[PATCH 5.4 333/411] tcp: tcp_rtx_synack() can be called from process context

Greg Kroah-Hartman
2022-06-13T11:41:44Z

From: Eric Dumazet <edumazet@google.com>

[ 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: 168a8f58059a ("tcp: TCP Fast Open Server - main code path")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Laurent Fasnacht <laurent.fasnacht@proton.ch>
Acked-by: Neal Cardwell <ncardwell@google.com>
Link: https://lore.kernel.org/r/20220530213713.601888-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/ipv4/tcp_output.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 67493ec6318a..739fc69cdcc6 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3869,8 +3869,8 @@ 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++;
trace_tcp_retransmit_synack(sk, req);
--
2.35.1