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BUG: KASAN: use-after-free in __ip6_append_data.isra.3+0x2859/0x3460 net/ipv6/ip6_output.c:1329
Read of size 2 at addr ffff8801c77ac23a by task syz-executor.1/19144

CPU: 0 PID: 19144 Comm: syz-executor.1 Not tainted 4.9.141+ #23
 ffff8801c4697430 ffffffff81b42e79 ffffea00071deb00 ffff8801c77ac23a
 0000000000000000 ffff8801c77ac23a 000000000000000e ffff8801c4697468
 ffffffff815009b8 ffff8801c77ac23a 0000000000000002 0000000000000000
Call Trace:
 [<ffffffff81b42e79>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81b42e79>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
 [<ffffffff815009b8>] print_address_description+0x6c/0x234 mm/kasan/report.c:256
 [<ffffffff81500dc2>] kasan_report_error mm/kasan/report.c:355 [inline]
 [<ffffffff81500dc2>] kasan_report.cold.6+0x242/0x2fe mm/kasan/report.c:412
 [<ffffffff814f3034>] __asan_report_load2_noabort+0x14/0x20 mm/kasan/report.c:431
 [<ffffffff82698d39>] __ip6_append_data.isra.3+0x2859/0x3460 net/ipv6/ip6_output.c:1329
 [<ffffffff82699b01>] ip6_append_data+0x1c1/0x2f0 net/ipv6/ip6_output.c:1645
 [<ffffffff827087ee>] udpv6_sendmsg+0x12fe/0x2430 net/ipv6/udp.c:1267
 [<ffffffff825952f3>] inet_sendmsg+0x203/0x4d0 net/ipv4/af_inet.c:770
 [<ffffffff822a063b>] sock_sendmsg_nosec net/socket.c:648 [inline]
 [<ffffffff822a063b>] sock_sendmsg+0xbb/0x110 net/socket.c:658
 [<ffffffff822a209a>] ___sys_sendmsg+0x47a/0x840 net/socket.c:1982
 [<ffffffff822a567c>] __sys_sendmmsg+0x23c/0x3d0 net/socket.c:2065
 [<ffffffff82390192>] C_SYSC_sendmmsg net/compat.c:742 [inline]
 [<ffffffff82390192>] compat_SyS_sendmmsg+0x32/0x40 net/compat.c:739
 [<ffffffff81006311>] do_syscall_32_irqs_on arch/x86/entry/common.c:328 [inline]
 [<ffffffff81006311>] do_fast_syscall_32+0x2f1/0xa10 arch/x86/entry/common.c:390
 [<ffffffff82818de0>] entry_SYSENTER_compat+0x90/0xa2 arch/x86/entry/entry_64_compat.S:137

The buggy address belongs to the page:
page:ffffea00071deb00 count:0 mapcount:-127 mapping:          (null) index:0x0
flags: 0x4000000000000000()
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801c77ac100: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff8801c77ac180: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff8801c77ac200: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                                        ^
 ffff8801c77ac280: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff8801c77ac300: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
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