tcp: tcp_rtx_synack() can be called from process context

[ Upstream commit 0a375c822497ed6ad6b5da0792a12a6f1af10c0b ]

Laurent reported the enclosed report [1]

This bug triggers with following coditions:

0) Kernel built with CONFIG_DEBUG_PREEMPT=y

1) A new passive FastOpen TCP socket is created.
   This FO socket waits for an ACK coming from client to be a complete
   ESTABLISHED one.
2) A socket operation on this socket goes through lock_sock()
   release_sock() dance.
3) While the socket is owned by the user in step 2),
   a retransmit of the SYN is received and stored in socket backlog.
4) At release_sock() time, the socket backlog is processed while
   in process context.
5) A SYNACK packet is cooked in response of the SYN retransmit.
6) -> tcp_rtx_synack() is called in process context.

Before blamed commit, tcp_rtx_synack() was always called from BH handler,
from a timer handler.

Fix this by using TCP_INC_STATS() & NET_INC_STATS()
which do not assume caller is in non preemptible context.

[1]
BUG: using __this_cpu_add() in preemptible [00000000] code: epollpep/2180
caller is tcp_rtx_synack.part.0+0x36/0xc0
CPU: 10 PID: 2180 Comm: epollpep Tainted: G           OE     5.16.0-0.bpo.4-amd64 #1  Debian 5.16.12-1~bpo11+1
Hardware name: Supermicro SYS-5039MC-H8TRF/X11SCD-F, BIOS 1.7 11/23/2021
Call Trace:
 <TASK>
 dump_stack_lvl+0x48/0x5e
 check_preemption_disabled+0xde/0xe0
 tcp_rtx_synack.part.0+0x36/0xc0
 tcp_rtx_synack+0x8d/0xa0
 ? kmem_cache_alloc+0x2e0/0x3e0
 ? apparmor_file_alloc_security+0x3b/0x1f0
 inet_rtx_syn_ack+0x16/0x30
 tcp_check_req+0x367/0x610
 tcp_rcv_state_process+0x91/0xf60
 ? get_nohz_timer_target+0x18/0x1a0
 ? lock_timer_base+0x61/0x80
 ? preempt_count_add+0x68/0xa0
 tcp_v4_do_rcv+0xbd/0x270
 __release_sock+0x6d/0xb0
 release_sock+0x2b/0x90
 sock_setsockopt+0x138/0x1140
 ? __sys_getsockname+0x7e/0xc0
 ? aa_sk_perm+0x3e/0x1a0
 __sys_setsockopt+0x198/0x1e0
 __x64_sys_setsockopt+0x21/0x30
 do_syscall_64+0x38/0xc0
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Fixes: 168a8f5 ("tcp: TCP Fast Open Server - main code path")
Signed-off-by: Eric Dumazet <[email protected]>
Reported-by: Laurent Fasnacht <[email protected]>
Acked-by: Neal Cardwell <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>

net/ipv4/tcp_output.c

@@ -3795,8 +3795,8 @@ int tcp_rtx_synack(const struct sock *sk, struct request_sock *req)
 	tcp_rsk(req)->txhash = net_tx_rndhash();
 	res = af_ops->send_synack(sk, NULL, &fl, req, NULL, TCP_SYNACK_NORMAL);
 	if (!res) {
-		__TCP_INC_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS);
-		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);
+		TCP_INC_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS);
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);
 		if (unlikely(tcp_passive_fastopen(sk)))
 			tcp_sk(sk)->total_retrans++;
 	}