netdev - [PATCH bpf-next v2 4/5] net/flow_dissector: correctly cap nhoff and thoff in case of BPF

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Message-Id: <20181204040100.243614-5-sdf@google.com>
Date:   Mon,  3 Dec 2018 20:00:59 -0800
From:   Stanislav Fomichev <sdf@...gle.com>
To:     netdev@...r.kernel.org
Cc:     davem@...emloft.net, ast@...nel.org, daniel@...earbox.net,
        simon.horman@...ronome.com, liu.song.a23@...il.com,
        Stanislav Fomichev <sdf@...gle.com>
Subject: [PATCH bpf-next v2 4/5] net/flow_dissector: correctly cap nhoff and
 thoff in case of BPF

We want to make sure that the following condition holds:
0 <= nhoff <= thoff <= skb->len

BPF program can set out-of-bounds nhoff and thoff, which is dangerous, see
recent commit d0c081b49137 ("flow_dissector: properly cap thoff field")'.

Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
---
 net/core/flow_dissector.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index ac19da6f390b..bb1a54747d64 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -716,6 +716,10 @@ bool __skb_flow_bpf_dissect(struct bpf_prog *prog,
 	/* Restore state */
 	memcpy(cb, &cb_saved, sizeof(cb_saved));
 
+	flow_keys->nhoff = clamp_t(u16, flow_keys->nhoff, 0, skb->len);
+	flow_keys->thoff = clamp_t(u16, flow_keys->thoff,
+				   flow_keys->nhoff, skb->len);
+
 	return result == BPF_OK;
 }
 
@@ -808,8 +812,6 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
 						     &flow_keys);
 			__skb_flow_bpf_to_target(&flow_keys, flow_dissector,
 						 target_container);
-			key_control->thoff = min_t(u16, key_control->thoff,
-						   skb->len);
 			rcu_read_unlock();
 			return ret;
 		}
-- 
2.20.0.rc1.387.gf8505762e3-goog