[PATCH v5 13/39] x86/mm: Start actually marking _PAGE_COW
From: Rick Edgecombe
Date: Thu Jan 19 2023 - 16:34:55 EST
The recently introduced _PAGE_COW should be used instead of the HW Dirty
bit whenever a PTE is Write=0, in order to not inadvertently create
shadow stack PTEs. Update pte_mk*() helpers to do this, and apply the same
changes to pmd and pud.
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
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>
---
v4:
- Break part patch for better bisectability
arch/x86/include/asm/pgtable.h | 125 ++++++++++++++++++++++++++++-----
1 file changed, 107 insertions(+), 18 deletions(-)
diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index c5047eb5f406..e96558abc8ec 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -124,9 +124,17 @@ 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;
+}
+
+static inline bool pte_shstk(pte_t pte)
+{
+ if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
+ return false;
+
+ return (pte_flags(pte) & (_PAGE_RW | _PAGE_DIRTY)) == _PAGE_DIRTY;
}
static inline int pte_young(pte_t pte)
@@ -134,9 +142,18 @@ 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 bool pmd_shstk(pmd_t pmd)
+{
+ if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
+ return false;
+
+ return (pmd_flags(pmd) & (_PAGE_RW | _PAGE_DIRTY | _PAGE_PSE)) ==
+ (_PAGE_DIRTY | _PAGE_PSE);
}
#define pmd_young pmd_young
@@ -145,9 +162,9 @@ 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)
@@ -157,13 +174,21 @@ static inline int pud_young(pud_t pud)
static inline int pte_write(pte_t pte)
{
- return pte_flags(pte) & _PAGE_RW;
+ /*
+ * Shadow stack pages are logically writable, but do not have
+ * _PAGE_RW. Check for them separately from _PAGE_RW itself.
+ */
+ return (pte_flags(pte) & _PAGE_RW) || pte_shstk(pte);
}
#define pmd_write pmd_write
static inline int pmd_write(pmd_t pmd)
{
- return pmd_flags(pmd) & _PAGE_RW;
+ /*
+ * Shadow stack pages are logically writable, but do not have
+ * _PAGE_RW. Check for them separately from _PAGE_RW itself.
+ */
+ return (pmd_flags(pmd) & _PAGE_RW) || pmd_shstk(pmd);
}
#define pud_write pud_write
@@ -374,7 +399,7 @@ 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)
@@ -384,7 +409,16 @@ static inline pte_t pte_mkold(pte_t pte)
static inline pte_t pte_wrprotect(pte_t pte)
{
- return pte_clear_flags(pte, _PAGE_RW);
+ pte = pte_clear_flags(pte, _PAGE_RW);
+
+ /*
+ * Blindly clearing _PAGE_RW might accidentally create
+ * a shadow stack PTE (Write=0,Dirty=1). Move the hardware
+ * dirty value to the software bit.
+ */
+ if (pte_dirty(pte))
+ pte = pte_mkcow(pte);
+ return pte;
}
static inline pte_t pte_mkexec(pte_t pte)
@@ -396,6 +430,10 @@ static inline pte_t __pte_mkdirty(pte_t pte, bool soft)
{
pteval_t dirty = _PAGE_DIRTY;
+ /* Avoid creating Dirty=1,Write=0 PTEs */
+ if (cpu_feature_enabled(X86_FEATURE_USER_SHSTK) && !pte_write(pte))
+ dirty = _PAGE_COW;
+
if (soft)
dirty |= _PAGE_SOFT_DIRTY;
@@ -407,6 +445,12 @@ static inline pte_t pte_mkdirty(pte_t pte)
return __pte_mkdirty(pte, true);
}
+static inline pte_t pte_mkwrite_shstk(pte_t pte)
+{
+ /* pte_clear_cow() also sets Dirty=1 */
+ return pte_clear_cow(pte);
+}
+
static inline pte_t pte_mkyoung(pte_t pte)
{
return pte_set_flags(pte, _PAGE_ACCESSED);
@@ -414,7 +458,12 @@ static inline pte_t pte_mkyoung(pte_t pte)
static inline pte_t pte_mkwrite(pte_t pte)
{
- return pte_set_flags(pte, _PAGE_RW);
+ pte = pte_set_flags(pte, _PAGE_RW);
+
+ if (pte_dirty(pte))
+ pte = pte_clear_cow(pte);
+
+ return pte;
}
static inline pte_t pte_mkhuge(pte_t pte)
@@ -505,18 +554,30 @@ 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)
{
- return pmd_clear_flags(pmd, _PAGE_RW);
+ pmd = pmd_clear_flags(pmd, _PAGE_RW);
+ /*
+ * 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 (pmd_dirty(pmd))
+ pmd = pmd_mkcow(pmd);
+ return pmd;
}
static inline pmd_t __pmd_mkdirty(pmd_t pmd, bool soft)
{
pmdval_t dirty = _PAGE_DIRTY;
+ /* Avoid creating (HW)Dirty=1, Write=0 PMDs */
+ if (cpu_feature_enabled(X86_FEATURE_USER_SHSTK) && !pmd_write(pmd))
+ dirty = _PAGE_COW;
+
if (soft)
dirty |= _PAGE_SOFT_DIRTY;
@@ -528,6 +589,11 @@ static inline pmd_t pmd_mkdirty(pmd_t pmd)
return __pmd_mkdirty(pmd, true);
}
+static inline pmd_t pmd_mkwrite_shstk(pmd_t pmd)
+{
+ return pmd_clear_cow(pmd);
+}
+
static inline pmd_t pmd_mkdevmap(pmd_t pmd)
{
return pmd_set_flags(pmd, _PAGE_DEVMAP);
@@ -545,7 +611,11 @@ static inline pmd_t pmd_mkyoung(pmd_t pmd)
static inline pmd_t pmd_mkwrite(pmd_t pmd)
{
- return pmd_set_flags(pmd, _PAGE_RW);
+ pmd = pmd_set_flags(pmd, _PAGE_RW);
+
+ if (pmd_dirty(pmd))
+ pmd = pmd_clear_cow(pmd);
+ return pmd;
}
static inline pud_t pud_set_flags(pud_t pud, pudval_t set)
@@ -589,17 +659,32 @@ 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)
{
- return pud_clear_flags(pud, _PAGE_RW);
+ pud = pud_clear_flags(pud, _PAGE_RW);
+
+ /*
+ * 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 (pud_dirty(pud))
+ pud = pud_mkcow(pud);
+ return pud;
}
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_USER_SHSTK) && !pud_write(pud))
+ dirty = _PAGE_COW;
+
+ return pud_set_flags(pud, dirty | _PAGE_SOFT_DIRTY);
}
static inline pud_t pud_mkdevmap(pud_t pud)
@@ -619,7 +704,11 @@ static inline pud_t pud_mkyoung(pud_t pud)
static inline pud_t pud_mkwrite(pud_t pud)
{
- return pud_set_flags(pud, _PAGE_RW);
+ pud = pud_set_flags(pud, _PAGE_RW);
+
+ if (pud_dirty(pud))
+ pud = pud_clear_cow(pud);
+ return pud;
}
#ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY
--
2.17.1