[PATCH] io_uring: Fix a null-ptr-deref in io_tctx_exit_cb()

[Harshit Mogalapalli]

Syzkaller reports a NULL deref bug as follows:

 BUG: KASAN: null-ptr-deref in io_tctx_exit_cb+0x53/0xd3
 Read of size 4 at addr 0000000000000138 by task file1/1955

 CPU: 1 PID: 1955 Comm: file1 Not tainted 6.1.0-rc7-00103-gef4d3ea40565 #75
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2.el7 04/01/2014
 Call Trace:
  <TASK>
  dump_stack_lvl+0xcd/0x134
  ? io_tctx_exit_cb+0x53/0xd3
  kasan_report+0xbb/0x1f0
  ? io_tctx_exit_cb+0x53/0xd3
  kasan_check_range+0x140/0x190
  io_tctx_exit_cb+0x53/0xd3
  task_work_run+0x164/0x250
  ? task_work_cancel+0x30/0x30
  get_signal+0x1c3/0x2440
  ? lock_downgrade+0x6e0/0x6e0
  ? lock_downgrade+0x6e0/0x6e0
  ? exit_signals+0x8b0/0x8b0
  ? do_raw_read_unlock+0x3b/0x70
  ? do_raw_spin_unlock+0x50/0x230
  arch_do_signal_or_restart+0x82/0x2470
  ? kmem_cache_free+0x260/0x4b0
  ? putname+0xfe/0x140
  ? get_sigframe_size+0x10/0x10
  ? do_execveat_common.isra.0+0x226/0x710
  ? lockdep_hardirqs_on+0x79/0x100
  ? putname+0xfe/0x140
  ? do_execveat_common.isra.0+0x238/0x710
  exit_to_user_mode_prepare+0x15f/0x250
  syscall_exit_to_user_mode+0x19/0x50
  do_syscall_64+0x42/0xb0
  entry_SYSCALL_64_after_hwframe+0x63/0xcd
 RIP: 0023:0x0
 Code: Unable to access opcode bytes at 0xffffffffffffffd6.
 RSP: 002b:00000000fffb7790 EFLAGS: 00000200 ORIG_RAX: 000000000000000b
 RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
 RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
 RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
 R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
 R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
  </TASK>
 Kernel panic - not syncing: panic_on_warn set ...

Add a NULL check on tctx to prevent this.

Fixes: d56d938b4bef ("io_uring: do ctx initiated file note removal")
Reported-by: syzkaller <syzkaller@xxxxxxxxxxxxxxxx>
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@xxxxxxxxxx>
---
Could not find the root cause of this.
---
 io_uring/io_uring.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 8840cf3e20f2..20f7d8655b50 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -2708,7 +2708,7 @@ static __cold void io_tctx_exit_cb(struct callback_head *cb)
 	 * When @in_idle, we're in cancellation and it's racy to remove the
 	 * node. It'll be removed by the end of cancellation, just ignore it.
 	 */
-	if (!atomic_read(&tctx->in_idle))
+	if (tctx && !atomic_read(&tctx->in_idle))
 		io_uring_del_tctx_node((unsigned long)work->ctx);
 	complete(&work->completion);
 }
-- 
2.38.1