[V9fs-developer] [PATCH] p9_check_errors() validate PDU length

[Tomas Bortoli <tomasbortoli@gmail.com>]

p9_check_errors() does not validate the size of the PDU read
in p9_parse_header(). Any size can be passed, provoking out-of-bound reads.

Signed-off-by: Tomas Bortoli <tomasbortoli@gmail.com>
Reported-by: syzbot+65c6b72f284a39d416b4@syzkaller.appspotmail.com
---
As suggested by Dominique:
https://lkml.org/lkml/2018/7/9/688
Such check is not enough as it will prevent to read more than how it has
been allocated but it won't prevent to read more than how it has been read
So this patch will require some more changes to prevent bad sizes.

 net/9p/client.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/9p/client.c b/net/9p/client.c
index 40f7c47f2f74..5b161b576b8a 100644
--- a/net/9p/client.c
+++ b/net/9p/client.c
@@ -520,10 +520,13 @@ EXPORT_SYMBOL(p9_parse_header);
 static int p9_check_errors(struct p9_client *c, struct p9_req_t *req)
 {
 	int8_t type;
+	int32_t size;
 	int err;
 	int ecode;
 
-	err = p9_parse_header(req->rc, NULL, &type, NULL, 0);
+	err = p9_parse_header(req->rc, &size, &type, NULL, 0);
+	if (size > req->rc->capacity)
+		return -EINVAL;
 	/*
 	 * dump the response from server
 	 * This should be after check errors which poplulate pdu_fcall.
-- 
2.11.0