On 24.03.23 23:27, Mike Kravetz wrote:
On 03/24/23 10:26, Peter Xu wrote:
This patch fixes an issue that a hugetlb uffd-wr-protected mapping can be
writable even with uffd-wp bit set. It only happens with hugetlb private
mappings, when someone firstly wr-protects a missing pte (which will
install a pte marker), then a write to the same page without any prior
access to the page.
Userfaultfd-wp trap for hugetlb was implemented in hugetlb_fault() before
reaching hugetlb_wp() to avoid taking more locks that userfault won't need.
However there's one CoW optimization path that can trigger hugetlb_wp()
inside hugetlb_no_page(), which will bypass the trap.
This patch skips hugetlb_wp() for CoW and retries the fault if uffd-wp bit
is detected. The new path will only trigger in the CoW optimization path
because generic hugetlb_fault() (e.g. when a present pte was wr-protected)
will resolve the uffd-wp bit already. Also make sure anonymous UNSHARE
won't be affected and can still be resolved, IOW only skip CoW not CoR.
This patch will be needed for v5.19+ hence copy stable.
Reported-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
Cc: linux-stable <stable@xxxxxxxxxxxxxxx>
Fixes: 166f3ecc0daf ("mm/hugetlb: hook page faults for uffd write protection")
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
Notes:
v2 is not on the list but in an attachment in the reply; this v3 is mostly
to make sure it's not the same as the patch used to be attached. Sorry
Andrew, we need to drop the queued one as I rewrote the commit message.
My appologies! I saw the code path missed in v2 and assumed you did not
think it applied. So, I said nothing. My bad!
Muhammad, I didn't attach your T-b because of the slight functional change.
Please feel free to re-attach if it still works for you (which I believe
should).
thanks,
---
mm/hugetlb.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 8bfd07f4c143..a58b3739ed4b 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5478,7 +5478,7 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
struct folio *pagecache_folio, spinlock_t *ptl)
{
const bool unshare = flags & FAULT_FLAG_UNSHARE;
- pte_t pte;
+ pte_t pte = huge_ptep_get(ptep);
struct hstate *h = hstate_vma(vma);
struct page *old_page;
struct folio *new_folio;
@@ -5487,6 +5487,17 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
unsigned long haddr = address & huge_page_mask(h);
struct mmu_notifier_range range;
+ /*
+ * Never handle CoW for uffd-wp protected pages. It should be only
+ * handled when the uffd-wp protection is removed.
+ *
+ * Note that only the CoW optimization path (in hugetlb_no_page())
+ * can trigger this, because hugetlb_fault() will always resolve
+ * uffd-wp bit first.
+ */
+ if (!unshare && huge_pte_uffd_wp(pte))
+ return 0;
This looks correct. However, since the previous version looked correct I must
ask. Can we have unshare set and huge_pte_uffd_wp true? If so, then it seems
we would need to possibly propogate that uffd_wp to the new pte as in v2
We can. A reproducer would share an anon hugetlb page because parent and
child. In the parent, we would uffd-wp that page. We could trigger
unsharing by R/O-pinning that page.
--
Thanks,
David / dhildenb
