[PATCH 5/9] x86/mm: Introduce _PAGE_DIRTY_SW

Yu-cheng Yu
2018-06-07T15:45:05Z

The PTE DIRTY bit indicates a few conditions:

(1) When the processor writes to a memory page, the page's
PTE is (R/W + _PAGE_DIRTY_HW);
(2) When a modified page is shared from fork(), its PTE is
(R/O + _PAGE_DIRTY_HW);
(3) When access_remote_vm() has tried to write to a read-
only page with (FOLL_FORCE | FOLL_WRITE), the PTE is
(R/O + _PAGE_DIRTY_HW);
(4) A shadow stack memory page is required to be set as
(R/O + _PAGE_DIRTY_HW);

In case (1) above, the DIRTY bit is set by the processor;
for other cases, it is set by the software. However, the
processor reads the DIRTY bit only in case (4) for ensuring
a valid shadow stack page.

To make (R/O + _PAGE_DIRTY_HW) exclusively for shadow stack,
we introduce _PAGE_BIT_DIRTY_SW, a spare bit of the 64-bit
PTE, to replace _PAGE_BIT_DIRTY for case (2), (3) and (4).

This results to the following possible PTE settings:

Modified PTE: (R/W + _PAGE_DIRTY_HW)
Modified and shared PTE: (R/O + _PAGE_DIRTY_SW)
R/O PTE was (FOLL_FORCE | FOLL_WRITE): (R/O + _PAGE_DIRTY_SW)
Shadow stack PTE: (R/O + _PAGE_DIRTY_HW)
Shared shadow stack PTE: (R/O + _PAGE_DIRTY_SW)

Note that _PAGE_BIT_DRITY_SW is only used in R/O PTEs but
not R/W PTEs.

When this patch is applied, there are six free bits left in
the 64-bit PTE. There is no more free bit 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@intel.com>
---
arch/x86/include/asm/pgtable.h | 91 ++++++++++++++++++++++++++++++++----
arch/x86/include/asm/pgtable_types.h | 14 +++++-
include/asm-generic/pgtable.h | 12 +++++
3 files changed, 105 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 00b5e79c09a6..0996f8a6979a 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -116,9 +116,9 @@ 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;
+ return pte_flags(pte) & _PAGE_DIRTY_BITS;
}

@@ -140,9 +140,9 @@ 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;
+ return pmd_flags(pmd) & _PAGE_DIRTY_BITS;
}

static inline int pmd_young(pmd_t pmd)
@@ -150,9 +150,9 @@ 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;
+ return pud_flags(pud) & _PAGE_DIRTY_BITS;
}

static inline int pud_young(pud_t pud)
@@ -281,9 +281,23 @@ static inline pte_t pte_clear_flags(pte_t pte, pteval_t clear)
return native_make_pte(v & ~clear);
}

+#if defined(CONFIG_X86_INTEL_SHADOW_STACK_USER)
+static inline pte_t pte_move_flags(pte_t pte, pteval_t from, pteval_t to)
+{
+ if (pte_flags(pte) & from)
+ pte = pte_set_flags(pte_clear_flags(pte, from), to);
+ return pte;
+}
+#else
+static inline pte_t pte_move_flags(pte_t pte, pteval_t from, pteval_t to)
+{
+ return pte;
+}
+#endif
+
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)
@@ -293,6 +307,7 @@ static inline pte_t pte_mkold(pte_t pte)

static inline pte_t pte_wrprotect(pte_t pte)
{
+ pte = pte_move_flags(pte, _PAGE_DIRTY_HW, _PAGE_DIRTY_SW);
return pte_clear_flags(pte, _PAGE_RW);
}

@@ -302,9 +317,18 @@ static inline pte_t pte_mkexec(pte_t pte)
}

static inline pte_t pte_mkdirty(pte_t pte)
+{
+ pteval_t dirty = (!IS_ENABLED(CONFIG_X86_INTEL_SHSTK_USER) ||
+ pte_write(pte)) ? _PAGE_DIRTY_HW:_PAGE_DIRTY_SW;
+ return pte_set_flags(pte, dirty | _PAGE_SOFT_DIRTY);
+}
+
+#ifdef CONFIG_ARCH_HAS_SHSTK
+static inline pte_t pte_mkdirty_shstk(pte_t pte)
{
return pte_set_flags(pte, _PAGE_DIRTY_HW | _PAGE_SOFT_DIRTY);
}
+#endif

static inline pte_t pte_mkyoung(pte_t pte)
{
@@ -313,6 +337,7 @@ static inline pte_t pte_mkyoung(pte_t pte)

static inline pte_t pte_mkwrite(pte_t pte)
{
+ pte = pte_move_flags(pte, _PAGE_DIRTY_SW, _PAGE_DIRTY_HW);
return pte_set_flags(pte, _PAGE_RW);
}

@@ -360,6 +385,20 @@ static inline pmd_t pmd_clear_flags(pmd_t pmd, pmdval_t clear)
return native_make_pmd(v & ~clear);
}

+#if defined(CONFIG_X86_INTEL_SHADOW_STACK_USER)
+static inline pmd_t pmd_move_flags(pmd_t pmd, pmdval_t from, pmdval_t to)
+{
+ if (pmd_flags(pmd) & from)
+ pmd = pmd_set_flags(pmd_clear_flags(pmd, from), to);
+ return pmd;
+}
+#else
+static inline pmd_t pmd_move_flags(pmd_t pmd, pmdval_t from, pmdval_t to)
+{
+ return pmd;
+}
+#endif
+
static inline pmd_t pmd_mkold(pmd_t pmd)
{
return pmd_clear_flags(pmd, _PAGE_ACCESSED);
@@ -367,18 +406,29 @@ 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)
{
+ pmd = pmd_move_flags(pmd, _PAGE_DIRTY_HW, _PAGE_DIRTY_SW);
return pmd_clear_flags(pmd, _PAGE_RW);
}

static inline pmd_t pmd_mkdirty(pmd_t pmd)
+{
+ pmdval_t dirty = (!IS_ENABLED(CONFIG_X86_INTEL_SHSTK_USER) ||
+ (pmd_flags(pmd) & _PAGE_RW)) ?
+ _PAGE_DIRTY_HW:_PAGE_DIRTY_SW;
+ return pmd_set_flags(pmd, dirty | _PAGE_SOFT_DIRTY);
+}
+
+#ifdef CONFIG_ARCH_HAS_SHSTK
+static inline pmd_t pmd_mkdirty_shstk(pmd_t pmd)
{
return pmd_set_flags(pmd, _PAGE_DIRTY_HW | _PAGE_SOFT_DIRTY);
}
+#endif

static inline pmd_t pmd_mkdevmap(pmd_t pmd)
{
@@ -397,6 +447,7 @@ static inline pmd_t pmd_mkyoung(pmd_t pmd)

static inline pmd_t pmd_mkwrite(pmd_t pmd)
{
+ pmd = pmd_move_flags(pmd, _PAGE_DIRTY_SW, _PAGE_DIRTY_HW);
return pmd_set_flags(pmd, _PAGE_RW);
}

@@ -419,6 +470,20 @@ static inline pud_t pud_clear_flags(pud_t pud, pudval_t clear)
return native_make_pud(v & ~clear);
}

+#if defined(CONFIG_X86_INTEL_SHADOW_STACK_USER)
+static inline pud_t pud_move_flags(pud_t pud, pudval_t from, pudval_t to)
+{
+ if (pud_flags(pud) & from)
+ pud = pud_set_flags(pud_clear_flags(pud, from), to);
+ return pud;
+}
+#else
+static inline pud_t pud_move_flags(pud_t pud, pudval_t from, pudval_t to)
+{
+ return pud;
+}
+#endif
+
static inline pud_t pud_mkold(pud_t pud)
{
return pud_clear_flags(pud, _PAGE_ACCESSED);
@@ -426,17 +491,22 @@ 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)
{
+ pud = pud_move_flags(pud, _PAGE_DIRTY_HW, _PAGE_DIRTY_SW);
return pud_clear_flags(pud, _PAGE_RW);
}

static inline pud_t pud_mkdirty(pud_t pud)
{
- return pud_set_flags(pud, _PAGE_DIRTY_HW | _PAGE_SOFT_DIRTY);
+ pudval_t dirty = (!IS_ENABLED(CONFIG_X86_INTEL_SHSTK_USER) ||
+ (pud_flags(pud) & _PAGE_RW)) ?
+ _PAGE_DIRTY_HW:_PAGE_DIRTY_SW;
+
+ return pud_set_flags(pud, dirty | _PAGE_SOFT_DIRTY);
}

static inline pud_t pud_mkdevmap(pud_t pud)
@@ -456,6 +526,7 @@ static inline pud_t pud_mkyoung(pud_t pud)

static inline pud_t pud_mkwrite(pud_t pud)
{
+ pud = pud_move_flags(pud, _PAGE_DIRTY_SW, _PAGE_DIRTY_HW);
return pud_set_flags(pud, _PAGE_RW);
}

diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
index 2ac5d46d7c49..0907adb56197 100644
--- a/arch/x86/include/asm/pgtable_types.h
+++ b/arch/x86/include/asm/pgtable_types.h
@@ -23,6 +23,7 @@
#define _PAGE_BIT_SOFTW2 10 /* " */
#define _PAGE_BIT_SOFTW3 11 /* " */
#define _PAGE_BIT_PAT_LARGE 12 /* On 2MB or 1GB pages */
+#define _PAGE_BIT_SOFTW5 57 /* available for programmer */
#define _PAGE_BIT_SOFTW4 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 */
@@ -34,6 +35,7 @@
#define _PAGE_BIT_CPA_TEST _PAGE_BIT_SOFTW1
#define _PAGE_BIT_SOFT_DIRTY _PAGE_BIT_SOFTW3 /* software dirty tracking */
#define _PAGE_BIT_DEVMAP _PAGE_BIT_SOFTW4
+#define _PAGE_BIT_DIRTY_SW _PAGE_BIT_SOFTW5 /* was written to */

/* If _PAGE_BIT_PRESENT is clear, we use these: */
/* - if the user mapped it with PROT_NONE; pte_present gives true */
@@ -109,6 +111,14 @@
#define _PAGE_DEVMAP (_AT(pteval_t, 0))
#endif

+#if defined(CONFIG_X86_INTEL_SHADOW_STACK_USER)
+#define _PAGE_DIRTY_SW (_AT(pteval_t, 1) << _PAGE_BIT_DIRTY_SW)
+#else
+#define _PAGE_DIRTY_SW (_AT(pteval_t, 0))
+#endif
+
+#define _PAGE_DIRTY_BITS (_PAGE_DIRTY_HW | _PAGE_DIRTY_SW)
+
#define _PAGE_PROTNONE (_AT(pteval_t, 1) << _PAGE_BIT_PROTNONE)

#define _PAGE_TABLE_NOENC (_PAGE_PRESENT | _PAGE_RW | _PAGE_USER |\
@@ -122,9 +132,9 @@
* 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 | \
+#define _PAGE_CHG_MASK (PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT | \
_PAGE_SPECIAL | _PAGE_ACCESSED | _PAGE_DIRTY_HW | \
- _PAGE_SOFT_DIRTY)
+ _PAGE_DIRTY_SW | _PAGE_SOFT_DIRTY)
#define _HPAGE_CHG_MASK (_PAGE_CHG_MASK | _PAGE_PSE)

/*
diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
index f59639afaa39..3f6f998509f0 100644
--- a/include/asm-generic/pgtable.h
+++ b/include/asm-generic/pgtable.h
@@ -1097,4 +1097,16 @@ static inline void init_espfix_bsp(void) { }
#endif
#endif

+#ifndef CONFIG_ARCH_HAS_SHSTK
+static inline pte_t pte_mkdirty_shstk(pte_t pte)
+{
+ return pte;
+}
+
+static inline pmd_t pmd_mkdirty_shstk(pmd_t pmd)
+{
+ return pmd;
+}
+#endif
+
#endif /* _ASM_GENERIC_PGTABLE_H */
--
2.15.1