[PATCH 1/1] io_uring: sqpoll: zero sqd->thread on tctx errors

[Pavel Begunkov <asml.silence@xxxxxxxxx>]

Syzkeller reports:

BUG: KASAN: slab-use-after-free in thread_group_cputime+0x409/0x700 kernel/sched/cputime.c:341
Read of size 8 at addr ffff88803578c510 by task syz.2.3223/27552
 Call Trace:
  <TASK>
  ...
  kasan_report+0x143/0x180 mm/kasan/report.c:602
  thread_group_cputime+0x409/0x700 kernel/sched/cputime.c:341
  thread_group_cputime_adjusted+0xa6/0x340 kernel/sched/cputime.c:639
  getrusage+0x1000/0x1340 kernel/sys.c:1863
  io_uring_show_fdinfo+0xdfe/0x1770 io_uring/fdinfo.c:197
  seq_show+0x608/0x770 fs/proc/fd.c:68
  ...

That's due to sqd->task not being cleared properly in cases where
SQPOLL task tctx setup fails.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 1251d2025c3e1 ("io_uring/sqpoll: early exit thread if task_context wasn't allocated")
Reported-by: syzbot+3d92cfcfa84070b0a470@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
---
 io_uring/sqpoll.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/io_uring/sqpoll.c b/io_uring/sqpoll.c
index 6df5e649c413..5768e31e99b1 100644
--- a/io_uring/sqpoll.c
+++ b/io_uring/sqpoll.c
@@ -268,8 +268,12 @@ static int io_sq_thread(void *data)
 	DEFINE_WAIT(wait);
 
 	/* offload context creation failed, just exit */
-	if (!current->io_uring)
+	if (!current->io_uring) {
+		mutex_lock(&sqd->lock);
+		sqd->thread = NULL;
+		mutex_unlock(&sqd->lock);
 		goto err_out;
+	}
 
 	snprintf(buf, sizeof(buf), "iou-sqp-%d", sqd->task_pid);
 	set_task_comm(current, buf);
-- 
2.47.1