On 1/21/2021 10:44 AM, Borislav Petkov wrote:
On Tue, Dec 29, 2020 at 01:30:35PM -0800, Yu-cheng Yu wrote:
[...]
@@ -343,6 +349,16 @@ 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)) {
+ pte.pte |= (pte.pte & _PAGE_DIRTY) >> _PAGE_BIT_DIRTY << _PAGE_BIT_COW;
Why the unreadable shifting when you can simply do:
if (pte.pte & _PAGE_DIRTY)
pte.pte |= _PAGE_COW;
?
It clears _PAGE_DIRTY and sets _PAGE_COW. That is,
if (pte.pte & _PAGE_DIRTY) {
pte.pte &= ~_PAGE_DIRTY;
pte.pte |= _PAGE_COW;
}
So, shifting makes resulting code more efficient.
@@ -434,16 +469,40 @@ 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)) {
+ pmdval_t v = native_pmd_val(pmd);
+
+ v |= (v & _PAGE_DIRTY) >> _PAGE_BIT_DIRTY << _PAGE_BIT_COW;
As above.
@@ -488,17 +554,35 @@ 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)) {
+ pudval_t v = native_pud_val(pud);
+
+ v |= (v & _PAGE_DIRTY) >> _PAGE_BIT_DIRTY << _PAGE_BIT_COW;
Ditto.
@@ -1131,6 +1222,12 @@ extern int pmdp_clear_flush_young(struct vm_area_struct *vma,
#define pmd_write pmd_write
static inline int pmd_write(pmd_t pmd)
{
+ /*
+ * If _PAGE_DIRTY is set, then the PMD must either have _PAGE_RW or
+ * be a shadow stack PMD, which is logically writable.
+ */
+ if (cpu_feature_enabled(X86_FEATURE_SHSTK))
+ return pmd_flags(pmd) & (_PAGE_RW | _PAGE_DIRTY);
else
return pmd_flags(pmd) & _PAGE_RW;
}
