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SubjectKASAN: null-ptr-deref Read in ip6_hold_safe

syzbot found the following crash on:

HEAD commit: b71acb0e3721 Merge branch 'linus' of git://
git tree: net-next
console output:
kernel config:
dashboard link:
compiler: gcc (GCC) 9.0.0 20181231 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:

kernel msg: ebtables bug: please report to author: EBT_ENTRY_OR_ENTRIES
shouldn't be set in distinguisher
BUG: KASAN: null-ptr-deref in atomic_read
include/asm-generic/atomic-instrumented.h:21 [inline]
BUG: KASAN: null-ptr-deref in atomic_fetch_add_unless
include/linux/atomic.h:575 [inline]
BUG: KASAN: null-ptr-deref in atomic_add_unless include/linux/atomic.h:597
BUG: KASAN: null-ptr-deref in dst_hold_safe include/net/dst.h:308 [inline]
BUG: KASAN: null-ptr-deref in ip6_hold_safe+0xca/0x620 net/ipv6/route.c:1021
Read of size 4 at addr 0000000000000068 by task syz-executor4/21421

CPU: 0 PID: 21421 Comm: syz-executor4 Not tainted 4.20.0+ #3
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+0x1db/0x2d0 lib/dump_stack.c:113
kasan_report_error mm/kasan/report.c:352 [inline]
kasan_report mm/kasan/report.c:412 [inline]
kasan_report.cold+0x19f/0x2ba mm/kasan/report.c:396
check_memory_region_inline mm/kasan/kasan.c:260 [inline]
check_memory_region+0x123/0x190 mm/kasan/kasan.c:267
kasan_check_read+0x11/0x20 mm/kasan/kasan.c:272
atomic_read include/asm-generic/atomic-instrumented.h:21 [inline]
atomic_fetch_add_unless include/linux/atomic.h:575 [inline]
atomic_add_unless include/linux/atomic.h:597 [inline]
dst_hold_safe include/net/dst.h:308 [inline]
ip6_hold_safe+0xca/0x620 net/ipv6/route.c:1021
rt6_get_pcpu_route net/ipv6/route.c:1241 [inline]
ip6_pol_route+0x3a3/0x1490 net/ipv6/route.c:1890
ip6_pol_route_output+0x54/0x70 net/ipv6/route.c:2066
fib6_rule_lookup+0x277/0x870 net/ipv6/fib6_rules.c:122
ip6_route_output_flags+0x2c4/0x350 net/ipv6/route.c:2095
ip6_route_output include/net/ip6_route.h:88 [inline]
ip6_dst_lookup_tail+0xe18/0x1d50 net/ipv6/ip6_output.c:958
ip6_dst_lookup_flow+0xca/0x270 net/ipv6/ip6_output.c:1086
ip6_sk_dst_lookup_flow net/ipv6/ip6_output.c:1124 [inline]
ip6_sk_dst_lookup_flow+0x5d6/0xbf0 net/ipv6/ip6_output.c:1114
udpv6_sendmsg+0x20d7/0x3550 net/ipv6/udp.c:1441
inet_sendmsg+0x1af/0x740 net/ipv4/af_inet.c:798
sock_sendmsg_nosec net/socket.c:621 [inline]
sock_sendmsg+0xdd/0x130 net/socket.c:631
___sys_sendmsg+0x7ec/0x910 net/socket.c:2116
__sys_sendmsg+0x112/0x270 net/socket.c:2154
__do_sys_sendmsg net/socket.c:2163 [inline]
__se_sys_sendmsg net/socket.c:2161 [inline]
__x64_sys_sendmsg+0x78/0xb0 net/socket.c:2161
protocol 88fb is buggy, dev hsr_slave_0
do_syscall_64+0x1a3/0x800 arch/x86/entry/common.c:290
protocol 88fb is buggy, dev hsr_slave_1
RIP: 0033:0x457ec9
Code: 6d b7 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 3b b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fb05ac4ac78 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000457ec9
RDX: 0000000000000000 RSI: 0000000020013000 RDI: 0000000000000004
RBP: 000000000073bf00 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007fb05ac4b6d4
R13: 00000000004c4d2e R14: 00000000004d8650 R15: 00000000ffffffff

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