Re: [syzbot] [kernel?] general protection fault in try_to_wake_up (3)

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

+Jens

On 2025-09-02 23:46:28 [+0200], Peter Zijlstra wrote:
> When I build the provided .config with clang-20, that a58 offset is
> exactly task_struct::pi_lock::lockdep_map, which nicely corresponds with
> the below stacktrace, and seems to suggest someone did:
> try_to_wake_up(NULL).

correct.

> >  try_to_wake_up+0x67/0x12b0 kernel/sched/core.c:4216
> >  requeue_pi_wake_futex+0x24b/0x2f0 kernel/futex/requeue.c:249
> 
> Trouble is, we've not changed the requeue bits in a fair while... So I'm
> somewhat confused on how this happens now ?!

This means syzkaller managed to invoke futex_wait_setup(…, NULL) in
order to get futex_q::task assigned to NULL. All users use current
except for io_futex_wait().

The syz-reproducer lists only:
| timer_create(0x0, &(0x7f0000000080)={0x0, 0x11, 0x0, @thr={0x0, 0x0}}, &(0x7f0000000000))
| timer_settime(0x0, 0x0, &(0x7f0000000240)={{0x0, 0x8}, {0x0, 0x9}}, 0x0)
| futex(&(0x7f000000cffc), 0x80000000000b, 0x0, 0x0, &(0x7f0000048000), 0x0)
| futex(&(0x7f000000cffc), 0xc, 0x1, 0x0, &(0x7f0000048000), 0x0)

and that is probably why it can't come up with C-reproducer.
The whole log has (filtered) the following lines:

| io_uring_setup(0x85a, &(0x7f0000000180)={0x0, 0x58b9, 0x1, 0x2, 0x383})
| syz_io_uring_setup(0x88f, &(0x7f0000000300)={0x0, 0xaedf, 0x0, 0x0, 0x25d}, &(0x7f0000000140)=<r0=>0x0, &(0x7f0000000280)=<r1=>0x0)
| syz_memcpy_off$IO_URING_METADATA_GENERIC(r0, 0x4, &(0x7f0000000080)=0xfffffffc, 0x0, 0x4)
| syz_io_uring_submit(r0, r1, &(0x7f00000001c0)=@IORING_OP_RECVMSG={0xa, 0x8, 0x1, r2, 0x0, &(0x7f0000000440)={0x0, 0x0, 0x0}, 0x0, 0x40000020, 0x1, {0x2}})

This should explain the how the waiter got NULL. There is no private
flag so that is how they interact with each other.
Do we want this:

diff --git a/kernel/futex/requeue.c b/kernel/futex/requeue.c
index c716a66f86929..0c98256ebdcb7 100644
--- a/kernel/futex/requeue.c
+++ b/kernel/futex/requeue.c
@@ -312,6 +312,8 @@ futex_proxy_trylock_atomic(u32 __user *pifutex, struct futex_hash_bucket *hb1,
 	if (!top_waiter->rt_waiter || top_waiter->pi_state)
 		return -EINVAL;
 
+	if (!top_waiter->task)
+		-EINVAL;
 	/* Ensure we requeue to the expected futex. */
 	if (!futex_match(top_waiter->requeue_pi_key, key2))
 		return -EINVAL;

?

Sebastian