[PATCH v7 14/41] x86/mm: Introduce _PAGE_SAVED_DIRTY

Rick Edgecombe
2023-02-27T22:32:31Z

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 avoid
inadvertently created shadow stack memory, in places where Linux normally
creates Write=0,Dirty=1, it can use the software-defined _PAGE_SAVED_DIRTY
in place of the hardware _PAGE_DIRTY. In other words, whenever Linux needs
to create Write=0,Dirty=1, it instead creates Write=0,SavedDirty=1 except
for shadow stack, which is Write=0,Dirty=1.

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

Implement only the infrastructure for _PAGE_SAVED_DIRTY. Changes to
actually begin creating _PAGE_SAVED_DIRTY PTEs will follow once other
pieces are in place.

Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Co-developed-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>

---
v7:
- Use lightly edited comment verbiage from (David Hildenbrand)
- Update commit log to reduce verbosity (David Hildenbrand)

v6:
- Rename _PAGE_COW to _PAGE_SAVED_DIRTY (David Hildenbrand)
- Add _PAGE_SAVED_DIRTY to _PAGE_CHG_MASK

v5:
- Fix log, comments and whitespace (Boris)
- Remove capitalization on shadow stack (Boris)

v4:
- Teach pte_flags_need_flush() about _PAGE_COW bit
- Break apart patch for better bisectability

v3:
- 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
(peterz)
- Fix swapping of Write/Dirty (PeterZ)
---
arch/x86/include/asm/pgtable.h | 79 ++++++++++++++++++++++++++++
arch/x86/include/asm/pgtable_types.h | 50 +++++++++++++++---
arch/x86/include/asm/tlbflush.h | 3 +-
3 files changed, 123 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 66c514808276..7360783f2140 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -301,6 +301,45 @@ static inline pte_t pte_clear_flags(pte_t pte, pteval_t clear)
return native_make_pte(v & ~clear);
}

+/*
+ * Write protection operations can result in Dirty=1,Write=0 PTEs. But in the
+ * case of X86_FEATURE_USER_SHSTK, the software SavedDirty 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 dirty, write-protected memory, a software bit is
+ * used _PAGE_BIT_SAVED_DIRTY. The following functions pte_mksaveddirty() and
+ * pte_clear_saveddirty() take a conventional dirty, write-protected PTE
+ * (Write=0,Dirty=1) and transition it to the shadow stack compatible
+ * version. (Write=0,SavedDirty=1).
+ */
+static inline pte_t pte_mksaveddirty(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_SAVED_DIRTY);
+}
+
+static inline pte_t pte_clear_saveddirty(pte_t pte)
+{
+ /*
+ * _PAGE_SAVED_DIRTY is unnecessary on !X86_FEATURE_USER_SHSTK kernels,
+ * since the HW dirty bit can be used without creating shadow stack
+ * memory. See the _PAGE_SAVED_DIRTY 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_SAVED_DIRTY);
+}
+
static inline pte_t pte_wrprotect(pte_t pte)
{
return pte_clear_flags(pte, _PAGE_RW);
@@ -420,6 +459,26 @@ static inline pmd_t pmd_clear_flags(pmd_t pmd, pmdval_t clear)
return native_make_pmd(v & ~clear);
}

+/* See comments above pte_mksaveddirty() */
+static inline pmd_t pmd_mksaveddirty(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_SAVED_DIRTY);
+}
+
+/* See comments above pte_mksaveddirty() */
+static inline pmd_t pmd_clear_saveddirty(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_SAVED_DIRTY);
+}
+
static inline pmd_t pmd_wrprotect(pmd_t pmd)
{
return pmd_clear_flags(pmd, _PAGE_RW);
@@ -491,6 +550,26 @@ static inline pud_t pud_clear_flags(pud_t pud, pudval_t clear)
return native_make_pud(v & ~clear);
}

+/* See comments above pte_mksaveddirty() */
+static inline pud_t pud_mksaveddirty(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_SAVED_DIRTY);
+}
+
+/* See comments above pte_mksaveddirty() */
+static inline pud_t pud_clear_saveddirty(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_SAVED_DIRTY);
+}
+
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..56b374d1bffb 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 */
#define _PAGE_BIT_DEVMAP _PAGE_BIT_SOFTW4

+/*
+ * Indicates a Saved Dirty bit page.
+ */
+#ifdef CONFIG_X86_USER_SHADOW_STACK
+#define _PAGE_BIT_SAVED_DIRTY _PAGE_BIT_SOFTW5 /* Saved Dirty bit */
+#else
+#define _PAGE_BIT_SAVED_DIRTY 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 +127,25 @@
#define _PAGE_SOFTW4 (_AT(pteval_t, 0))
#endif

+/*
+ * The hardware requires shadow stack to be Write=0,Dirty=1. However,
+ * there are valid cases where the kernel might create read-only PTEs that
+ * are dirty (e.g., fork(), mprotect(), uffd-wp(), soft-dirty tracking). In
+ * this case, the _PAGE_SAVED_DIRTY bit is used instead of the HW-dirty bit,
+ * to avoid creating a wrong "shadow stack" PTEs. Such PTEs have
+ * (Write=0,SavedDirty=1,Dirty=0) set.
+ *
+ * Note that on processors without shadow stack support, the
+ * _PAGE_SAVED_DIRTY remains unused.
+ */
+#ifdef CONFIG_X86_USER_SHADOW_STACK
+#define _PAGE_SAVED_DIRTY (_AT(pteval_t, 1) << _PAGE_BIT_SAVED_DIRTY)
+#else
+#define _PAGE_SAVED_DIRTY (_AT(pteval_t, 0))
+#endif
+
+#define _PAGE_DIRTY_BITS (_PAGE_DIRTY | _PAGE_SAVED_DIRTY)
+
#define _PAGE_PROTNONE (_AT(pteval_t, 1) << _PAGE_BIT_PROTNONE)

/*
@@ -125,9 +154,9 @@
* instance, and is *not* included in this mask since
* pte_modify() does modify it.
*/
-#define _PAGE_CHG_MASK (PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT | \
- _PAGE_SPECIAL | _PAGE_ACCESSED | _PAGE_DIRTY | \
- _PAGE_SOFT_DIRTY | _PAGE_DEVMAP | _PAGE_ENC | \
+#define _PAGE_CHG_MASK (PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT | \
+ _PAGE_SPECIAL | _PAGE_ACCESSED | _PAGE_DIRTY_BITS | \
+ _PAGE_SOFT_DIRTY | _PAGE_DEVMAP | _PAGE_ENC | \
_PAGE_UFFD_WP)
#define _HPAGE_CHG_MASK (_PAGE_CHG_MASK | _PAGE_PSE)

@@ -186,12 +215,17 @@ enum page_cache_mode {
#define PAGE_READONLY __pg(__PP| 0|_USR|___A|__NX| 0| 0| 0)
#define PAGE_READONLY_EXEC __pg(__PP| 0|_USR|___A| 0| 0| 0| 0)

-#define __PAGE_KERNEL (__PP|__RW| 0|___A|__NX|___D| 0|___G)
-#define __PAGE_KERNEL_EXEC (__PP|__RW| 0|___A| 0|___D| 0|___G)
-#define _KERNPG_TABLE_NOENC (__PP|__RW| 0|___A| 0|___D| 0| 0)
-#define _KERNPG_TABLE (__PP|__RW| 0|___A| 0|___D| 0| 0| _ENC)
+/*
+ * Page tables needs to have Write=1 in order for any lower PTEs to be
+ * writable. This includes shadow stack memory (Write=0, Dirty=1)
+ */
#define _PAGE_TABLE_NOENC (__PP|__RW|_USR|___A| 0|___D| 0| 0)
#define _PAGE_TABLE (__PP|__RW|_USR|___A| 0|___D| 0| 0| _ENC)
+#define _KERNPG_TABLE_NOENC (__PP|__RW| 0|___A| 0|___D| 0| 0)
+#define _KERNPG_TABLE (__PP|__RW| 0|___A| 0|___D| 0| 0| _ENC)
+
+#define __PAGE_KERNEL (__PP|__RW| 0|___A|__NX|___D| 0|___G)
+#define __PAGE_KERNEL_EXEC (__PP|__RW| 0|___A| 0|___D| 0|___G)
#define __PAGE_KERNEL_RO (__PP| 0| 0|___A|__NX| 0| 0|___G)
#define __PAGE_KERNEL_ROX (__PP| 0| 0|___A| 0| 0| 0|___G)
#define __PAGE_KERNEL_NOCACHE (__PP|__RW| 0|___A|__NX|___D| 0|___G| __NC)
diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
index cda3118f3b27..6c5ef14060a8 100644
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -273,7 +273,8 @@ static inline bool pte_flags_need_flush(unsigned long oldflags,
const pteval_t flush_on_clear = _PAGE_DIRTY | _PAGE_PRESENT |
_PAGE_ACCESSED;
const pteval_t software_flags = _PAGE_SOFTW1 | _PAGE_SOFTW2 |
- _PAGE_SOFTW3 | _PAGE_SOFTW4;
+ _PAGE_SOFTW3 | _PAGE_SOFTW4 |
+ _PAGE_SAVED_DIRTY;
const pteval_t flush_on_change = _PAGE_RW | _PAGE_USER | _PAGE_PWT |
_PAGE_PCD | _PAGE_PSE | _PAGE_GLOBAL | _PAGE_PAT |
_PAGE_PAT_LARGE | _PAGE_PKEY_BIT0 | _PAGE_PKEY_BIT1 |
--
2.17.1