linux-kernel - [PATCH 4.15 048/122] net: sched: report if filter is too large to dump

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Message-Id: <20180307191735.949703125@linuxfoundation.org>
Date:   Wed,  7 Mar 2018 11:37:40 -0800
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Roman Kapl <code@...pl.cz>,
        Jiri Pirko <jiri@...lanox.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: [PATCH 4.15 048/122] net: sched: report if filter is too large to dump

4.15-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Roman Kapl <code@...pl.cz>

[ Upstream commit 5ae437ad5a2ed573b1ebb04e0afa70b8869f88dd ]

So far, if the filter was too large to fit in the allocated skb, the
kernel did not return any error and stopped dumping. Modify the dumper
so that it returns -EMSGSIZE when a filter fails to dump and it is the
first filter in the skb. If we are not first, we will get a next chance
with more room.

I understand this is pretty near to being an API change, but the
original design (silent truncation) can be considered a bug.

Note: The error case can happen pretty easily if you create a filter
with 32 actions and have 4kb pages. Also recent versions of iproute try
to be clever with their buffer allocation size, which in turn leads to

Signed-off-by: Roman Kapl <code@...pl.cz>
Acked-by: Jiri Pirko <jiri@...lanox.com>
Acked-by: Cong Wang <xiyou.wangcong@...il.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 net/sched/cls_api.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/net/sched/cls_api.c
+++ b/net/sched/cls_api.c
@@ -1054,13 +1054,18 @@ static int tc_dump_tfilter(struct sk_buf
 		    nla_get_u32(tca[TCA_CHAIN]) != chain->index)
 			continue;
 		if (!tcf_chain_dump(chain, q, parent, skb, cb,
-				    index_start, &index))
+				    index_start, &index)) {
+			err = -EMSGSIZE;
 			break;
+		}
 	}

 	cb->args[0] = index;

 out:
+	/* If we did no progress, the error (EMSGSIZE) is real */
+	if (skb->len == 0 && err)
+		return err;
 	return skb->len;
 }