From 454143aeb4c6c85ca7d50df36d1a66a64d88283e Mon Sep 17 00:00:00 2001 From: "Cen Zhang (Microsoft)" Date: Mon, 12 Oct 2026 16:15:14 -0400 Subject: [PATCH] userfaultfd: fix UFFDIO_MOVE livelock on large folio with extra references 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//, 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: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI") Signed-off-by: Cen Zhang (Microsoft) Signed-off-by: Andrew Morton Reported-by: Acked-by: David Hildenbrand (Arm) Assisted-by: LLM Cc: Mike Rapoport Cc: Peter Xu Cc: Suren Baghdasaryan Cc: Andrea Arcangeli Cc: Cc: --- mm/userfaultfd.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index 405f2d72f204..669f71d485d0 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -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);