io_uring/sqpoll: zero sqd->thread on tctx errors

JIRA: https://issues.redhat.com/browse/RHEL-74645
CVE: CVE-2025-21633

commit 4b7cfa8
Author: Pavel Begunkov <asml.silence@gmail.com>
Date:   Fri Jan 10 14:31:23 2025 +0000

    io_uring/sqpoll: zero sqd->thread on tctx errors
    
    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, which can essentially only happen with
    fault injection to insert allocation errors.
    
    Cc: stable@vger.kernel.org
    Fixes: 1251d20 ("io_uring/sqpoll: early exit thread if task_context wasn't allocated")
    Reported-by: syzbot+3d92cfcfa84070b0a470@syzkaller.appspotmail.com
    Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
    Link: https://lore.kernel.org/r/efc7ec7010784463b2e7466d7b5c02c2cb381635.1736519461.git.asml.silence@gmail.com
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>

Author: JeffMoyer

io_uring/sqpoll.c

@@ -278,8 +278,12 @@ 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);