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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Pavel Begunkov <asml.silence@gmail.com>,
        Jens Axboe <axboe@kernel.dk>
Subject: [PATCH 5.13 192/223] io_uring: explicitly count entries for poll reqs
Date:   Mon, 26 Jul 2021 17:39:44 +0200
Message-Id: <20210726153852.478104432@linuxfoundation.org>
In-Reply-To: <20210726153846.245305071@linuxfoundation.org>
References: <20210726153846.245305071@linuxfoundation.org>
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From: Pavel Begunkov <asml.silence@gmail.com>

commit 68b11e8b1562986c134764433af64e97d30c9fc0 upstream.

If __io_queue_proc() fails to add a second poll entry, e.g. kmalloc()
failed, but it goes on with a third waitqueue, it may succeed and
overwrite the error status. Count the number of poll entries we added,
so we can set pt->error to zero at the beginning and find out when the
mentioned scenario happens.

Cc: stable@vger.kernel.org
Fixes: 18bceab101add ("io_uring: allow POLL_ADD with double poll_wait() users")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/9d6b9e561f88bcc0163623b74a76c39f712151c3.1626774457.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 fs/io_uring.c |   16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -4805,6 +4805,7 @@ IO_NETOP_FN(recv);
 struct io_poll_table {
 	struct poll_table_struct pt;
 	struct io_kiocb *req;
+	int nr_entries;
 	int error;
 };
 
@@ -5002,11 +5003,11 @@ static void __io_queue_proc(struct io_po
 	struct io_kiocb *req = pt->req;
 
 	/*
-	 * If poll->head is already set, it's because the file being polled
-	 * uses multiple waitqueues for poll handling (eg one for read, one
-	 * for write). Setup a separate io_poll_iocb if this happens.
+	 * The file being polled uses multiple waitqueues for poll handling
+	 * (e.g. one for read, one for write). Setup a separate io_poll_iocb
+	 * if this happens.
 	 */
-	if (unlikely(poll->head)) {
+	if (unlikely(pt->nr_entries)) {
 		struct io_poll_iocb *poll_one = poll;
 
 		/* already have a 2nd entry, fail a third attempt */
@@ -5034,7 +5035,7 @@ static void __io_queue_proc(struct io_po
 		*poll_ptr = poll;
 	}
 
-	pt->error = 0;
+	pt->nr_entries++;
 	poll->head = head;
 
 	if (poll->events & EPOLLEXCLUSIVE)
@@ -5112,9 +5113,12 @@ static __poll_t __io_arm_poll_handler(st
 
 	ipt->pt._key = mask;
 	ipt->req = req;
-	ipt->error = -EINVAL;
+	ipt->error = 0;
+	ipt->nr_entries = 0;
 
 	mask = vfs_poll(req->file, &ipt->pt) & poll->events;
+	if (unlikely(!ipt->nr_entries) && !ipt->error)
+		ipt->error = -EINVAL;
 
 	spin_lock_irq(&ctx->completion_lock);
 	if (likely(poll->head)) {