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

[Stanislav Fomichev]

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 <s...@google.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