netdev - [Patch net] net_sched: fix datalen for ematch

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Date:   Wed, 22 Jan 2020 15:42:02 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     netdev@...r.kernel.org
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        syzbot+5af9a90dad568aa9f611@...kaller.appspotmail.com,
        syzbot+2f07903a5b05e7f36410@...kaller.appspotmail.com,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: [Patch net] net_sched: fix datalen for ematch

syzbot reported an out-of-bound access in em_nbyte. As initially
analyzed by Eric, this is because em_nbyte sets its own em->datalen
in em_nbyte_change() other than the one specified by user, but this
value gets overwritten later by its caller tcf_em_validate().
We should leave em->datalen untouched to respect their choices.

I audit all the in-tree ematch users, all of those implement
->change() set em->datalen, so we can just avoid setting it twice
in this case.

Reported-and-tested-by: syzbot+5af9a90dad568aa9f611@...kaller.appspotmail.com
Reported-by: syzbot+2f07903a5b05e7f36410@...kaller.appspotmail.com
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: Eric Dumazet <eric.dumazet@...il.com>
Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
---
 net/sched/ematch.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sched/ematch.c b/net/sched/ematch.c
index 8f2ad706784d..d0140a92694a 100644
--- a/net/sched/ematch.c
+++ b/net/sched/ematch.c
@@ -263,12 +263,12 @@ static int tcf_em_validate(struct tcf_proto *tp,
 				}
 				em->data = (unsigned long) v;
 			}
+			em->datalen = data_len;
 		}
 	}
 
 	em->matchid = em_hdr->matchid;
 	em->flags = em_hdr->flags;
-	em->datalen = data_len;
 	em->net = net;
 
 	err = 0;
-- 
2.21.1