On Mon, 24 Jan 2022 at 15:13, Maxim Mikityanskiy <maximmi@xxxxxxxxxx> 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@xxxxxxxxxx>
> Reviewed-by: Tariq Toukan <tariqt@xxxxxxxxxx>
> ---
> 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?
--
Lorenz Bauer | Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK
www.cloudflare.com
