KASAN: slab-out-of-bounds Read in mqueue_get_tree

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    7c1b097f27bf Add linux-next specific files for 20180912
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=10ce5081400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=5980033172920ec0
dashboard link: https://syzkaller.appspot.com/bug?extid=fc036355de0dd1b312b1
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1167ea56400000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=110b36ae400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+fc036355de0dd1b312b1@xxxxxxxxxxxxxxxxxxxxxxxxx

random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
==================================================================
BUG: KASAN: slab-out-of-bounds in mqueue_get_tree+0x2ac/0x2e0  
ipc/mqueue.c:362
Read of size 8 at addr ffff8801d8bce3e0 by task syz-executor656/5564

CPU: 0 PID: 5564 Comm: syz-executor656 Not tainted  
4.19.0-rc3-next-20180912+ #72
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+0x1d3/0x2c4 lib/dump_stack.c:113
 print_address_description.cold.8+0x9/0x1ff mm/kasan/report.c:256
 kasan_report_error mm/kasan/report.c:354 [inline]
 kasan_report.cold.9+0x242/0x309 mm/kasan/report.c:412
 __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:433
 mqueue_get_tree+0x2ac/0x2e0 ipc/mqueue.c:362
 vfs_get_tree+0x1cb/0x5c0 fs/super.c:1787
 mq_create_mount+0xe3/0x190 ipc/mqueue.c:415
 mq_init_ns+0x15a/0x210 ipc/mqueue.c:1621
 create_ipc_ns ipc/namespace.c:58 [inline]
 copy_ipcs+0x3d2/0x580 ipc/namespace.c:84
 create_new_namespaces+0x376/0x900 kernel/nsproxy.c:87
 unshare_nsproxy_namespaces+0xc3/0x1f0 kernel/nsproxy.c:206
 ksys_unshare+0x79c/0x10b0 kernel/fork.c:2535
 __do_sys_unshare kernel/fork.c:2603 [inline]
 __se_sys_unshare kernel/fork.c:2601 [inline]
 __x64_sys_unshare+0x31/0x40 kernel/fork.c:2601
 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x4447f7
Code: 00 00 00 b8 63 00 00 00 0f 05 48 3d 01 f0 ff ff 0f 83 ed df fb ff c3  
66 2e 0f 1f 84 00 00 00 00 00 66 90 b8 10 01 00 00 0f 05 <48> 3d 01 f0 ff  
ff 0f 83 cd df fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ffe5aa7e2a8 EFLAGS: 00000217 ORIG_RAX: 0000000000000110
RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 00000000004447f7
RDX: 0000000000000000 RSI: 00007ffe5aa7e2b0 RDI: 0000000008000000
RBP: 00000000006cc018 R08: 0000000000000000 R09: 00000000019a3880
R10: 0000000000000000 R11: 0000000000000217 R12: 0000000000402c50
R13: 0000000000402ce0 R14: 0000000000000000 R15: 0000000000000000

Allocated by task 3455:
 save_stack+0x43/0xd0 mm/kasan/kasan.c:448
 set_track mm/kasan/kasan.c:460 [inline]
 kasan_kmalloc+0xc7/0xe0 mm/kasan/kasan.c:553
 kmem_cache_alloc_trace+0x152/0x750 mm/slab.c:3620
 kmalloc include/linux/slab.h:513 [inline]
 kzalloc include/linux/slab.h:707 [inline]
 kernfs_iop_get_link+0x6d/0x660 fs/kernfs/symlink.c:133
 vfs_readlink+0x2ac/0x4c0 fs/namei.c:4736
 do_readlinkat+0x359/0x410 fs/stat.c:409
 __do_sys_readlink fs/stat.c:430 [inline]
 __se_sys_readlink fs/stat.c:427 [inline]
 __x64_sys_readlink+0x78/0xb0 fs/stat.c:427
 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Freed by task 3455:
 save_stack+0x43/0xd0 mm/kasan/kasan.c:448
 set_track mm/kasan/kasan.c:460 [inline]
 __kasan_slab_free+0x102/0x150 mm/kasan/kasan.c:521
 kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:528
 __cache_free mm/slab.c:3498 [inline]
 kfree+0xcf/0x230 mm/slab.c:3813
 kfree_link+0x15/0x20 fs/libfs.c:1123
 do_delayed_call include/linux/delayed_call.h:28 [inline]
 vfs_readlink+0x207/0x4c0 fs/namei.c:4741
 do_readlinkat+0x359/0x410 fs/stat.c:409
 __do_sys_readlink fs/stat.c:430 [inline]
 __se_sys_readlink fs/stat.c:427 [inline]
 __x64_sys_readlink+0x78/0xb0 fs/stat.c:427
 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

The buggy address belongs to the object at ffff8801d8bce740
 which belongs to the cache kmalloc-4096 of size 4096
The buggy address is located 864 bytes to the left of
 4096-byte region [ffff8801d8bce740, ffff8801d8bcf740)
The buggy address belongs to the page:
page:ffffea000762f380 count:1 mapcount:0 mapping:ffff8801da800dc0 index:0x0  
compound_mapcount: 0
flags: 0x2fffc0000008100(slab|head)
raw: 02fffc0000008100 ffffea0007628988 ffffea0007623008 ffff8801da800dc0
raw: 0000000000000000 ffff8801d8bce740 0000000100000001 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801d8bce280: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff8801d8bce300: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff8801d8bce380: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
                                                       ^
 ffff8801d8bce400: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff8801d8bce480: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================


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