bpf: Fix out-of-bounds read of rtt_min in sock_ops

In the Linux kernel, the following vulnerability has been resolved:

bpf: Fix out-of-bounds read of rtt_min in sock_ops

A sockops prog reading skops->rtt_min never checks the sk type: on the
tcp_conn_request() path sock_ops->sk is a request_sock (non-full), and the
ctx rewrite casts it to a tcp_sock (full) and reads rtt_min past the end of
the request_sock, returning dirty adjacent memory.

SEC("sockops")
int prog(struct bpf_sock_ops *skops)
{
switch (skops->op) {
case BPF_SOCK_OPS_RWND_INIT:
leak = skops->rtt_min; /* reads the request_sock OOB */
...
}
}

For instance one such read returned rtt_min=0xffff8881, the high half of a
leaked kernel pointer.

Guarding that cast is exactly what SOCK_OPS_GET_FIELD() does -- it checks
is_locked_tcp_sock and returns 0 when sock_ops->sk is not a locked full
socket. Every other tcp_sock field in sock_ops goes through it; rtt_min is
the only one open-coded, so it skips the check.

Read rtt_min through SOCK_OPS_GET_FIELD() too. rtt_min is a bit special:
it is a struct minmax and we only want the current min, so pass
rtt_min.s[0].v. That is equivalent to the old hand-computed offset

offsetof(struct tcp_sock, rtt_min) + sizeof_field(struct minmax_sample, t)

(s[0] sits at rtt_min + 0 and .v at + sizeof(.t), i.e. what minmax_get()
returns), so the loaded field is unchanged and only the full-sock guard is
added. The two BUILD_BUG_ON()s that protected the hand-computed offset
are no longer needed.

Before patch:

0: r1 = *(u64 *)(r1 +0) ; r1 = skops->sk
1: r1 = *(u32 *)(r1 +2324) ; ((tcp_sock *)sk)->rtt_min.s[0].v

After patch:

0: *(u64 *)(r1 +56) = r9
1: r9 = *(u8 *)(r1 +50) ; is_locked_tcp_sock
2: if r9 == 0 goto pc+4 ; not a locked full sock -> 0
3: r9 = *(u64 *)(r1 +56)
4: r1 = *(u64 *)(r1 +0) ; r1 = skops->sk
5: r1 = *(u32 *)(r1 +2324) ; rtt_min.s[0].v
6: goto pc+2
7: r9 = *(u64 *)(r1 +56)
8: r1 = 0