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348736955e
[ Upstream commit b549113738e8c751b613118032a724b772aa83f2 ]
syzbot managed to trigger the following race:
T1 T2
futex_wait_requeue_pi()
futex_do_wait()
schedule()
futex_requeue()
futex_proxy_trylock_atomic()
futex_requeue_pi_prepare()
requeue_pi_wake_futex()
futex_requeue_pi_complete()
/* preempt */
* timeout/ signal wakes T1 *
futex_requeue_pi_wakeup_sync() // Q_REQUEUE_PI_LOCKED
futex_hash_put()
// back to userland, on stack futex_q is garbage
/* back */
wake_up_state(q->task, TASK_NORMAL);
In this scenario futex_wait_requeue_pi() is able to leave without using
futex_q::lock_ptr for synchronization.
This can be prevented by reading futex_q::task before updating the
futex_q::requeue_state. A reference on the task_struct is not needed
because requeue_pi_wake_futex() is invoked with a spinlock_t held which
implies a RCU read section.
Even if T1 terminates immediately after, the task_struct will remain valid
during T2's wake_up_state(). A READ_ONCE on futex_q::task before
futex_requeue_pi_complete() is enough because it ensures that the variable
is read before the state is updated.
Read futex_q::task before updating the requeue state, use it for the
following wakeup.
Fixes:
|
||
|---|---|---|
| .. | ||
| core.c | ||
| futex.h | ||
| Makefile | ||
| pi.c | ||
| requeue.c | ||
| syscalls.c | ||
| waitwake.c | ||