On Fri, Aug 1, 2025 at 9:23 AM Peter Xu peterx@redhat.com wrote:
On Fri, Aug 01, 2025 at 08:28:38AM -0700, Suren Baghdasaryan wrote:
On Fri, Aug 1, 2025 at 7:16 AM Peter Xu peterx@redhat.com wrote:
On Fri, Aug 01, 2025 at 09:21:30AM +0200, David Hildenbrand wrote:
On 31.07.25 17:44, Suren Baghdasaryan wrote:
Hi!
Did you mean in you patch description:
"userfaultfd: fix a crash in UFFDIO_MOVE with some non-present PMDs"
Talking about THP holes is very very confusing.
When UFFDIO_MOVE is used with UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES and it encounters a non-present THP, it fails to properly recognize an unmapped
You mean a "non-present PMD that is not a migration entry".
hole and tries to access a non-existent folio, resulting in a crash. Add a check to skip non-present THPs.
That makes sense. The code we have after this patch is rather complicated and hard to read.
Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI") Reported-by: syzbot+b446dbe27035ef6bd6c2@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/68794b5c.a70a0220.693ce.0050.GAE@google.com/ Signed-off-by: Suren Baghdasaryan surenb@google.com Cc: stable@vger.kernel.org
Changes since v1 [1]
- Fixed step size calculation, per Lokesh Gidra
- Added missing check for UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES, per Lokesh Gidra
[1] https://lore.kernel.org/all/20250730170733.3829267-1-surenb@google.com/
mm/userfaultfd.c | 45 +++++++++++++++++++++++++++++---------------- 1 file changed, 29 insertions(+), 16 deletions(-)
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index cbed91b09640..b5af31c22731 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -1818,28 +1818,41 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start, ptl = pmd_trans_huge_lock(src_pmd, src_vma); if (ptl) {
/* Check if we can move the pmd without splitting it. */if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||!pmd_none(dst_pmdval)) {struct folio *folio = pmd_folio(*src_pmd);
if (pmd_present(*src_pmd) || is_pmd_migration_entry(*src_pmd)) {[1]
/* Check if we can move the pmd without splitting it. */if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||!pmd_none(dst_pmdval)) {if (pmd_present(*src_pmd)) {[2]
struct folio *folio = pmd_folio(*src_pmd);[3]
if (!folio || (!is_huge_zero_folio(folio) &&!PageAnonExclusive(&folio->page))) {spin_unlock(ptl);err = -EBUSY;break;}}... in particular that. Is there some way to make this code simpler / easier to read? Like moving that whole last folio-check thingy into a helper?
One question might be relevant is, whether the check above [1] can be dropped.
The thing is __pmd_trans_huge_lock() does double check the pmd to be !none before returning the ptl. I didn't follow closely on the recent changes on mm side on possible new pmd swap entries, if migration is the only possible one then it looks like [1] can be avoided.
Hi Peter, is_swap_pmd() check in __pmd_trans_huge_lock() allows for (!pmd_none() && !pmd_present()) PMD to pass and that's when this crash is hit.
First for all, thanks for looking into the issue with Lokesh; I am still catching up with emails after taking weeks off.
I didn't yet read into the syzbot report, but I thought the bug was about referencing the folio on top of a swap entry after reading your current patch, which has:
if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) || !pmd_none(dst_pmdval)) { struct folio *folio = pmd_folio(*src_pmd); <----Here looks like *src_pmd can be a migration entry. Is my understanding correct?
Correct.
If we drop the check at [1] then the path that takes us to
If my above understanding is correct, IMHO it should be [2] above that makes sure the reference won't happen on a swap entry, not necessarily [1]?
Yes, in case of migration entry this is what protects us.
split_huge_pmd() will bail out inside split_huge_pmd_locked() with no indication that split did not happen. Afterwards we will retry
So we're talking about the case where it's a swap pmd entry, right?
Hmm, my understanding is that it's being treated as a swap entry but in reality is not. I thought THPs are always split before they get swapped, no?
Could you elaborate why the split would fail?
Just looking at the code, split_huge_pmd_locked() checks for (pmd_trans_huge(*pmd) || is_pmd_migration_entry(*pmd)). pmd_trans_huge() is false if !pmd_present() and it's not a migration entry, so __split_huge_pmd_locked() will be skipped.
AFAIU, split_huge_pmd_locked() should still work for a migration pmd entry.
That is correct because the above mentioned is_pmd_migration_entry() check will pass.
Thanks,
thinking that PMD got split and leaving further remapping to move_pages_pte() (see the comment before "continue"). I think in this case it will end up in the same path again instead (infinite loop). I didn't test this but from the code I think that's what would happen. Does that make sense?
And it also looks applicable to also drop the "else" later, because in "if (ptl)" it cannot hit pmd_none().
Thanks,
-- Peter Xu
-- Peter Xu