ipv6: avoid use-after-free in ip6_fragment()

In the Linux kernel, the following vulnerability has been resolved:

ipv6: avoid use-after-free in ip6_fragment()

Blamed commit claimed rcu_read_lock() was held by ip6_fragment() callers.

It seems to not be always true, at least for UDP stack.

syzbot reported:

BUG: KASAN: use-after-free in ip6_dst_idev include/net/ip6_fib.h:245 [inline]
BUG: KASAN: use-after-free in ip6_fragment+0x2724/0x2770 net/ipv6/ip6_output.c:951
Read of size 8 at addr ffff88801d403e80 by task syz-executor.3/7618

CPU: 1 PID: 7618 Comm: syz-executor.3 Not tainted 6.1.0-rc6-syzkaller-00012-g4312098baf37 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/26/2022
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0xd1/0x138 lib/dump_stack.c:106
print_address_description mm/kasan/report.c:284 [inline]
print_report+0x15e/0x45d mm/kasan/report.c:395
kasan_report+0xbf/0x1f0 mm/kasan/report.c:495
ip6_dst_idev include/net/ip6_fib.h:245 [inline]
ip6_fragment+0x2724/0x2770 net/ipv6/ip6_output.c:951
__ip6_finish_output net/ipv6/ip6_output.c:193 [inline]
ip6_finish_output+0x9a3/0x1170 net/ipv6/ip6_output.c:206
NF_HOOK_COND include/linux/netfilter.h:291 [inline]
ip6_output+0x1f1/0x540 net/ipv6/ip6_output.c:227
dst_output include/net/dst.h:445 [inline]
ip6_local_out+0xb3/0x1a0 net/ipv6/output_core.c:161
ip6_send_skb+0xbb/0x340 net/ipv6/ip6_output.c:1966
udp_v6_send_skb+0x82a/0x18a0 net/ipv6/udp.c:1286
udp_v6_push_pending_frames+0x140/0x200 net/ipv6/udp.c:1313
udpv6_sendmsg+0x18da/0x2c80 net/ipv6/udp.c:1606
inet6_sendmsg+0x9d/0xe0 net/ipv6/af_inet6.c:665
sock_sendmsg_nosec net/socket.c:714 [inline]
sock_sendmsg+0xd3/0x120 net/socket.c:734
sock_write_iter+0x295/0x3d0 net/socket.c:1108
call_write_iter include/linux/fs.h:2191 [inline]
new_sync_write fs/read_write.c:491 [inline]
vfs_write+0x9ed/0xdd0 fs/read_write.c:584
ksys_write+0x1ec/0x250 fs/read_write.c:637
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd
RIP: 0033:0x7fde3588c0d9
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 f1 19 00 00 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 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fde365b6168 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007fde359ac050 RCX: 00007fde3588c0d9
RDX: 000000000000ffdc RSI: 00000000200000c0 RDI: 000000000000000a
RBP: 00007fde358e7ae9 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fde35acfb1f R14: 00007fde365b6300 R15: 0000000000022000
</TASK>

Allocated by task 7618:
kasan_save_stack+0x22/0x40 mm/kasan/common.c:45
kasan_set_track+0x25/0x30 mm/kasan/common.c:52
__kasan_slab_alloc+0x82/0x90 mm/kasan/common.c:325
kasan_slab_alloc include/linux/kasan.h:201 [inline]
slab_post_alloc_hook mm/slab.h:737 [inline]
slab_alloc_node mm/slub.c:3398 [inline]
slab_alloc mm/slub.c:3406 [inline]
__kmem_cache_alloc_lru mm/slub.c:3413 [inline]
kmem_cache_alloc+0x2b4/0x3d0 mm/slub.c:3422
dst_alloc+0x14a/0x1f0 net/core/dst.c:92
ip6_dst_alloc+0x32/0xa0 net/ipv6/route.c:344
ip6_rt_pcpu_alloc net/ipv6/route.c:1369 [inline]
rt6_make_pcpu_route net/ipv6/route.c:1417 [inline]
ip6_pol_route+0x901/0x1190 net/ipv6/route.c:2254
pol_lookup_func include/net/ip6_fib.h:582 [inline]
fib6_rule_lookup+0x52e/0x6f0 net/ipv6/fib6_rules.c:121
ip6_route_output_flags_noref+0x2e6/0x380 net/ipv6/route.c:2625
ip6_route_output_flags+0x76/0x320 net/ipv6/route.c:2638
ip6_route_output include/net/ip6_route.h:98 [inline]
ip6_dst_lookup_tail+0x5ab/0x1620 net/ipv6/ip6_output.c:1092
ip6_dst_lookup_flow+0x90/0x1d0 net/ipv6/ip6_output.c:1222
ip6_sk_dst_lookup_flow+0x553/0x980 net/ipv6/ip6_output.c:1260
udpv6_sendmsg+0x151d/0x2c80 net/ipv6/udp.c:1554
inet6_sendmsg+0x9d/0xe0 net/ipv6/af_inet6.c:665
sock_sendmsg_nosec n
---truncated---