userfaultfd-fix-uffdio_move-livelock-on-large-folio-with-extra-references.patch removed from -mm tree

The quilt patch titled
Subject: userfaultfd: fix UFFDIO_MOVE livelock on large folio with extra references
has been removed from the -mm tree. Its filename was
userfaultfd-fix-uffdio_move-livelock-on-large-folio-with-extra-references.patch

This patch was dropped because it was merged into the mm-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: "Cen Zhang (Microsoft)" <[email protected]>
Subject: userfaultfd: fix UFFDIO_MOVE livelock on large folio with extra references
Date: Mon, 12 Oct 2026 16:15:14 -0400

To move one page out of a large folio, move_pages_ptes() first has to split the folio. __folio_split() refuses if the folio has any reference beyond its page table mappings and the caller's own, and it reports the refusal as -EAGAIN. move_pages() treats returned -EAGAIN as a transient race and keeps retrying the same address, holding the VMA read locks the whole time. This is incorrect since in some cases userspace can hold such a reference indefinitely, such as by registering the page as an io_uring buffer.

An unprivileged user can trigger this with three threads: first holds such a reference on a page of a THP, second calls UFFDIO_MOVE on another page of the same THP and spins in the kernel forever holding the VMA read locks, and third calls fork() or mprotect(), which takes mmap_lock for writing and then waits forever for those read locks. From then on every mmap_lock user of that mm hangs, including other processes inspecting it through /proc/<pid>/, until the process is SIGKILLed. The hung task detector reports the blocked tasks (third one), and with kernel.hung_task_panic=1 the machine panics:

  INFO: task wedge:95 blocked for more than 122 seconds.
  task:wedge  state:D stack:12528 pid:95 tgid:95 ppid:94
  Call Trace:
   __schedule+0x596/0x1110
   schedule+0x36/0xf0
   __vma_start_exclude_readers+0x100/0x170
   __vma_start_write+0x51/0xd0
   __split_vma+0x208/0x3d0
   vma_modify+0x6a8/0x840
   vma_modify_flags+0xce/0x110
   mprotect_fixup+0xbe/0x480
   do_mprotect_pkey+0x2d9/0x580
   __x64_sys_mprotect+0x1a/0x30

Fix by returning -EBUSY instead of -EAGAIN in move_pages_ptes(), the caller of split_folio(), when the split fails. Thus, move_pages() does not retry -EBUSY and the ioctl drops its locks and returns to userspace instead of spinning.

---

 mm/userfaultfd.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

@@ -1635,8 +1635,12 @@ retry:
 			pte_unmap(dst_pte);
 			src_pte = dst_pte = NULL;
 			ret = split_folio(src_folio);
-			if (ret)
+			if (ret) {
+				/* do not retry, extra refs may be held forever */
+				if (ret == -EAGAIN)
+					ret = -EBUSY;
 				goto out;
+			}
 			/* have to reacquire the folio after it got split */
 			folio_unlock(src_folio);
 			folio_put(src_folio);

_

Patches currently in -mm which might be from [email protected] are