flow_dissector: fix clamping of BPF flow_keys for non-zero nhoff

Don't allow BPF program to set flow_keys->nhoff to less than initial
value. We currently don't read the value afterwards in anything but
the tests, but it's still a good practice to return consistent
values to the test programs.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

1 files changed, 2 insertions, 1 deletions

diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 9b84250039df..94a450b2191a 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -717,7 +717,8 @@ 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->nhoff = clamp_t(u16, flow_keys->nhoff,
+                                   skb_network_offset(skb), skb->len);
         flow_keys->thoff = clamp_t(u16, flow_keys->thoff,
                                    flow_keys->nhoff, skb->len);