netdev - [PATCH] 9p/trans_fd: always use O

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Message-ID: <345de429-a88b-7097-d177-adecf9fed342@I-love.SAKURA.ne.jp>
Date:   Sat, 27 Aug 2022 00:27:46 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Eric Van Hensbergen <ericvh@...il.com>,
        Latchesar Ionkov <lucho@...kov.net>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Christian Schoenebeck <linux_oss@...debyte.com>
Cc:     syzbot <syzbot+8b41a1365f1106fd0f33@...kaller.appspotmail.com>,
        v9fs-developer@...ts.sourceforge.net,
        syzkaller-bugs@...glegroups.com, netdev@...r.kernel.org
Subject: [PATCH] 9p/trans_fd: always use O_NONBLOCK read/write

syzbot is reporting hung task at p9_fd_close() [1], for p9_mux_poll_stop()
 from p9_conn_destroy() from p9_fd_close() is failing to interrupt already
started kernel_read() from p9_fd_read() from p9_read_work() and/or
kernel_write() from p9_fd_write() from p9_write_work() requests.

Since p9_socket_open() sets O_NONBLOCK flag, p9_mux_poll_stop() does not
need to interrupt kernel_read()/kernel_write(). However, since p9_fd_open()
does not set O_NONBLOCK flag, but pipe blocks unless signal is pending,
p9_mux_poll_stop() needs to interrupt kernel_read()/kernel_write() when
the file descriptor refers to a pipe. In other words, pipe file descriptor
needs to be handled as if socket file descriptor.

We somehow need to interrupt kernel_read()/kernel_write() on pipes.

A minimal change, which this patch is doing, is to set O_NONBLOCK flag
 from p9_fd_open(), for O_NONBLOCK flag does not affect reading/writing
of regular files. But this approach changes O_NONBLOCK flag on userspace-
supplied file descriptors (which might break userspace programs), and
O_NONBLOCK flag could be changed by userspace. It would be possible to set
O_NONBLOCK flag every time p9_fd_read()/p9_fd_write() is invoked, but still
remains small race window for clearing O_NONBLOCK flag.

If we don't want to manipulate O_NONBLOCK flag, we might be able to
surround kernel_read()/kernel_write() with set_thread_flag(TIF_SIGPENDING)
and recalc_sigpending(). Since p9_read_work()/p9_write_work() works are
processed by kernel threads which process global system_wq workqueue,
signals could not be delivered from remote threads when p9_mux_poll_stop()
 from p9_conn_destroy() from p9_fd_close() is called. Therefore, calling
set_thread_flag(TIF_SIGPENDING)/recalc_sigpending() every time would be
needed if we count on signals for making kernel_read()/kernel_write()
non-blocking.

Link: https://syzkaller.appspot.com/bug?extid=8b41a1365f1106fd0f33 [1]
Reported-by: syzbot <syzbot+8b41a1365f1106fd0f33@...kaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Tested-by: syzbot <syzbot+8b41a1365f1106fd0f33@...kaller.appspotmail.com>
---
Although syzbot tested that this patch solves hung task problem, syzbot
cannot verify that this patch will not break functionality of p9 users.
Please test before applying this patch.

 net/9p/trans_fd.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/9p/trans_fd.c b/net/9p/trans_fd.c
index e758978b44be..9870597da583 100644
--- a/net/9p/trans_fd.c
+++ b/net/9p/trans_fd.c
@@ -821,11 +821,13 @@ static int p9_fd_open(struct p9_client *client, int rfd, int wfd)
 		goto out_free_ts;
 	if (!(ts->rd->f_mode & FMODE_READ))
 		goto out_put_rd;
+	ts->rd->f_flags |= O_NONBLOCK;
 	ts->wr = fget(wfd);
 	if (!ts->wr)
 		goto out_put_rd;
 	if (!(ts->wr->f_mode & FMODE_WRITE))
 		goto out_put_wr;
+	ts->wr->f_flags |= O_NONBLOCK;

 	client->trans = ts;
 	client->status = Connected;
-- 
2.18.4