futex: Prevent use-after-free during requeue-PI

Overview

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

futex: Prevent use-after-free during requeue-PI

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.

Affected

• Linux >= 07d91ef510fb16a2e0ca7453222105835b7ba3b8 < cb5d19a61274b51b49601214a87af573b43d60fa
• Linux >= 07d91ef510fb16a2e0ca7453222105835b7ba3b8 < 348736955ed6ca6e99ca24b93b1d3fbfe352c181
• Linux >= 07d91ef510fb16a2e0ca7453222105835b7ba3b8 < a170b9c0dde83312b8b58ccc91509c7c15711641
• Linux >= 07d91ef510fb16a2e0ca7453222105835b7ba3b8 < d824b2dbdcfe3c390278dd9652ea526168ef6850
• Linux >= 07d91ef510fb16a2e0ca7453222105835b7ba3b8 < b549113738e8c751b613118032a724b772aa83f2
• Linux 5.15

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.