From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
When clearing _PAGE_PRESENT on a huge page, we need to be careful
to also clear _PAGE_PSE, otherwise it might still get confused
for a valid large page table entry.
We do that near the spot where we *set* _PAGE_PSE. That's fine,
but it's unnecessary. pgprot_large_2_4k() already did it.
BTW, I also noticed that pgprot_large_2_4k() and
pgprot_4k_2_large() are not symmetric. pgprot_large_2_4k() clears
_PAGE_PSE (because it is aliased to _PAGE_PAT) but
pgprot_4k_2_large() does not put _PAGE_PSE back. Bummer.
Also, add some comments and change "promote" to "move". "Promote"
seems an odd word to move when we are logically moving a bit to a
lower bit position. Also add an extra line return to make it clear
to which line the comment applies.
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: Nadav Amit <namit@xxxxxxxxxx>
---
b/arch/x86/mm/pageattr.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)
diff -puN arch/x86/mm/pageattr.c~kpti-undo-double-_PAGE_PSE-clear arch/x86/mm/pageattr.c
--- a/arch/x86/mm/pageattr.c~kpti-undo-double-_PAGE_PSE-clear 2018-04-02 10:26:43.152661217 -0700
+++ b/arch/x86/mm/pageattr.c 2018-04-02 10:26:43.156661217 -0700
@@ -583,6 +583,7 @@ try_preserve_large_page(pte_t *kpte, uns
* up accordingly.
*/
old_pte = *kpte;
+ /* Clear PSE (aka _PAGE_PAT) and move PAT bit to correct position */
req_prot = pgprot_large_2_4k(old_prot);
pgprot_val(req_prot) &= ~pgprot_val(cpa->mask_clr);
@@ -597,8 +598,6 @@ try_preserve_large_page(pte_t *kpte, uns
req_prot = pgprot_clear_protnone_bits(req_prot);
if (pgprot_val(req_prot) & _PAGE_PRESENT)
pgprot_val(req_prot) |= _PAGE_PSE;
- else
- pgprot_val(req_prot) &= ~_PAGE_PSE;
req_prot = canon_pgprot(req_prot);
/*
@@ -684,8 +683,12 @@ __split_large_page(struct cpa_data *cpa,
switch (level) {
case PG_LEVEL_2M:
ref_prot = pmd_pgprot(*(pmd_t *)kpte);
- /* clear PSE and promote PAT bit to correct position */
+ /*
+ * Clear PSE (aka _PAGE_PAT) and move
+ * PAT bit to correct position.
+ */
ref_prot = pgprot_large_2_4k(ref_prot);
+
ref_pfn = pmd_pfn(*(pmd_t *)kpte);
break;
_
