[PATCH kernel] 9p/trans_fd: Check file mode at opening

[Alexey Kardashevskiy]

The "fd" transport layer uses 2 file descriptors passed externally
and calls kernel_write()/kernel_read() on these. If files were opened
without FMODE_WRITE/FMODE_READ, WARN_ON_ONCE() will fire.

This adds file mode checking in p9_fd_open; this returns -EBADF to
preserve the original behavior.

Found by syzkaller.

Signed-off-by: Alexey Kardashevskiy <aik@xxxxxxxxx>
---
 net/9p/trans_fd.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/9p/trans_fd.c b/net/9p/trans_fd.c
index 13cd683a658a..62cdfbd01f0a 100644
--- a/net/9p/trans_fd.c
+++ b/net/9p/trans_fd.c
@@ -797,6 +797,7 @@ static int parse_opts(char *params, struct p9_fd_opts *opts)
 
 static int p9_fd_open(struct p9_client *client, int rfd, int wfd)
 {
+	bool perm;
 	struct p9_trans_fd *ts = kzalloc(sizeof(struct p9_trans_fd),
 					   GFP_KERNEL);
 	if (!ts)
@@ -804,12 +805,16 @@ static int p9_fd_open(struct p9_client *client, int rfd, int wfd)
 
 	ts->rd = fget(rfd);
 	ts->wr = fget(wfd);
-	if (!ts->rd || !ts->wr) {
+	perm = ts->rd && (ts->rd->f_mode & FMODE_READ) &&
+	       ts->wr && (ts->wr->f_mode & FMODE_WRITE);
+	if (!ts->rd || !ts->wr || !perm) {
 		if (ts->rd)
 			fput(ts->rd);
 		if (ts->wr)
 			fput(ts->wr);
 		kfree(ts);
+		if (!perm)
+			return -EBADF;
 		return -EIO;
 	}
 
-- 
2.17.1