We had many syzbot reports that seem to be caused by use-after-free
of struct fib6_info.

ip6_dst_destroy(), fib6_drop_pcpu_from() and rt6_remove_exception()
are writers vs rt->from, and use non consistent synchronization among
themselves.

Switching to xchg() will solve the issues with no possible
lockdep issues.

BUG: KASAN: user-memory-access in atomic_dec_and_test include/asm-generic/atomic-instrumented.h:747 [inline]
BUG: KASAN: user-memory-access in fib6_info_release include/net/ip6_fib.h:294 [inline]
BUG: KASAN: user-memory-access in fib6_info_release include/net/ip6_fib.h:292 [inline]
BUG: KASAN: user-memory-access in fib6_drop_pcpu_from net/ipv6/ip6_fib.c:927 [inline]
BUG: KASAN: user-memory-access in fib6_purge_rt+0x4f6/0x670 net/ipv6/ip6_fib.c:960
Write of size 4 at addr 0000000000ffffb4 by task syz-executor.1/7649

CPU: 0 PID: 7649 Comm: syz-executor.1 Not tainted 5.1.0-rc6+ #183
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x172/0x1f0 lib/dump_stack.c:113
 kasan_report.cold+0x5/0x40 mm/kasan/report.c:321
 check_memory_region_inline mm/kasan/generic.c:185 [inline]
 check_memory_region+0x123/0x190 mm/kasan/generic.c:191
 kasan_check_write+0x14/0x20 mm/kasan/common.c:108
 atomic_dec_and_test include/asm-generic/atomic-instrumented.h:747 [inline]
 fib6_info_release include/net/ip6_fib.h:294 [inline]
 fib6_info_release include/net/ip6_fib.h:292 [inline]
 fib6_drop_pcpu_from net/ipv6/ip6_fib.c:927 [inline]
 fib6_purge_rt+0x4f6/0x670 net/ipv6/ip6_fib.c:960
 fib6_del_route net/ipv6/ip6_fib.c:1813 [inline]
 fib6_del+0xac2/0x10a0 net/ipv6/ip6_fib.c:1844
 fib6_clean_node+0x3a8/0x590 net/ipv6/ip6_fib.c:2006
 fib6_walk_continue+0x495/0x900 net/ipv6/ip6_fib.c:1928
 fib6_walk+0x9d/0x100 net/ipv6/ip6_fib.c:1976
 fib6_clean_tree+0xe0/0x120 net/ipv6/ip6_fib.c:2055
 __fib6_clean_all+0x118/0x2a0 net/ipv6/ip6_fib.c:2071
 fib6_clean_all+0x2b/0x40 net/ipv6/ip6_fib.c:2082
 rt6_sync_down_dev+0x134/0x150 net/ipv6/route.c:4057
 rt6_disable_ip+0x27/0x5f0 net/ipv6/route.c:4062
 addrconf_ifdown+0xa2/0x1220 net/ipv6/addrconf.c:3705
 addrconf_notify+0x19a/0x2260 net/ipv6/addrconf.c:3630
 notifier_call_chain+0xc7/0x240 kernel/notifier.c:93
 __raw_notifier_call_chain kernel/notifier.c:394 [inline]
 raw_notifier_call_chain+0x2e/0x40 kernel/notifier.c:401
 call_netdevice_notifiers_info+0x3f/0x90 net/core/dev.c:1753
 call_netdevice_notifiers_extack net/core/dev.c:1765 [inline]
 call_netdevice_notifiers net/core/dev.c:1779 [inline]
 dev_close_many+0x33f/0x6f0 net/core/dev.c:1522
 rollback_registered_many+0x43b/0xfd0 net/core/dev.c:8177
 rollback_registered+0x109/0x1d0 net/core/dev.c:8242
 unregister_netdevice_queue net/core/dev.c:9289 [inline]
 unregister_netdevice_queue+0x1ee/0x2c0 net/core/dev.c:9282
 unregister_netdevice include/linux/netdevice.h:2658 [inline]
 __tun_detach+0xd5b/0x1000 drivers/net/tun.c:727
 tun_detach drivers/net/tun.c:744 [inline]
 tun_chr_close+0xe0/0x180 drivers/net/tun.c:3443
 __fput+0x2e5/0x8d0 fs/file_table.c:278
 ____fput+0x16/0x20 fs/file_table.c:309
 task_work_run+0x14a/0x1c0 kernel/task_work.c:113
 exit_task_work include/linux/task_work.h:22 [inline]
 do_exit+0x90a/0x2fa0 kernel/exit.c:876
 do_group_exit+0x135/0x370 kernel/exit.c:980
 __do_sys_exit_group kernel/exit.c:991 [inline]
 __se_sys_exit_group kernel/exit.c:989 [inline]
 __x64_sys_exit_group+0x44/0x50 kernel/exit.c:989
 do_syscall_64+0x103/0x610 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x458da9
Code: ad b8 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 7b b8 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ffeafc2a6a8 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
RAX: ffffffffffffffda RBX: 000000000000001c RCX: 0000000000458da9
RDX: 0000000000412a80 RSI: 0000000000a54ef0 RDI: 0000000000000043
RBP: 00000000004be552 R08: 000000000000000c R09: 000000000004c0d1
R10: 0000000002341940 R11: 0000000000000246 R12: 00000000ffffffff
R13: 00007ffeafc2a7f0 R14: 000000000004c065 R15: 00007ffeafc2a800

Fixes: a68886a6 ("net/ipv6: Make from in rt6_info rcu protected")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: David Ahern <dsahern@gmail.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Wei Wang <weiwan@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>