From: Jens Axboe <axboe@xxxxxxxxx> commit 607ec89ed18f49ca59689572659b9c0076f1991f upstream. IORING_OP_CLOSE is special in terms of cancelation, since it has an intermediate state where we've removed the file descriptor but hasn't closed the file yet. For that reason, it's currently marked with IO_WQ_WORK_NO_CANCEL to prevent cancelation. This ensures that the op is always run even if canceled, to prevent leaving us with a live file but an fd that is gone. However, with SQPOLL, since a cancel request doesn't carry any resources on behalf of the request being canceled, if we cancel before any of the close op has been run, we can end up with io-wq not having the ->files assigned. This can result in the following oops reported by Joseph: BUG: kernel NULL pointer dereference, address: 00000000000000d8 PGD 800000010b76f067 P4D 800000010b76f067 PUD 10b462067 PMD 0 Oops: 0000 [#1] SMP PTI CPU: 1 PID: 1788 Comm: io_uring-sq Not tainted 5.11.0-rc4 #1 Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011 RIP: 0010:__lock_acquire+0x19d/0x18c0 Code: 00 00 8b 1d fd 56 dd 08 85 db 0f 85 43 05 00 00 48 c7 c6 98 7b 95 82 48 c7 c7 57 96 93 82 e8 9a bc f5 ff 0f 0b e9 2b 05 00 00 <48> 81 3f c0 ca 67 8a b8 00 00 00 00 41 0f 45 c0 89 04 24 e9 81 fe RSP: 0018:ffffc90001933828 EFLAGS: 00010002 RAX: 0000000000000001 RBX: 0000000000000001 RCX: 0000000000000000 RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00000000000000d8 RBP: 0000000000000246 R08: 0000000000000001 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000 R13: 0000000000000000 R14: ffff888106e8a140 R15: 00000000000000d8 FS: 0000000000000000(0000) GS:ffff88813bd00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00000000000000d8 CR3: 0000000106efa004 CR4: 00000000003706e0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace: lock_acquire+0x31a/0x440 ? close_fd_get_file+0x39/0x160 ? __lock_acquire+0x647/0x18c0 _raw_spin_lock+0x2c/0x40 ? close_fd_get_file+0x39/0x160 close_fd_get_file+0x39/0x160 io_issue_sqe+0x1334/0x14e0 ? lock_acquire+0x31a/0x440 ? __io_free_req+0xcf/0x2e0 ? __io_free_req+0x175/0x2e0 ? find_held_lock+0x28/0xb0 ? io_wq_submit_work+0x7f/0x240 io_wq_submit_work+0x7f/0x240 io_wq_cancel_cb+0x161/0x580 ? io_wqe_wake_worker+0x114/0x360 ? io_uring_get_socket+0x40/0x40 io_async_find_and_cancel+0x3b/0x140 io_issue_sqe+0xbe1/0x14e0 ? __lock_acquire+0x647/0x18c0 ? __io_queue_sqe+0x10b/0x5f0 __io_queue_sqe+0x10b/0x5f0 ? io_req_prep+0xdb/0x1150 ? mark_held_locks+0x6d/0xb0 ? mark_held_locks+0x6d/0xb0 ? io_queue_sqe+0x235/0x4b0 io_queue_sqe+0x235/0x4b0 io_submit_sqes+0xd7e/0x12a0 ? _raw_spin_unlock_irq+0x24/0x30 ? io_sq_thread+0x3ae/0x940 io_sq_thread+0x207/0x940 ? do_wait_intr_irq+0xc0/0xc0 ? __ia32_sys_io_uring_enter+0x650/0x650 kthread+0x134/0x180 ? kthread_create_worker_on_cpu+0x90/0x90 ret_from_fork+0x1f/0x30 Fix this by moving the IO_WQ_WORK_NO_CANCEL until _after_ we've modified the fdtable. Canceling before this point is totally fine, and running it in the io-wq context _after_ that point is also fine. For 5.12, we'll handle this internally and get rid of the no-cancel flag, as IORING_OP_CLOSE is the only user of it. Cc: stable@xxxxxxxxxxxxxxx Fixes: b5dba59e0cf7 ("io_uring: add support for IORING_OP_CLOSE") Reported-by: "Abaci <abaci@xxxxxxxxxxxxxxxxx>" Reviewed-and-tested-by: Joseph Qi <joseph.qi@xxxxxxxxxxxxxxxxx> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- fs/io_uring.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) --- a/fs/io_uring.c +++ b/fs/io_uring.c @@ -4250,7 +4250,6 @@ static int io_close_prep(struct io_kiocb * io_wq_work.flags, so initialize io_wq_work firstly. */ io_req_init_async(req); - req->work.flags |= IO_WQ_WORK_NO_CANCEL; if (unlikely(req->ctx->flags & (IORING_SETUP_IOPOLL|IORING_SETUP_SQPOLL))) return -EINVAL; @@ -4283,6 +4282,8 @@ static int io_close(struct io_kiocb *req /* if the file has a flush method, be safe and punt to async */ if (close->put_file->f_op->flush && force_nonblock) { + /* not safe to cancel at this point */ + req->work.flags |= IO_WQ_WORK_NO_CANCEL; /* was never set, but play safe */ req->flags &= ~REQ_F_NOWAIT; /* avoid grabbing files - we don't need the files */