Re: [PATCH bpf v2 2/4] bpf: Support dual-stack sockets in bpf_tcp_check_syncookie

[Maxim Mikityanskiy <maximmi@nvidia.com>]

On 2022-01-26 11:49, Lorenz Bauer wrote:
> On Mon, 24 Jan 2022 at 15:13, Maxim Mikityanskiy <maximmi@nvidia.com> wrote:
>>
>> bpf_tcp_gen_syncookie looks at the IP version in the IP header and
>> validates the address family of the socket. It supports IPv4 packets in
>> AF_INET6 dual-stack sockets.
>>
>> On the other hand, bpf_tcp_check_syncookie looks only at the address
>> family of the socket, ignoring the real IP version in headers, and
>> validates only the packet size. This implementation has some drawbacks:
>>
>> 1. Packets are not validated properly, allowing a BPF program to trick
>>     bpf_tcp_check_syncookie into handling an IPv6 packet on an IPv4
>>     socket.
>>
>> 2. Dual-stack sockets fail the checks on IPv4 packets. IPv4 clients end
>>     up receiving a SYNACK with the cookie, but the following ACK gets
>>     dropped.
>>
>> This patch fixes these issues by changing the checks in
>> bpf_tcp_check_syncookie to match the ones in bpf_tcp_gen_syncookie. IP
>> version from the header is taken into account, and it is validated
>> properly with address family.
>>
>> Fixes: 399040847084 ("bpf: add helper to check for a valid SYN cookie")
>> Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
>> Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
>> ---
>>   net/core/filter.c | 17 +++++++++++++----
>>   1 file changed, 13 insertions(+), 4 deletions(-)
>>
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index 05efa691b796..780e635fb52a 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -6774,24 +6774,33 @@ BPF_CALL_5(bpf_tcp_check_syncookie, struct sock *, sk, void *, iph, u32, iph_len
>>          if (!th->ack || th->rst || th->syn)
>>                  return -ENOENT;
>>
>> +       if (unlikely(iph_len < sizeof(struct iphdr)))
>> +               return -EINVAL;
>> +
>>          if (tcp_synq_no_recent_overflow(sk))
>>                  return -ENOENT;
>>
>>          cookie = ntohl(th->ack_seq) - 1;
>>
>> -       switch (sk->sk_family) {
>> -       case AF_INET:
>> -               if (unlikely(iph_len < sizeof(struct iphdr)))
>> +       /* Both struct iphdr and struct ipv6hdr have the version field at the
>> +        * same offset so we can cast to the shorter header (struct iphdr).
>> +        */
>> +       switch (((struct iphdr *)iph)->version) {
>> +       case 4:
>> +               if (sk->sk_family == AF_INET6 && ipv6_only_sock(sk))
>>                          return -EINVAL;
> 
> Wouldn't this allow an arbitrary value for sk->sk_family, since there
> is no further check that sk_family is AF_INET?

It relies on the assumption that sk_family is either AF_INET or 
AF_INET6, when sk_protocol is IPPROTO_TCP (checked above). The same 
assumption is used in bpf_tcp_gen_syncookie. Do you think there are 
cases when it doesn't hold, and we must verify sk_family? If yes, then 
bpf_tcp_gen_syncookie should also be fixed.