Re: KASAN: slab-out-of-bounds Read in decode_session6

[syzbot]

syzbot has found a reproducer for the following issue on:

HEAD commit:    c43fd36f net: bridge: mcast: fix stub definition of br_mul..
git tree:       net-next
console output: https://syzkaller.appspot.com/x/log.txt?x=12677e92500000
kernel config:  https://syzkaller.appspot.com/x/.config?x=eac680ae76558a0e
dashboard link: https://syzkaller.appspot.com/bug?extid=2bcc71839223ec82f056
compiler:       gcc (GCC) 10.1.0-syz 20200507
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15e1b946500000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1143c192500000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+2bcc71839223ec82f056@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KASAN: slab-out-of-bounds in decode_session6+0xe7c/0x1580 net/xfrm/xfrm_policy.c:3393
Read of size 1 at addr ffff8880247cb8af by task syz-executor222/8528

CPU: 0 PID: 8528 Comm: syz-executor222 Not tainted 5.10.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 <IRQ>
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x107/0x163 lib/dump_stack.c:118
 print_address_description.constprop.0.cold+0xae/0x4c8 mm/kasan/report.c:385
 __kasan_report mm/kasan/report.c:545 [inline]
 kasan_report.cold+0x1f/0x37 mm/kasan/report.c:562
 decode_session6+0xe7c/0x1580 net/xfrm/xfrm_policy.c:3393
 __xfrm_decode_session net/xfrm/xfrm_policy.c:3485 [inline]
 __xfrm_policy_check+0x2fa/0x2850 net/xfrm/xfrm_policy.c:3540
 __xfrm_policy_check2 include/net/xfrm.h:1097 [inline]
 xfrm_policy_check include/net/xfrm.h:1106 [inline]
 sctp_rcv+0x12b0/0x2e30 net/sctp/input.c:202
 sctp6_rcv+0x22/0x40 net/sctp/ipv6.c:1078
 ip6_protocol_deliver_rcu+0x2e8/0x1680 net/ipv6/ip6_input.c:433
 ip6_input_finish+0x7f/0x160 net/ipv6/ip6_input.c:474
 NF_HOOK include/linux/netfilter.h:301 [inline]
 NF_HOOK include/linux/netfilter.h:295 [inline]
 ip6_input+0x9c/0xd0 net/ipv6/ip6_input.c:483
 dst_input include/net/dst.h:449 [inline]
 ip6_rcv_finish net/ipv6/ip6_input.c:76 [inline]
 NF_HOOK include/linux/netfilter.h:301 [inline]
 NF_HOOK include/linux/netfilter.h:295 [inline]
 ipv6_rcv+0x28e/0x3c0 net/ipv6/ip6_input.c:307
 __netif_receive_skb_one_core+0x114/0x180 net/core/dev.c:5315
 __netif_receive_skb+0x27/0x1c0 net/core/dev.c:5429
 process_backlog+0x232/0x6c0 net/core/dev.c:6319
 napi_poll net/core/dev.c:6763 [inline]
 net_rx_action+0x4dc/0x1100 net/core/dev.c:6833
 __do_softirq+0x2a0/0x9f6 kernel/softirq.c:298
 asm_call_irq_on_stack+0xf/0x20
 </IRQ>
 __run_on_irqstack arch/x86/include/asm/irq_stack.h:26 [inline]
 run_on_irqstack_cond arch/x86/include/asm/irq_stack.h:77 [inline]
 do_softirq_own_stack+0xaa/0xd0 arch/x86/kernel/irq_64.c:77
 do_softirq kernel/softirq.c:343 [inline]
 do_softirq+0xb5/0xe0 kernel/softirq.c:330
 __local_bh_enable_ip+0xf0/0x110 kernel/softirq.c:195
 local_bh_enable include/linux/bottom_half.h:32 [inline]
 rcu_read_unlock_bh include/linux/rcupdate.h:730 [inline]
 ip6_finish_output2+0x71f/0x16c0 net/ipv6/ip6_output.c:118
 __ip6_finish_output net/ipv6/ip6_output.c:143 [inline]
 __ip6_finish_output+0x447/0xab0 net/ipv6/ip6_output.c:128
 ip6_finish_output+0x34/0x1f0 net/ipv6/ip6_output.c:153
 NF_HOOK_COND include/linux/netfilter.h:290 [inline]
 ip6_output+0x1db/0x520 net/ipv6/ip6_output.c:176
 dst_output include/net/dst.h:443 [inline]
 NF_HOOK include/linux/netfilter.h:301 [inline]
 NF_HOOK include/linux/netfilter.h:295 [inline]
 ip6_xmit+0x1258/0x1e80 net/ipv6/ip6_output.c:280
 sctp_v6_xmit+0xbf3/0xfe0 net/sctp/ipv6.c:223
 sctp_packet_transmit+0x1f44/0x32f0 net/sctp/output.c:627
 sctp_packet_singleton net/sctp/outqueue.c:773 [inline]
 sctp_outq_flush_ctrl.constprop.0+0x6d3/0xc40 net/sctp/outqueue.c:904
 sctp_outq_flush+0xf3/0x2580 net/sctp/outqueue.c:1186
 sctp_cmd_interpreter net/sctp/sm_sideeffect.c:1801 [inline]
 sctp_side_effects net/sctp/sm_sideeffect.c:1185 [inline]
 sctp_do_sm+0x74e/0x5130 net/sctp/sm_sideeffect.c:1156
 sctp_primitive_ASSOCIATE+0x98/0xc0 net/sctp/primitive.c:73
 sctp_sendmsg_to_asoc+0xb5b/0x2140 net/sctp/socket.c:1823
 sctp_sendmsg+0x103b/0x1d30 net/sctp/socket.c:2013
 inet_sendmsg+0x99/0xe0 net/ipv4/af_inet.c:817
 sock_sendmsg_nosec net/socket.c:651 [inline]
 sock_sendmsg+0xcf/0x120 net/socket.c:671
 __sys_sendto+0x21c/0x320 net/socket.c:1992
 __do_sys_sendto net/socket.c:2004 [inline]
 __se_sys_sendto net/socket.c:2000 [inline]
 __x64_sys_sendto+0xdd/0x1b0 net/socket.c:2000
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x441759
Code: 18 89 d0 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 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 13 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ffc188919d8 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 00007ffc188919f0 RCX: 0000000000441759
RDX: 0000000000034000 RSI: 0000000020847fff RDI: 0000000000000004
RBP: 0000000000000000 R08: 000000002005ffe4 R09: 000000000000001c
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000402f60
R13: 0000000000402ff0 R14: 0000000000000000 R15: 0000000000000000

Allocated by task 1:
 kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
 kasan_set_track mm/kasan/common.c:56 [inline]
 __kasan_kmalloc.constprop.0+0xc2/0xd0 mm/kasan/common.c:461
 kmalloc include/linux/slab.h:557 [inline]
 tomoyo_realpath_from_path+0xc3/0x620 security/tomoyo/realpath.c:254
 tomoyo_get_realpath security/tomoyo/file.c:151 [inline]
 tomoyo_path_perm+0x21b/0x400 security/tomoyo/file.c:822
 security_inode_getattr+0xcf/0x140 security/security.c:1279
 vfs_getattr fs/stat.c:121 [inline]
 vfs_statx+0x164/0x390 fs/stat.c:189
 vfs_fstatat fs/stat.c:207 [inline]
 vfs_lstat include/linux/fs.h:3109 [inline]
 __do_sys_newlstat+0x91/0x110 fs/stat.c:362
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Freed by task 1:
 kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
 kasan_set_track+0x1c/0x30 mm/kasan/common.c:56
 kasan_set_free_info+0x1b/0x30 mm/kasan/generic.c:355
 __kasan_slab_free+0x102/0x140 mm/kasan/common.c:422
 slab_free_hook mm/slub.c:1544 [inline]
 slab_free_freelist_hook+0x5d/0x150 mm/slub.c:1577
 slab_free mm/slub.c:3142 [inline]
 kfree+0xdb/0x360 mm/slub.c:4124
 tomoyo_realpath_from_path+0x191/0x620 security/tomoyo/realpath.c:291
 tomoyo_get_realpath security/tomoyo/file.c:151 [inline]
 tomoyo_path_perm+0x21b/0x400 security/tomoyo/file.c:822
 security_inode_getattr+0xcf/0x140 security/security.c:1279
 vfs_getattr fs/stat.c:121 [inline]
 vfs_statx+0x164/0x390 fs/stat.c:189
 vfs_fstatat fs/stat.c:207 [inline]
 vfs_lstat include/linux/fs.h:3109 [inline]
 __do_sys_newlstat+0x91/0x110 fs/stat.c:362
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

The buggy address belongs to the object at ffff8880247ca000
 which belongs to the cache kmalloc-4k of size 4096
The buggy address is located 2223 bytes to the right of
 4096-byte region [ffff8880247ca000, ffff8880247cb000)
The buggy address belongs to the page:
page:00000000ecac6d17 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x247c8
head:00000000ecac6d17 order:3 compound_mapcount:0 compound_pincount:0
flags: 0xfff00000010200(slab|head)
raw: 00fff00000010200 dead000000000100 dead000000000122 ffff888010042140
raw: 0000000000000000 0000000000040004 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8880247cb780: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff8880247cb800: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff8880247cb880: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
                                  ^
 ffff8880247cb900: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff8880247cb980: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================