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)) {struct folio *folio = pmd_folio(*src_pmd);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.
And it also looks applicable to also drop the "else" later, because in "if (ptl)" it cannot hit pmd_none().
Thanks,