net: add a READ_ONCE() in skb_peek_tail()
skb_peek_tail() can be used without protection of a lock, as spotted by KCSAN [1] In order to avoid load-stearing, add a READ_ONCE() Note that the corresponding WRITE_ONCE() are already there. [1] BUG: KCSAN: data-race in sk_wait_data / skb_queue_tail read to 0xffff8880b36a4118 of 8 bytes by task 20426 on cpu 1: skb_peek_tail include/linux/skbuff.h:1784 [inline] sk_wait_data+0x15b/0x250 net/core/sock.c:2477 kcm_wait_data+0x112/0x1f0 net/kcm/kcmsock.c:1103 kcm_recvmsg+0xac/0x320 net/kcm/kcmsock.c:1130 sock_recvmsg_nosec net/socket.c:871 [inline] sock_recvmsg net/socket.c:889 [inline] sock_recvmsg+0x92/0xb0 net/socket.c:885 ___sys_recvmsg+0x1a0/0x3e0 net/socket.c:2480 do_recvmmsg+0x19a/0x5c0 net/socket.c:2601 __sys_recvmmsg+0x1ef/0x200 net/socket.c:2680 __do_sys_recvmmsg net/socket.c:2703 [inline] __se_sys_recvmmsg net/socket.c:2696 [inline] __x64_sys_recvmmsg+0x89/0xb0 net/socket.c:2696 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290 entry_SYSCALL_64_after_hwframe+0x44/0xa9 write to 0xffff8880b36a4118 of 8 bytes by task 451 on cpu 0: __skb_insert include/linux/skbuff.h:1852 [inline] __skb_queue_before include/linux/skbuff.h:1958 [inline] __skb_queue_tail include/linux/skbuff.h:1991 [inline] skb_queue_tail+0x7e/0xc0 net/core/skbuff.c:3145 kcm_queue_rcv_skb+0x202/0x310 net/kcm/kcmsock.c:206 kcm_rcv_strparser+0x74/0x4b0 net/kcm/kcmsock.c:370 __strp_recv+0x348/0xf50 net/strparser/strparser.c:309 strp_recv+0x84/0xa0 net/strparser/strparser.c:343 tcp_read_sock+0x174/0x5c0 net/ipv4/tcp.c:1639 strp_read_sock+0xd4/0x140 net/strparser/strparser.c:366 do_strp_work net/strparser/strparser.c:414 [inline] strp_work+0x9a/0xe0 net/strparser/strparser.c:423 process_one_work+0x3d4/0x890 kernel/workqueue.c:2269 worker_thread+0xa0/0x800 kernel/workqueue.c:2415 kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352 Reported by Kernel Concurrency Sanitizer on: CPU: 0 PID: 451 Comm: kworker/u4:3 Not tainted 5.4.0-rc3+ #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 Workqueue: kstrp strp_work Signed-off-by:Eric Dumazet <edumazet@google.com> Reported-by: