Revert "ipv6: fix double refcount of fib6_metrics"

This reverts commit e70a3aad44.

This change causes use-after-free on dst->_metrics.
The crash trace looks like this:
[   97.763269] BUG: KASAN: use-after-free in ip6_mtu+0x116/0x140
[   97.769038] Read of size 4 at addr ffff881781d2cf84 by task svw_NetThreadEv/8801

[   97.777954] CPU: 76 PID: 8801 Comm: svw_NetThreadEv Not tainted 4.15.0-smp-DEV #11
[   97.777956] Hardware name: Default string Default string/Indus_QC_02, BIOS 5.46.4 03/29/2018
[   97.777957] Call Trace:
[   97.777971]  [<ffffffff895709db>] dump_stack+0x4d/0x72
[   97.777985]  [<ffffffff881651df>] print_address_description+0x6f/0x260
[   97.777997]  [<ffffffff88165747>] kasan_report+0x257/0x370
[   97.778001]  [<ffffffff894488e6>] ? ip6_mtu+0x116/0x140
[   97.778004]  [<ffffffff881658b9>] __asan_report_load4_noabort+0x19/0x20
[   97.778008]  [<ffffffff894488e6>] ip6_mtu+0x116/0x140
[   97.778013]  [<ffffffff892bb91e>] tcp_current_mss+0x12e/0x280
[   97.778016]  [<ffffffff892bb7f0>] ? tcp_mtu_to_mss+0x2d0/0x2d0
[   97.778022]  [<ffffffff887b45b8>] ? depot_save_stack+0x138/0x4a0
[   97.778037]  [<ffffffff87c38985>] ? __mmdrop+0x145/0x1f0
[   97.778040]  [<ffffffff881643b1>] ? save_stack+0xb1/0xd0
[   97.778046]  [<ffffffff89264c82>] tcp_send_mss+0x22/0x220
[   97.778059]  [<ffffffff89273a49>] tcp_sendmsg_locked+0x4f9/0x39f0
[   97.778062]  [<ffffffff881642b4>] ? kasan_check_write+0x14/0x20
[   97.778066]  [<ffffffff89273550>] ? tcp_sendpage+0x60/0x60
[   97.778070]  [<ffffffff881cb359>] ? rw_copy_check_uvector+0x69/0x280
[   97.778075]  [<ffffffff8873c65f>] ? import_iovec+0x9f/0x430
[   97.778078]  [<ffffffff88164be7>] ? kasan_slab_free+0x87/0xc0
[   97.778082]  [<ffffffff8873c5c0>] ? memzero_page+0x140/0x140
[   97.778085]  [<ffffffff881642b4>] ? kasan_check_write+0x14/0x20
[   97.778088]  [<ffffffff89276f6c>] tcp_sendmsg+0x2c/0x50
[   97.778092]  [<ffffffff89276f6c>] ? tcp_sendmsg+0x2c/0x50
[   97.778098]  [<ffffffff89352d43>] inet_sendmsg+0x103/0x480
[   97.778102]  [<ffffffff89352c40>] ? inet_gso_segment+0x15b0/0x15b0
[   97.778105]  [<ffffffff890294da>] sock_sendmsg+0xba/0xf0
[   97.778108]  [<ffffffff8902ab6a>] ___sys_sendmsg+0x6ca/0x8e0
[   97.778113]  [<ffffffff87dccac1>] ? hrtimer_try_to_cancel+0x71/0x3b0
[   97.778116]  [<ffffffff8902a4a0>] ? copy_msghdr_from_user+0x3d0/0x3d0
[   97.778119]  [<ffffffff881646d1>] ? memset+0x31/0x40
[   97.778123]  [<ffffffff87a0cff5>] ? schedule_hrtimeout_range_clock+0x165/0x380
[   97.778127]  [<ffffffff87a0ce90>] ? hrtimer_nanosleep_restart+0x250/0x250
[   97.778130]  [<ffffffff87dcc700>] ? __hrtimer_init+0x180/0x180
[   97.778133]  [<ffffffff87dd1f82>] ? ktime_get_ts64+0x172/0x200
[   97.778137]  [<ffffffff8822b8ec>] ? __fget_light+0x8c/0x2f0
[   97.778141]  [<ffffffff8902d5c6>] __sys_sendmsg+0xe6/0x190
[   97.778144]  [<ffffffff8902d5c6>] ? __sys_sendmsg+0xe6/0x190
[   97.778147]  [<ffffffff8902d4e0>] ? SyS_shutdown+0x20/0x20
[   97.778152]  [<ffffffff87cd4370>] ? wake_up_q+0xe0/0xe0
[   97.778155]  [<ffffffff8902d670>] ? __sys_sendmsg+0x190/0x190
[   97.778158]  [<ffffffff8902d683>] SyS_sendmsg+0x13/0x20
[   97.778162]  [<ffffffff87a1600c>] do_syscall_64+0x2ac/0x430
[   97.778166]  [<ffffffff87c17515>] ? do_page_fault+0x35/0x3d0
[   97.778171]  [<ffffffff8960131f>] ? page_fault+0x2f/0x50
[   97.778174]  [<ffffffff89600071>] entry_SYSCALL_64_after_hwframe+0x3d/0xa2
[   97.778177] RIP: 0033:0x7f83fa36000d
[   97.778178] RSP: 002b:00007f83ef9229e0 EFLAGS: 00000293 ORIG_RAX: 000000000000002e
[   97.778180] RAX: ffffffffffffffda RBX: 0000000000000001 RCX: 00007f83fa36000d
[   97.778182] RDX: 0000000000004000 RSI: 00007f83ef922f00 RDI: 0000000000000036
[   97.778183] RBP: 00007f83ef923040 R08: 00007f83ef9231f8 R09: 00007f83ef923168
[   97.778184] R10: 0000000000000000 R11: 0000000000000293 R12: 00007f83f69c5b40
[   97.778185] R13: 000000000000001c R14: 0000000000000001 R15: 0000000000004000

[   97.779684] Allocated by task 5919:
[   97.783185]  save_stack+0x46/0xd0
[   97.783187]  kasan_kmalloc+0xad/0xe0
[   97.783189]  kmem_cache_alloc_trace+0xdf/0x580
[   97.783190]  ip6_convert_metrics.isra.79+0x7e/0x190
[   97.783192]  ip6_route_info_create+0x60a/0x2480
[   97.783193]  ip6_route_add+0x1d/0x80
[   97.783195]  inet6_rtm_newroute+0xdd/0xf0
[   97.783198]  rtnetlink_rcv_msg+0x641/0xb10
[   97.783200]  netlink_rcv_skb+0x27b/0x3e0
[   97.783202]  rtnetlink_rcv+0x15/0x20
[   97.783203]  netlink_unicast+0x4be/0x720
[   97.783204]  netlink_sendmsg+0x7bc/0xbf0
[   97.783205]  sock_sendmsg+0xba/0xf0
[   97.783207]  ___sys_sendmsg+0x6ca/0x8e0
[   97.783208]  __sys_sendmsg+0xe6/0x190
[   97.783209]  SyS_sendmsg+0x13/0x20
[   97.783211]  do_syscall_64+0x2ac/0x430
[   97.783213]  entry_SYSCALL_64_after_hwframe+0x3d/0xa2

[   97.784709] Freed by task 0:
[   97.785056] knetbase: Error: /proc/sys/net/core/txcs_enable does not exist
[   97.794497]  save_stack+0x46/0xd0
[   97.794499]  kasan_slab_free+0x71/0xc0
[   97.794500]  kfree+0x7c/0xf0
[   97.794501]  fib6_info_destroy_rcu+0x24f/0x310
[   97.794504]  rcu_process_callbacks+0x38b/0x1730
[   97.794506]  __do_softirq+0x1c8/0x5d0

Reported-by: John Sperbeck <jsperbeck@google.com>
Signed-off-by: Wei Wang <weiwan@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -976,6 +976,10 @@ static void rt6_set_from(struct rt6_info *rt, struct fib6_info *from)
 	rt->rt6i_flags &= ~RTF_EXPIRES;
 	rcu_assign_pointer(rt->from, from);
 	dst_init_metrics(&rt->dst, from->fib6_metrics->metrics, true);
+	if (from->fib6_metrics != &dst_default_metrics) {
+		rt->dst._metrics |= DST_METRICS_REFCOUNTED;
+		refcount_inc(&from->fib6_metrics->refcnt);
+	}
 }
 
 /* Caller must already hold reference to @ort */