On Thu, 2018-08-30 at 17:49 +0200, Jann Horn wrote:
On Thu, Aug 30, 2018 at 4:43 PM Yu-cheng Yu [off-list ref]
wrote:
quoted
When Shadow Stack is enabled, the read-only and PAGE_DIRTY_HW PTE
setting is reserved only for the Shadow Stack. To track dirty of
non-Shadow Stack read-only PTEs, we use PAGE_DIRTY_SW.
Update ptep_set_wrprotect() and pmdp_set_wrprotect().
Signed-off-by: Yu-cheng Yu <redacted>
---
arch/x86/include/asm/pgtable.h | 42
++++++++++++++++++++++++++++++++++
1 file changed, 42 insertions(+)
diff --git a/arch/x86/include/asm/pgtable.h
b/arch/x86/include/asm/pgtable.h
index 4d50de77ea96..556ef258eeff 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -1203,7 +1203,28 @@ static inline pte_t
ptep_get_and_clear_full(struct mm_struct *mm,
static inline void ptep_set_wrprotect(struct mm_struct *mm,
unsigned long addr, pte_t
*ptep)
{
+ pte_t pte;
+
clear_bit(_PAGE_BIT_RW, (unsigned long *)&ptep->pte);
+ pte = *ptep;
+
+ /*
+ * Some processors can start a write, but ending up seeing
+ * a read-only PTE by the time they get to the Dirty bit.
+ * In this case, they will set the Dirty bit, leaving a
+ * read-only, Dirty PTE which looks like a Shadow Stack
PTE.
+ *
+ * However, this behavior has been improved and will not
occur
+ * on processors supporting Shadow Stacks. Without this
+ * guarantee, a transition to a non-present PTE and flush
the
+ * TLB would be needed.
+ *
+ * When change a writable PTE to read-only and if the PTE
has
+ * _PAGE_DIRTY_HW set, we move that bit to _PAGE_DIRTY_SW
so
+ * that the PTE is not a valid Shadow Stack PTE.
+ */
+ pte = pte_move_flags(pte, _PAGE_DIRTY_HW, _PAGE_DIRTY_SW);
+ set_pte_at(mm, addr, ptep, pte);
}
I don't understand why it's okay that you first atomically clear the
RW bit, then atomically switch from DIRTY_HW to DIRTY_SW. Doesn't
that
mean that between the two atomic writes, another core can
incorrectly
see a shadow stack?
Yes, we had that concern earlier and checked.
On processors supporting Shadow Stacks, that will not happen.
Yu-cheng