netdev - [PATCH bpf 2/2] net/flow_dissector: correctly cap nhoff and thoff in case of BPF

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Message-Id: <33711939695bf620703d60064c6242a8e4f4b64a.1544071106.git.sdf@google.com>
Date:   Wed,  5 Dec 2018 20:40:48 -0800
From:   Stanislav Fomichev <sdf@...gle.com>
To:     netdev@...r.kernel.org, ast@...nel.org
Cc:     davem@...emloft.net, daniel@...earbox.net,
        simon.horman@...ronome.com, liu.song.a23@...il.com,
        Stanislav Fomichev <sdf@...gle.com>
Subject: [PATCH bpf 2/2] 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 | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index ff5556d80570..af68207ee56c 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -791,9 +791,12 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
 		/* 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);
+
 		__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 result == BPF_OK;
 	}
-- 
2.20.0.rc1.387.gf8505762e3-goog