syzkaller reported a read of uninit req->syncookie. [0]

Originally, req->syncookie was used only in tcp_conn_request()
to indicate if we need to encode SYN cookie in SYN+ACK, so the
field remains uninitialised in other places.

The commit 695751e3 ("bpf: tcp: Handle BPF SYN Cookie in
cookie_v[46]_check().") added another meaning in ACK path;
req->syncookie is set true if SYN cookie is validated by BPF
kfunc.

After the change, cookie_v[46]_check() always read req->syncookie,
but it is not initialised in the normal SYN cookie case as reported
by KMSAN.

Let's make sure we always initialise req->syncookie in reqsk_alloc().

[0]:
BUG: KMSAN: uninit-value in cookie_v4_check+0x22b7/0x29e0
 net/ipv4/syncookies.c:477
 cookie_v4_check+0x22b7/0x29e0 net/ipv4/syncookies.c:477
 tcp_v4_cookie_check net/ipv4/tcp_ipv4.c:1855 [inline]
 tcp_v4_do_rcv+0xb17/0x10b0 net/ipv4/tcp_ipv4.c:1914
 tcp_v4_rcv+0x4ce4/0x5420 net/ipv4/tcp_ipv4.c:2322
 ip_protocol_deliver_rcu+0x2a3/0x13d0 net/ipv4/ip_input.c:205
 ip_local_deliver_finish+0x332/0x500 net/ipv4/ip_input.c:233
 NF_HOOK include/linux/netfilter.h:314 [inline]
 ip_local_deliver+0x21f/0x490 net/ipv4/ip_input.c:254
 dst_input include/net/dst.h:460 [inline]
 ip_rcv_finish+0x4a2/0x520 net/ipv4/ip_input.c:449
 NF_HOOK include/linux/netfilter.h:314 [inline]
 ip_rcv+0xcd/0x380 net/ipv4/ip_input.c:569
 __netif_receive_skb_one_core net/core/dev.c:5538 [inline]
 __netif_receive_skb+0x319/0x9e0 net/core/dev.c:5652
 process_backlog+0x480/0x8b0 net/core/dev.c:5981
 __napi_poll+0xe7/0x980 net/core/dev.c:6632
 napi_poll net/core/dev.c:6701 [inline]
 net_rx_action+0x89d/0x1820 net/core/dev.c:6813
 __do_softirq+0x1c0/0x7d7 kernel/softirq.c:554
 do_softirq+0x9a/0x100 kernel/softirq.c:455
 __local_bh_enable_ip+0x9f/0xb0 kernel/softirq.c:382
 local_bh_enable include/linux/bottom_half.h:33 [inline]
 rcu_read_unlock_bh include/linux/rcupdate.h:820 [inline]
 __dev_queue_xmit+0x2776/0x52c0 net/core/dev.c:4362
 dev_queue_xmit include/linux/netdevice.h:3091 [inline]
 neigh_hh_output include/net/neighbour.h:526 [inline]
 neigh_output include/net/neighbour.h:540 [inline]
 ip_finish_output2+0x187a/0x1b70 net/ipv4/ip_output.c:235
 __ip_finish_output+0x287/0x810
 ip_finish_output+0x4b/0x550 net/ipv4/ip_output.c:323
 NF_HOOK_COND include/linux/netfilter.h:303 [inline]
 ip_output+0x15f/0x3f0 net/ipv4/ip_output.c:433
 dst_output include/net/dst.h:450 [inline]
 ip_local_out net/ipv4/ip_output.c:129 [inline]
 __ip_queue_xmit+0x1e93/0x2030 net/ipv4/ip_output.c:535
 ip_queue_xmit+0x60/0x80 net/ipv4/ip_output.c:549
 __tcp_transmit_skb+0x3c70/0x4890 net/ipv4/tcp_output.c:1462
 tcp_transmit_skb net/ipv4/tcp_output.c:1480 [inline]
 tcp_write_xmit+0x3ee1/0x8900 net/ipv4/tcp_output.c:2792
 __tcp_push_pending_frames net/ipv4/tcp_output.c:2977 [inline]
 tcp_send_fin+0xa90/0x12e0 net/ipv4/tcp_output.c:3578
 tcp_shutdown+0x198/0x1f0 net/ipv4/tcp.c:2716
 inet_shutdown+0x33f/0x5b0 net/ipv4/af_inet.c:923
 __sys_shutdown_sock net/socket.c:2425 [inline]
 __sys_shutdown net/socket.c:2437 [inline]
 __do_sys_shutdown net/socket.c:2445 [inline]
 __se_sys_shutdown+0x2a4/0x440 net/socket.c:2443
 __x64_sys_shutdown+0x6c/0xa0 net/socket.c:2443
 do_syscall_64+0xd5/0x1f0
 entry_SYSCALL_64_after_hwframe+0x6d/0x75

Uninit was stored to memory at:
 reqsk_alloc include/net/request_sock.h:148 [inline]
 inet_reqsk_alloc+0x651/0x7a0 net/ipv4/tcp_input.c:6978
 cookie_tcp_reqsk_alloc+0xd4/0x900 net/ipv4/syncookies.c:328
 cookie_tcp_check net/ipv4/syncookies.c:388 [inline]
 cookie_v4_check+0x289f/0x29e0 net/ipv4/syncookies.c:420
 tcp_v4_cookie_check net/ipv4/tcp_ipv4.c:1855 [inline]
 tcp_v4_do_rcv+0xb17/0x10b0 net/ipv4/tcp_ipv4.c:1914
 tcp_v4_rcv+0x4ce4/0x5420 net/ipv4/tcp_ipv4.c:2322
 ip_protocol_deliver_rcu+0x2a3/0x13d0 net/ipv4/ip_input.c:205
 ip_local_deliver_finish+0x332/0x500 net/ipv4/ip_input.c:233
 NF_HOOK include/linux/netfilter.h:314 [inline]
 ip_local_deliver+0x21f/0x490 net/ipv4/ip_input.c:254
 dst_input include/net/dst.h:460 [inline]
 ip_rcv_finish+0x4a2/0x520 net/ipv4/ip_input.c:449
 NF_HOOK include/linux/netfilter.h:314 [inline]
 ip_rcv+0xcd/0x380 net/ipv4/ip_input.c:569
 __netif_receive_skb_one_core net/core/dev.c:5538 [inline]
 __netif_receive_skb+0x319/0x9e0 net/core/dev.c:5652
 process_backlog+0x480/0x8b0 net/core/dev.c:5981
 __napi_poll+0xe7/0x980 net/core/dev.c:6632
 napi_poll net/core/dev.c:6701 [inline]
 net_rx_action+0x89d/0x1820 net/core/dev.c:6813
 __do_softirq+0x1c0/0x7d7 kernel/softirq.c:554

Uninit was created at:
 __alloc_pages+0x9a7/0xe00 mm/page_alloc.c:4592
 __alloc_pages_node include/linux/gfp.h:238 [inline]
 alloc_pages_node include/linux/gfp.h:261 [inline]
 alloc_slab_page mm/slub.c:2175 [inline]
 allocate_slab mm/slub.c:2338 [inline]
 new_slab+0x2de/0x1400 mm/slub.c:2391
 ___slab_alloc+0x1184/0x33d0 mm/slub.c:3525
 __slab_alloc mm/slub.c:3610 [inline]
 __slab_alloc_node mm/slub.c:3663 [inline]
 slab_alloc_node mm/slub.c:3835 [inline]
 kmem_cache_alloc+0x6d3/0xbe0 mm/slub.c:3852
 reqsk_alloc include/net/request_sock.h:131 [inline]
 inet_reqsk_alloc+0x66/0x7a0 net/ipv4/tcp_input.c:6978
 tcp_conn_request+0x484/0x44e0 net/ipv4/tcp_input.c:7135
 tcp_v4_conn_request+0x16f/0x1d0 net/ipv4/tcp_ipv4.c:1716
 tcp_rcv_state_process+0x2e5/0x4bb0 net/ipv4/tcp_input.c:6655
 tcp_v4_do_rcv+0xbfd/0x10b0 net/ipv4/tcp_ipv4.c:1929
 tcp_v4_rcv+0x4ce4/0x5420 net/ipv4/tcp_ipv4.c:2322
 ip_protocol_deliver_rcu+0x2a3/0x13d0 net/ipv4/ip_input.c:205
 ip_local_deliver_finish+0x332/0x500 net/ipv4/ip_input.c:233
 NF_HOOK include/linux/netfilter.h:314 [inline]
 ip_local_deliver+0x21f/0x490 net/ipv4/ip_input.c:254
 dst_input include/net/dst.h:460 [inline]
 ip_sublist_rcv_finish net/ipv4/ip_input.c:580 [inline]
 ip_list_rcv_finish net/ipv4/ip_input.c:631 [inline]
 ip_sublist_rcv+0x15f3/0x17f0 net/ipv4/ip_input.c:639
 ip_list_rcv+0x9ef/0xa40 net/ipv4/ip_input.c:674
 __netif_receive_skb_list_ptype net/core/dev.c:5581 [inline]
 __netif_receive_skb_list_core+0x15c5/0x1670 net/core/dev.c:5629
 __netif_receive_skb_list net/core/dev.c:5681 [inline]
 netif_receive_skb_list_internal+0x106c/0x16f0 net/core/dev.c:5773
 gro_normal_list include/net/gro.h:438 [inline]
 napi_complete_done+0x425/0x880 net/core/dev.c:6113
 virtqueue_napi_complete drivers/net/virtio_net.c:465 [inline]
 virtnet_poll+0x149d/0x2240 drivers/net/virtio_net.c:2211
 __napi_poll+0xe7/0x980 net/core/dev.c:6632
 napi_poll net/core/dev.c:6701 [inline]
 net_rx_action+0x89d/0x1820 net/core/dev.c:6813
 __do_softirq+0x1c0/0x7d7 kernel/softirq.c:554

CPU: 0 PID: 16792 Comm: syz-executor.2 Not tainted 6.8.0-syzkaller-05562-g61387b8d #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/29/2024

Fixes: 695751e3 ("bpf: tcp: Handle BPF SYN Cookie in cookie_v[46]_check().")
Reported-by: syzkaller <syzkaller@googlegroups.com>
Reported-by: Eric Dumazet <edumazet@google.com>
Closes: https://lore.kernel.org/bpf/CANn89iKdN9c+C_2JAUbc+VY3DDQjAQukMtiBbormAmAk9CdvQA@mail.gmail.com/Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20240315224710.55209-1-kuniyu@amazon.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>