On 2022-01-26 11:45, Lorenz Bauer wrote:

On Mon, 24 Jan 2022 at 15:13, Maxim Mikityanskiy [off-list ref] wrote:

quoted

bpf_tcp_gen_syncookie and bpf_tcp_check_syncookie expect the full length
of the TCP header (with all extensions). Fix the documentation that says
it should be sizeof(struct tcphdr).

I don't understand this change, sorry. Are you referring to the fact
that the check is len < sizeof(*th) instead of len != sizeof(*th)?

Your commit message makes me think that the helpers will access data
in the extension headers, which isn't true as far as I can tell.

Yes, they will. See bpf_tcp_gen_syncookie -> tcp_v4_get_syncookie -> 
tcp_get_syncookie_mss -> tcp_parse_mss_option, which iterates over the 
TCP options ("extensions" wasn't the best word I used here). Moreover, 
bpf_tcp_gen_syncookie even checks that th_len == th->doff * 4.

Although bpf_tcp_check_syncookie doesn't need the TCP options and 
doesn't enforce them to be passed, it's still allowed.

That
would be a problem in fact, since it could be used to read memory that
the verifier hasn't deemed safe.

It's safe, because bpf_tcp_gen_syncookie reads up to th_len, which is 
ARG_CONST_SIZE for the TCP header.