[PATCH v2] 9p/trans_fd: perform read/write with TIF_SIGPENDING set

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

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,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,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,write}() on pipes.

If we can tolerate "possibility of breaking userspace program by setting
O_NONBLOCK flag on userspace-supplied file descriptors" and "possibility
of race window that userspace program clears O_NONBLOCK flag between after
automatically setting O_NONBLOCK flag and before calling
kernel_{read,write}()", we could automatically set O_NONBLOCK flag
immediately before calling kernel_{read,write}().

A different approach, which this patch is doing, is to surround
kernel_{read,write}() with set_thread_flag(TIF_SIGPENDING) and
recalc_sigpending(). This might be ugly and bit costly, but does not
touch userspace-supplied file descriptors.

Link: https://syzkaller.appspot.com/bug?extid=8b41a1365f1106fd0f33 [1]
Reported-by: syzbot <syzbot+8b41a1365f1106fd0f33@syzkaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Tested-by: syzbot <syzbot+8b41a1365f1106fd0f33@syzkaller.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 | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/net/9p/trans_fd.c b/net/9p/trans_fd.c
index e758978b44be..e2f4e3245a80 100644
--- a/net/9p/trans_fd.c
+++ b/net/9p/trans_fd.c
@@ -256,11 +256,13 @@ static int p9_fd_read(struct p9_client *client, void *v, int len)
 	if (!ts)
 		return -EREMOTEIO;
 
-	if (!(ts->rd->f_flags & O_NONBLOCK))
-		p9_debug(P9_DEBUG_ERROR, "blocking read ...\n");
-
 	pos = ts->rd->f_pos;
+	/* Force non-blocking read() even without O_NONBLOCK. */
+	set_thread_flag(TIF_SIGPENDING);
 	ret = kernel_read(ts->rd, v, len, &pos);
+	spin_lock_irq(&current->sighand->siglock);
+	recalc_sigpending();
+	spin_unlock_irq(&current->sighand->siglock);
 	if (ret <= 0 && ret != -ERESTARTSYS && ret != -EAGAIN)
 		client->status = Disconnected;
 	return ret;
@@ -423,10 +425,12 @@ static int p9_fd_write(struct p9_client *client, void *v, int len)
 	if (!ts)
 		return -EREMOTEIO;
 
-	if (!(ts->wr->f_flags & O_NONBLOCK))
-		p9_debug(P9_DEBUG_ERROR, "blocking write ...\n");
-
+	/* Force non-blocking write() even without O_NONBLOCK. */
+	set_thread_flag(TIF_SIGPENDING);
 	ret = kernel_write(ts->wr, v, len, &ts->wr->f_pos);
+	spin_lock_irq(&current->sighand->siglock);
+	recalc_sigpending();
+	spin_unlock_irq(&current->sighand->siglock);
 	if (ret <= 0 && ret != -ERESTARTSYS && ret != -EAGAIN)
 		client->status = Disconnected;
 	return ret;
-- 
2.18.4