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userfaultfd: fix UFFDIO_MOVE livelock on large folio with extra references

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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.

Link: https://lore.kernel.org/20261012201514.97015-1-cenzhang@linux.microsoft.com
Fixes: adef440691 ("userfaultfd: UFFDIO_MOVE uABI")
Signed-off-by: Cen Zhang (Microsoft) <cenzhang@linux.microsoft.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: <AutonomousCodeSecurity@microsoft.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Assisted-by: LLM
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: <tgopinath@linux.microsoft.com>
Cc: <stable@vger.kernel.org>
This commit is contained in:
Cen Zhang (Microsoft) 2026-10-12 16:15:14 -04:00 committed by Andrew Morton
parent 3ae91e3079
commit 454143aeb4
1 changed files with 5 additions and 1 deletions
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6
mm/userfaultfd.c
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@ -1635,8 +1635,12 @@ static long move_pages_ptes(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd
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);