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BUG: KCSAN: data-race in sctp_poll / sctp_wfree

write to 0xffff88803335d888 of 4 bytes by task 26551 on cpu 1:
 sctp_wfree+0x127/0x450 net/sctp/socket.c:8983
 skb_release_head_state+0xb3/0x170 net/core/skbuff.c:712
 skb_release_all net/core/skbuff.c:723 [inline]
 __kfree_skb+0x14/0x150 net/core/skbuff.c:739
 consume_skb+0x4b/0x160 net/core/skbuff.c:895
 sctp_chunk_destroy net/sctp/sm_make_chunk.c:1478 [inline]
 sctp_chunk_put+0xd2/0x130 net/sctp/sm_make_chunk.c:1505
 sctp_datamsg_destroy net/sctp/chunk.c:107 [inline]
 sctp_datamsg_put+0x2d7/0x340 net/sctp/chunk.c:128
 sctp_chunk_free+0x30/0x50 net/sctp/sm_make_chunk.c:1490
 sctp_outq_sack+0xb8b/0xdf0 net/sctp/outqueue.c:1359
 sctp_cmd_process_sack net/sctp/sm_sideeffect.c:798 [inline]
 sctp_cmd_interpreter net/sctp/sm_sideeffect.c:1354 [inline]
 sctp_side_effects net/sctp/sm_sideeffect.c:1185 [inline]
 sctp_do_sm+0x1092/0x3110 net/sctp/sm_sideeffect.c:1156
 sctp_assoc_bh_rcv+0x2de/0x480 net/sctp/associola.c:1048
 sctp_inq_push+0x10f/0x120 net/sctp/inqueue.c:80
 sctp_backlog_rcv+0xf0/0x680 net/sctp/input.c:344
 sk_backlog_rcv include/net/sock.h:1016 [inline]
 __release_sock+0xf5/0x270 net/core/sock.c:2556
 release_sock+0x40/0x110 net/core/sock.c:3080
 sctp_recvmsg+0x50e/0x600 net/sctp/socket.c:2176
 inet_recvmsg+0xa3/0x210 net/ipv4/af_inet.c:852
 ____sys_recvmsg+0x15d/0x310 include/linux/uio.h:234
 ___sys_recvmsg net/socket.c:2611 [inline]
 do_recvmmsg+0x34c/0x9e0 net/socket.c:2705
 __sys_recvmmsg net/socket.c:2784 [inline]
 __do_sys_recvmmsg net/socket.c:2807 [inline]
 __se_sys_recvmmsg net/socket.c:2800 [inline]
 __x64_sys_recvmmsg+0xcf/0x150 net/socket.c:2800
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88803335d888 of 4 bytes by task 26536 on cpu 0:
 sctp_writeable net/sctp/socket.c:9140 [inline]
 sctp_poll+0x259/0x400 net/sctp/socket.c:8506
 sock_poll+0x23e/0x260 net/socket.c:1246
 vfs_poll include/linux/poll.h:90 [inline]
 do_pollfd fs/select.c:869 [inline]
 do_poll fs/select.c:917 [inline]
 do_sys_poll+0x622/0xc40 fs/select.c:1011
 __do_sys_ppoll fs/select.c:1117 [inline]
 __se_sys_ppoll+0x169/0x1b0 fs/select.c:1097
 __x64_sys_ppoll+0x63/0x70 fs/select.c:1097
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 26536 Comm: syz-executor.1 Tainted: G        W         5.12.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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