On 2022-01-26 11:49, Lorenz Bauer wrote:
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?
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.
