Affected: Linux

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

tcp: Add preempt_{disable,enable}_nested() in reqsk_queue_hash_req().

syzbot reported a weird reqsk->rsk_refcnt underflow in __inet_csk_reqsk_queue_drop().

The captured reqsk_put() in __inet_csk_reqsk_queue_drop() is called only when it successfully removes reqsk from ehash.

Moreover, reqsk_timer_handler() calls another reqsk_put() after that.

This indicates that the reqsk was missing both refcnts for ehash and the timer itself.

Since all the syzbot reports had PREEMPT_RT enabled, the only possible scenario is that reqsk_queue_hash_req() is preempted after mod_timer() and before refcount_set(), and then the timer triggered after 1s aborts the reqsk due to its listener's close().

Let's wrap mod_timer() and refcount_set() with preempt_disable_nested() and preempt_enable_nested().

Note that inet_ehash_insert() holds the normal spin_lock() (mutex in PREEMPT_RT), so it must be called outside of preempt_disable_nested(), but this is fine.

The lookup path just ignores 0 sk_refcnt entries in ehash and tries to create another reqsk, but this will fail at inet_ehash_insert().

[0]: refcount_t: underflow; use-after-free. WARNING: lib/refcount.c:28 at refcount_warn_saturate+0xb2/0x110 lib/refcount.c:28, CPU#0: ktimers/0/16 Modules linked in: CPU: 0 UID: 0 PID: 16 Comm: ktimers/0 Tainted: G L syzkaller #0 PREEMPT_{RT,(full)} Tainted: [L]=SOFTLOCKUP Hardware name: Google Google Compute Engine/Goo…