792bb6eb86233 ("io_uring: don't take uring_lock during iowq cancel")
inadvertently fixed this issue in v5.12.  This patch cherry-picks the
hunk of commit which does so.

Syzbot vomit (snipped):

  ==================================================================
  BUG: KASAN: double-free or invalid-free in slab_free mm/slub.c:3195 [inline]
  BUG: KASAN: double-free or invalid-free in kfree+0xca/0x310 mm/slub.c:4183

  CPU: 1 PID: 431 Comm: syz-executor823 Not tainted 5.10.75-syzkaller-01082-g234d53d2bb60 #0
  Hardware name: Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
  Call Trace:
   __dump_stack lib/dump_stack.c:77 [inline]
   dump_stack_lvl+0x1e2/0x24b lib/dump_stack.c:118
   print_address_description+0x8d/0x3d0 mm/kasan/report.c:233
   kasan_report_invalid_free+0x58/0x130 mm/kasan/report.c:358
   ____kasan_slab_free+0x14b/0x170 mm/kasan/common.c:362
   __kasan_slab_free+0x11/0x20 mm/kasan/common.c:368
   kasan_slab_free include/linux/kasan.h:235 [inline]
   slab_free_hook mm/slub.c:1596 [inline]
   slab_free_freelist_hook+0xb2/0x180 mm/slub.c:1621
   slab_free mm/slub.c:3195 [inline]
   kfree+0xca/0x310 mm/slub.c:4183
   __io_queue_deferred fs/io_uring.c:1541 [inline]
   io_commit_cqring+0x76a/0xa00 fs/io_uring.c:1587
   io_iopoll_complete fs/io_uring.c:2378 [inline]
   io_do_iopoll+0x1e18/0x23f0 fs/io_uring.c:2431
   io_iopoll_try_reap_events+0x116/0x290 fs/io_uring.c:2470
   io_ring_ctx_wait_and_kill+0x295/0x670 fs/io_uring.c:8575
   io_uring_release+0x5b/0x70 fs/io_uring.c:8602
   __fput+0x348/0x7d0 fs/file_table.c:281
   ____fput+0x15/0x20 fs/file_table.c:314
   task_work_run+0x147/0x1b0 kernel/task_work.c:154
   exit_task_work include/linux/task_work.h:30 [inline]
   do_exit+0x70e/0x23a0 kernel/exit.c:813
   do_group_exit+0x16a/0x2d0 kernel/exit.c:910
   __do_sys_exit_group+0x17/0x20 kernel/exit.c:921
   __se_sys_exit_group+0x14/0x20 kernel/exit.c:919
   __x64_sys_exit_group+0x3b/0x40 kernel/exit.c:919
   do_syscall_64+0x31/0x70 arch/x86/entry/common.c:46
   entry_SYSCALL_64_after_hwframe+0x44/0xa9

Cc: stable <redacted> # 5.10.x
Reported-by: syzbot+59d8a1f4e60c20c066cf@syzkaller.appspotmail.com
Signed-off-by: Lee Jones <redacted>
---
 fs/io_uring.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 26753d0cb4312..361f8ae96c36f 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c

@@ -2075,7 +2075,9 @@ static void io_req_task_cancel(struct callback_head *cb)
 	struct io_kiocb *req = container_of(cb, struct io_kiocb, task_work);
 	struct io_ring_ctx *ctx = req->ctx;
 
+	mutex_lock(&ctx->uring_lock);
 	__io_req_task_cancel(req, -ECANCELED);
+	mutex_unlock(&ctx->uring_lock);
 	percpu_ref_put(&ctx->refs);
 }
 

-- 
2.33.0.1079.g6e70778dc9-goog