bpf, sockmap: Fix self-redirect copied_seq double-counting

When a BPF stream_verdict program redirects an skb back to the same
socket (self-redirect with BPF_F_INGRESS), sk_psock_verdict_apply()
calls tcp_eat_skb() which advances tcp_sk->copied_seq. However, the
skb is then delivered to the socket's psock ingress queue and later
read by tcp_bpf_recvmsg_parser(), which also advances copied_seq via
the copied_from_self accounting path. This double-counting causes
copied_seq to advance by 2x the actual data length, triggering:

  TCP recvmsg seq # bug 2: copied BF2E806, seq BF2E7FD, \
			   rcvnxt BF2E806, fl 0
  WARNING: net/ipv4/tcp.c:2745 at tcp_recvmsg_locked+0x72b/0x2640
  Call Trace:
   tcp_recvmsg+0x10a/0x500
   sock_recvmsg+0x168/0x1d0
   __sys_recvfrom+0x19a/0x2a0
   __x64_sys_recvfrom+0xe4/0x1f0
   do_syscall_64+0xf7/0x530
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

  cleanup rbuf bug: copied BF2E806 seq BF2E806 rcvnxt BF2E806
  WARNING: net/ipv4/tcp.c:1609 at tcp_cleanup_rbuf+0xf2/0x1c0
  Call Trace:
   tcp_recvmsg_locked+0x8d1/0x2640
   tcp_recvmsg+0x10a/0x500
   sock_recvmsg+0x168/0x1d0
   __sys_recvfrom+0x19a/0x2a0
   __x64_sys_recvfrom+0xe4/0x1f0
   do_syscall_64+0xf7/0x530
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

Fix this by converting self-redirect verdict to __SK_PASS at the
beginning of sk_psock_verdict_apply(). This bypasses the
__SK_REDIRECT case entirely (which calls sk_psock_eat_skb), letting
the __SK_PASS path queue the skb to the psock ingress queue. The
data is then read via tcp_bpf_recvmsg_parser(), which advances
copied_seq exactly once through copied_from_self. Cross-socket
redirects continue through __SK_REDIRECT with sk_psock_eat_skb()
unchanged.

Fixes: e5c6de5fa0 ("bpf, sockmap: Incorrectly handling copied_seq")
Suggested-by: Jakub Sitnicki <[email protected]>
Suggested-by: Jiayuan Chen <[email protected]>
Signed-off-by: Geliang Tang <[email protected]>
Reviewed-by: Emil Tsalapatis <[email protected]>
Reviewed-by: Jiayuan Chen <[email protected]>
Link: https://lore.kernel.org/r/1a8e797a1b26e2f695aaac22ac644c2862f63466.1788858299.git.tanggeliang@kylinos.cn
Signed-off-by: Alexei Starovoitov <[email protected]>

net/core/skmsg.c

@@ -1000,6 +1000,10 @@ static int sk_psock_verdict_apply(struct sk_psock *psock, struct sk_buff *skb,
 	int err = 0;
 	u32 len, off;
 
+	if (verdict == __SK_REDIRECT && skb_bpf_ingress(skb) &&
+	    skb_bpf_redirect_fetch(skb) == psock->sk)
+		verdict = __SK_PASS;
+
 	switch (verdict) {
 	case __SK_PASS:
 		err = -EIO;