[PATCH backport 5.10] io_uring: close a small race gap for files cancel

[PATCH backport 5.10] io_uring: close a small race gap for files cancel

[Pavel Begunkov]

commit dfea9fce29fda6f2f91161677e0e0d9b671bc099 upstream.

The purpose of io_uring_cancel_files() is to wait for all requests
matching ->files to go/be cancelled. We should first drop files of a
request in io_req_drop_files() and only then make it undiscoverable for
io_uring_cancel_files.

First drop, then delete from list. It's ok to leave req->id->files
dangling, because it's not dereferenced by cancellation code, only
compared against. It would potentially go to sleep and be awaken by
following in io_req_drop_files() wake_up().

Fixes: 0f2122045b946 ("io_uring: don't rely on weak ->files references")
Cc: <stable@vger.kernel.org> # 5.5+
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/io_uring.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 86dac2b2e276..dfe33b0b148f 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -5854,15 +5854,15 @@ static void io_req_drop_files(struct io_kiocb *req)
 	struct io_ring_ctx *ctx = req->ctx;
 	unsigned long flags;
 
+	put_files_struct(req->work.identity->files);
+	put_nsproxy(req->work.identity->nsproxy);
 	spin_lock_irqsave(&ctx->inflight_lock, flags);
 	list_del(&req->inflight_entry);
-	if (waitqueue_active(&ctx->inflight_wait))
-		wake_up(&ctx->inflight_wait);
 	spin_unlock_irqrestore(&ctx->inflight_lock, flags);
 	req->flags &= ~REQ_F_INFLIGHT;
-	put_files_struct(req->work.identity->files);
-	put_nsproxy(req->work.identity->nsproxy);
 	req->work.flags &= ~IO_WQ_WORK_FILES;
+	if (waitqueue_active(&ctx->inflight_wait))
+		wake_up(&ctx->inflight_wait);
 }
 
 static void __io_clean_op(struct io_kiocb *req)
-- 
2.24.0

Re: [PATCH backport 5.10] io_uring: close a small race gap for files cancel

[Greg KH]

On Tue, Dec 29, 2020 at 04:35:49AM +0000, Pavel Begunkov wrote:
> commit dfea9fce29fda6f2f91161677e0e0d9b671bc099 upstream.
> 
> The purpose of io_uring_cancel_files() is to wait for all requests
> matching ->files to go/be cancelled. We should first drop files of a
> request in io_req_drop_files() and only then make it undiscoverable for
> io_uring_cancel_files.
> 
> First drop, then delete from list. It's ok to leave req->id->files
> dangling, because it's not dereferenced by cancellation code, only
> compared against. It would potentially go to sleep and be awaken by
> following in io_req_drop_files() wake_up().
> 
> Fixes: 0f2122045b946 ("io_uring: don't rely on weak ->files references")
> Cc: <stable@vger.kernel.org> # 5.5+
> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
>  fs/io_uring.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)

Now applied, thanks.

greg k-h