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mm/userfaultfd: write-lock the VMA before waking waiters when unregistering

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userfaultfd_unregister() wakes pending missing faults before clearing the
VMA's userfaultfd registration. With per-VMA fault locking, holding
mmap_lock for write does not prevent another fault from queuing after
the wakeup but before userfaultfd_clear_vma() write-locks the VMA. That
waiter can then remain asleep indefinitely.

Call vma_start_write() before waking waiters. Existing fault handlers
queue themselves before dropping the VMA read lock, and new readers are
excluded until mmap_write_unlock().

Fixes: 29a22b9e08 ("mm: handle userfaults under VMA lock")
Cc: stable@vger.kernel.org
Reported-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Link: https://patch.msgid.link/20261031-fixes_core-v3-23-d6437171ecc3@kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
This commit is contained in:
David Hildenbrand (Arm) 2026-10-31 10:39:52 +01:00
parent 21a3888d3d
commit 35f7ecd55c
1 changed files with 3 additions and 0 deletions
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3
mm/userfaultfd.c
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@ -3991,6 +3991,9 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
start = vma->vm_start;
vma_end = min(end, vma->vm_end);
/* Prevent new userfaults from being queued after the wakeup. */
vma_start_write(vma);
if (userfaultfd_missing(vma)) {
/*
* Wake any concurrent pending userfault while