[PATCH net] net: __skb_flow_dissect() must cap its return value

[Eric Dumazet]

From: Eric Dumazet <eduma...@google.com>

After Tom patch, thoff field could point past the end of the buffer,
this could fool some callers.

If an skb was provided, skb->len should be the upper limit.
If not, hlen is supposed to be the upper limit.

Fixes: a6e544b0a88b ("flow_dissector: Jump to exit code in __skb_flow_dissect")
Signed-off-by: Eric Dumazet <eduma...@google.com>
Reported-by: Yibin Yang <yiby...@cisco.com
Acked-by: Alexander Duyck <alexander.h.du...@intel.com>
Acked-by: Willem de Bruijn <will...@google.com>
Acked-by: Alexei Starovoitov <a...@kernel.org>
---
 net/core/flow_dissector.c |   11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 44e6ba9d3a6b..5a908c534bec 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -122,7 +122,7 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
        struct flow_dissector_key_keyid *key_keyid;
        bool skip_vlan = false;
        u8 ip_proto = 0;
-       bool ret = false;
+       bool ret;
 
        if (!data) {
                data = skb->data;
@@ -549,12 +549,17 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
 out_good:
        ret = true;
 
-out_bad:
+       key_control->thoff = (u16)nhoff;
+out:
        key_basic->n_proto = proto;
        key_basic->ip_proto = ip_proto;
-       key_control->thoff = (u16)nhoff;
 
        return ret;
+
+out_bad:
+       ret = false;
+       key_control->thoff = min_t(u16, nhoff, skb ? skb->len : hlen);
+       goto out;
 }
 EXPORT_SYMBOL(__skb_flow_dissect);