linux-kernel - [PATCH AUTOSEL 5.5 41/41] io_uring: fix lockup with timeouts

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Date:   Sun, 15 Mar 2020 22:33:19 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Pavel Begunkov <asml.silence@...il.com>,
        Jens Axboe <axboe@...nel.dk>, Sasha Levin <sashal@...nel.org>,
        linux-fsdevel@...r.kernel.org, io-uring@...r.kernel.org
Subject: [PATCH AUTOSEL 5.5 41/41] io_uring: fix lockup with timeouts

From: Pavel Begunkov <asml.silence@...il.com>

[ Upstream commit f0e20b8943509d81200cef5e30af2adfddba0f5c ]

There is a recipe to deadlock the kernel: submit a timeout sqe with a
linked_timeout (e.g.  test_single_link_timeout_ception() from liburing),
and SIGKILL the process.

Then, io_kill_timeouts() takes @ctx->completion_lock, but the timeout
isn't flagged with REQ_F_COMP_LOCKED, and will try to double grab it
during io_put_free() to cancel the linked timeout. Probably, the same
can happen with another io_kill_timeout() call site, that is
io_commit_cqring().

Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 fs/io_uring.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 60a4832089982..fd28f85677225 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -688,6 +688,7 @@ static void io_kill_timeout(struct io_kiocb *req)
 	if (ret != -1) {
 		atomic_inc(&req->ctx->cq_timeouts);
 		list_del_init(&req->list);
+		req->flags |= REQ_F_COMP_LOCKED;
 		io_cqring_fill_event(req, 0);
 		io_put_req(req);
 	}
-- 
2.20.1