoung(pmd_t pmd)

static inline pmd_t pmd_mkwrite(pmd_t pmd)
{
+ if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
+ if (pmd_flags(pmd) & _PAGE_COW) {
+ pmd = pmd_clear_flags(pmd, _PAGE_COW);
+ pmd = pmd_set_flags(pmd, _PAGE_DIRTY);
+ }
+ }
+
return pmd_set_flags(pmd, _PAGE_RW);
}

@@ -488,17 +567,35 @@ static inline pud_t pud_mkold(pud_t pud)

static inline pud_t pud_mkclean(pud_t pud)
{
- return pud_clear_flags(pud, _PAGE_DIRTY);
+ return pud_clear_flags(pud, _PAGE_DIRTY_BITS);
}

static inline pud_t pud_wrprotect(pud_t pud)
{
+ /*
+ * Blindly clearing _PAGE_RW might accidentally create
+ * a shadow stack PUD (RW=0, Dirty=1). Move the hardware
+ * dirty value to the software bit.
+ */
+ if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
+ if (pud_flags(pud) & _PAGE_DIRTY) {
+ pud = pud_clear_flags(pud, _PAGE_DIRTY);
+ pud = pud_set_flags(pud, _PAGE_COW);
+ }
+ }
+
return pud_clear_flags(pud, _PAGE_RW);
}

static inline pud_t pud_mkdirty(pud_t pud)
{
- return pud_set_flags(pud, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
+ pudval_t dirty = _PAGE_DIRTY;
+
+ /* Avoid creating (HW)Dirty=1, Write=0 PUDs */
+ if (cpu_feature_enabled(X86_FEATURE_SHSTK) && !(pud_flags(pud) & _PAGE_RW))
+ dirty = _PAGE_COW;
+
+ return pud_set_flags(pud, dirty | _PAGE_SOFT_DIRTY);
}

static inline pud_t pud_mkdevmap(pud_t pud)
@@ -518,6 +615,13 @@ static inline pud_t pud_mkyoung(pud_t pud)

static inline pud_t pud_mkwrite(pud_t pud)
{
+ if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
+ if (pud_flags(pud) & _PAGE_COW) {
+ pud = pud_clear_flags(pud, _PAGE_COW);
+ pud = pud_set_flags(pud, _PAGE_DIRTY);
+ }
+ }
+
return pud_set_flags(pud, _PAGE_RW);
}

@@ -1131,7 +1235,15 @@ extern int pmdp_clear_flush_young(struct vm_area_struct *vma,
#define pmd_write pmd_write
static inline int pmd_write(pmd_t pmd)
{
- return pmd_flags(pmd) & _PAGE_RW;
+ /*
+ * When shadow stack is enabled, a directly writable PMD has RW=1.
+ * A shadow stack PMD is logically writable and has RW=0, Dirty=1.
+ * See comments for _PAGE_COW.
+ */
+ if (cpu_feature_enabled(X86_FEATURE_SHSTK))
+ return pmd_flags(pmd) & (_PAGE_RW | _PAGE_DIRTY);
+ else
+ return pmd_flags(pmd) & _PAGE_RW;
}

#define __HAVE_ARCH_PMDP_HUGE_GET_AND_CLEAR
diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
index b8b79d618bbc..437d7ff0ae80 100644
--- a/arch/x86/include/asm/pgtable_types.h
+++ b/arch/x86/include/asm/pgtable_types.h
@@ -23,7 +23,8 @@
#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 */
@@ -36,6 +37,15 @@
#define _PAGE_BIT_SOFT_DIRTY _PAGE_BIT_SOFTW3 /* software dirty tracking */
#define _PAGE_BIT_DEVMAP _PAGE_BIT_SOFTW4

+/*
+ * Indicates a copy-on-write page.
+ */
+#ifdef CONFIG_X86_CET
+#define _PAGE_BIT_COW _PAGE_BIT_SOFTW5 /* copy-on-write */
+#else
+#define _PAGE_BIT_COW 0
+#endif
+
/* If _PAGE_BIT_PRESENT is clear, we use these: */
/* - if the user mapped it with PROT_NONE; pte_present gives true */
#define _PAGE_BIT_PROTNONE _PAGE_BIT_GLOBAL
@@ -117,6 +127,36 @@
#define _PAGE_DEVMAP (_AT(pteval_t, 0))
#endif

+/*
+ * 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:
+ * (a) A modified, copy-on-write (COW) page: (Write=0, Cow=1)
+ * (b) A R/O page that has been COW'ed: (Write=0, Cow=1)
+ * The user page is in a R/O VMA, and get_user_pages() needs a
+ * writable copy. The page fault handler creates a copy of the page
+ * and sets the new copy's PTE as Write=0, Cow=1.
+ * (c) A shadow stack PTE: (Write=0, Dirty=1)
+ * (d) A shared (copy-on-access) shadow stack PTE: (Write=0, Cow=1)
+ * When a shadow stack page is being shared among processes (this
+ * happens at fork()), its PTE is cleared of _PAGE_DIRTY, so the next
+ * shadow stack access causes a fault, and the page is duplicated and
+ * _PAGE_DIRTY is set again. This is the COW equivalent for shadow
+ * stack pages, even though it's copy-on-access rather than
+ * copy-on-write.
+ * (e) A page where the processor observed a Write=1 PTE, started a write,
+ * set Dirty=1, but then observed a Write=0 PTE (changed by another
+ * thread). That's possible today, but will not happen on processors
+ * that support shadow stack.
+ */
+#ifdef CONFIG_X86_CET
+#define _PAGE_COW (_AT(pteval_t, 1) << _PAGE_BIT_COW)
+#else
+#define _PAGE_COW (_AT(pteval_t, 0))
+#endif
+
+#define _PAGE_DIRTY_BITS (_PAGE_DIRTY | _PAGE_COW)
+
#define _PAGE_PROTNONE (_AT(pteval_t, 1) << _PAGE_BIT_PROTNONE)

/*
--
2.21.0

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