ipv6: fix data-race in fib6_info_hw_flags_set / fib6_purge_rt

ipv6: fix data-race in fib6_info_hw_flags_set / fib6_purge_rt

Because fib6_info_hw_flags_set() is called without any synchronization,
all accesses to gi6->offload, fi->trap and fi->offload_failed
need some basic protection like READ_ONCE()/WRITE_ONCE().

BUG: KCSAN: data-race in fib6_info_hw_flags_set / fib6_purge_rt

read to 0xffff8881087d5886 of 1 bytes by task 13953 on cpu 0:
 fib6_drop_pcpu_from net/ipv6/ip6_fib.c:1007 [inline]
 fib6_purge_rt+0x4f/0x580 net/ipv6/ip6_fib.c:1033
 fib6_del_route net/ipv6/ip6_fib.c:1983 [inline]
 fib6_del+0x696/0x890 net/ipv6/ip6_fib.c:2028
 __ip6_del_rt net/ipv6/route.c:3876 [inline]
 ip6_del_rt+0x83/0x140 net/ipv6/route.c:3891
 __ipv6_dev_ac_dec+0x2b5/0x370 net/ipv6/anycast.c:374
 ipv6_dev_ac_dec net/ipv6/anycast.c:387 [inline]
 __ipv6_sock_ac_close+0x141/0x200 net/ipv6/anycast.c:207
 ipv6_sock_ac_close+0x79/0x90 net/ipv6/anycast.c:220
 inet6_release+0x32/0x50 net/ipv6/af_inet6.c:476
 __sock_release net/socket.c:650 [inline]
 sock_close+0x6c/0x150 net/socket.c:1318
 __fput+0x295/0x520 fs/file_table.c:280
 ____fput+0x11/0x20 fs/file_table.c:313
 task_work_run+0x8e/0x110 kernel/task_work.c:164
 tracehook_notify_resume include/linux/tracehook.h:189 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:175 [inline]
 exit_to_user_mode_prepare+0x160/0x190 kernel/entry/common.c:207
 __syscall_exit_to_user_mode_work kernel/entry/common.c:289 [inline]
 syscall_exit_to_user_mode+0x20/0x40 kernel/entry/common.c:300
 do_syscall_64+0x50/0xd0 arch/x86/entry/common.c:86
 entry_SYSCALL_64_after_hwframe+0x44/0xae

write to 0xffff8881087d5886 of 1 bytes by task 1912 on cpu 1:
 fib6_info_hw_flags_set+0x155/0x3b0 net/ipv6/route.c:6230
 nsim_fib6_rt_hw_flags_set drivers/net/netdevsim/fib.c:668 [inline]
 nsim_fib6_rt_add drivers/net/netdevsim/fib.c:691 [inline]
 nsim_fib6_rt_insert drivers/net/netdevsim/fib.c:756 [inline]
 nsim_fib6_event drivers/net/netdevsim/fib.c:853 [inline]
 nsim_fib_event drivers/net/netdevsim/fib.c:886 [inline]
 nsim_fib_event_work+0x284f/0x2cf0 drivers/net/netdevsim/fib.c:1477
 process_one_work+0x3f6/0x960 kernel/workqueue.c:2307
 worker_thread+0x616/0xa70 kernel/workqueue.c:2454
 kthread+0x2c7/0x2e0 kernel/kthread.c:327
 ret_from_fork+0x1f/0x30

value changed: 0x22 -> 0x2a

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 1912 Comm: kworker/1:3 Not tainted 5.16.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events nsim_fib_event_work

Fixes: 0c5fcf9e249e ("IPv6: Add "offload failed" indication to routes")
Fixes: bb3c4ab93e44 ("ipv6: Add "offload" and "trap" indications to routes")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Amit Cohen <amcohen@nvidia.com>
Cc: Ido Schimmel <idosch@nvidia.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Link: https://lore.kernel.org/r/20220216173217.3792411-2-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>