In the Linux kernel, the following vulnerability has been resolved:

xfrm: iptfs: fix ABBA deadlock in iptfs_destroy_state()

iptfs_destroy_state() calls hrtimer_cancel() while holding a spinlock
that the timer callback also acquires, leading to an ABBA deadlock on
SMP systems.

For the output timer (iptfs_timer):
  - iptfs_destroy_state() holds x->lock, calls hrtimer_cancel()
  - iptfs_delay_timer() callback takes x->lock

For the drop timer (drop_timer):
  - iptfs_destroy_state() holds drop_lock, calls hrtimer_cancel()
  - iptfs_drop_timer() callback takes drop_lock

Both timers use HRTIMER_MODE_REL_SOFT, so their callbacks run in softirq
context.  When hrtimer_cancel() is called for a soft timer that is
currently executing on another CPU, hrtimer_cancel_wait_running() spins
on softirq_expiry_lock -- the same lock held by the softirq running the
callback.  If the callback is blocked waiting for the spinlock held by
the caller of hrtimer_cancel(), a circular dependency forms:

  CPU 0: holds lock_A -> waits for softirq_expiry_lock
  CPU 1: holds softirq_expiry_lock -> waits for lock_A

Fix by calling hrtimer_cancel() before acquiring the respective locks.
hrtimer_cancel() is safe to call without holding any lock and will wait
for any in-progress callback to complete.  For the output timer, the
lock is still acquired afterwards to drain the packet queue.  For the
drop timer, the lock/unlock pair is removed entirely since it only
existed to serialize with the timer callback, which hrtimer_cancel()
already guarantees.

Found by source code audit.