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

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

quoted

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 <redacted>
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.