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

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: f313b43be461 Merge branch 'i2c/for-current' of git://git.k..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=16c1da3c400000
kernel config: https://syzkaller.appspot.com/x/.config?x=152cb8ccd35b1f70
dashboard link: https://syzkaller.appspot.com/bug?extid=87c3c541582e56943277
compiler: gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=11f38e22400000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=165d5c02400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+87c3c5...@syzkaller.appspotmail.com

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 _autofs_dev_ioctl+0x8f5/0x990
fs/autofs/dev-ioctl.c:670
Read of size 4 at addr ffff8801d98bc340 by task syz-executor124/4446

CPU: 1 PID: 4446 Comm: syz-executor124 Not tainted 4.18.0-rc8+ #183
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+0x1c9/0x2b4 lib/dump_stack.c:113
print_address_description+0x6c/0x20b mm/kasan/report.c:256
kasan_report_error mm/kasan/report.c:354 [inline]
kasan_report.cold.7+0x242/0x2fe mm/kasan/report.c:412
__asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:432
_autofs_dev_ioctl+0x8f5/0x990 fs/autofs/dev-ioctl.c:670
autofs_dev_ioctl+0x1b/0x30 fs/autofs/dev-ioctl.c:704
vfs_ioctl fs/ioctl.c:46 [inline]
file_ioctl fs/ioctl.c:500 [inline]
do_vfs_ioctl+0x1de/0x1720 fs/ioctl.c:684
ksys_ioctl+0xa9/0xd0 fs/ioctl.c:701
__do_sys_ioctl fs/ioctl.c:708 [inline]
__se_sys_ioctl fs/ioctl.c:706 [inline]
__x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:706
do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x440139
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 fb 13 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fff94b42b78 EFLAGS: 00000217 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0030656c69662f2e RCX: 0000000000440139
RDX: 0000000020000180 RSI: 800000000000937e RDI: 0000000000000005
RBP: 00000000006ca018 R08: 0000000000000000 R09: 00000000004002c8
R10: 0000000000000000 R11: 0000000000000217 R12: 00000000004019c0
R13: 0000000000401a50 R14: 0000000000000000 R15: 0000000000000000

Allocated by task 4446:
save_stack+0x43/0xd0 mm/kasan/kasan.c:448
set_track mm/kasan/kasan.c:460 [inline]
kasan_kmalloc+0xc4/0xe0 mm/kasan/kasan.c:553
kmem_cache_alloc_trace+0x152/0x780 mm/slab.c:3620
kmalloc include/linux/slab.h:513 [inline]
kzalloc include/linux/slab.h:707 [inline]
ramfs_fill_super+0xc4/0x580 fs/ramfs/inode.c:226
mount_nodev+0x6b/0x110 fs/super.c:1220
ramfs_mount+0x2c/0x40 fs/ramfs/inode.c:253
mount_fs+0xae/0x328 fs/super.c:1277
vfs_kern_mount.part.34+0xdc/0x4e0 fs/namespace.c:1037
vfs_kern_mount fs/namespace.c:1027 [inline]
do_new_mount fs/namespace.c:2518 [inline]
do_mount+0x581/0x30e0 fs/namespace.c:2848
ksys_mount+0x12d/0x140 fs/namespace.c:3064
__do_sys_mount fs/namespace.c:3078 [inline]
__se_sys_mount fs/namespace.c:3075 [inline]
__x64_sys_mount+0xbe/0x150 fs/namespace.c:3075
do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x49/0xbe

Freed by task 2730:
save_stack+0x43/0xd0 mm/kasan/kasan.c:448
set_track mm/kasan/kasan.c:460 [inline]
__kasan_slab_free+0x11a/0x170 mm/kasan/kasan.c:521
kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:528
__cache_free mm/slab.c:3498 [inline]
kfree+0xd9/0x260 mm/slab.c:3813
single_release+0x8f/0xb0 fs/seq_file.c:609
__fput+0x355/0x8b0 fs/file_table.c:209
____fput+0x15/0x20 fs/file_table.c:243
task_work_run+0x1ec/0x2a0 kernel/task_work.c:113
tracehook_notify_resume include/linux/tracehook.h:192 [inline]
exit_to_usermode_loop+0x313/0x370 arch/x86/entry/common.c:166
prepare_exit_to_usermode arch/x86/entry/common.c:197 [inline]
syscall_return_slowpath arch/x86/entry/common.c:268 [inline]
do_syscall_64+0x6be/0x820 arch/x86/entry/common.c:293
entry_SYSCALL_64_after_hwframe+0x49/0xbe

The buggy address belongs to the object at ffff8801d98bc340
which belongs to the cache kmalloc-32 of size 32
The buggy address is located 0 bytes inside of
32-byte region [ffff8801d98bc340, ffff8801d98bc360)
The buggy address belongs to the page:
page:ffffea0007662f00 count:1 mapcount:0 mapping:ffff8801dac001c0
index:0xffff8801d98bcfc1
flags: 0x2fffc0000000100(slab)
raw: 02fffc0000000100 ffffea00072a0088 ffffea00072fccc8 ffff8801dac001c0
raw: ffff8801d98bcfc1 ffff8801d98bc000 0000000100000022 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff8801d98bc200: fb fb fb fb fc fc fc fc fb fb fb fb fc fc fc fc
ffff8801d98bc280: fb fb fb fb fc fc fc fc 00 00 fc fc fc fc fc fc
> ffff8801d98bc300: fb fb fb fb fc fc fc fc 02 fc fc fc fc fc fc fc
^
ffff8801d98bc380: fb fb fb fb fc fc fc fc fb fb fb fb fc fc fc fc
ffff8801d98bc400: fb fb fb fb fc fc fc fc fb fb fb fb fc fc fc fc
==================================================================


---
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[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger
crash:

Reported-and-tested-by:
syzbot+87c3c5...@syzkaller.appspotmail.com

Tested on:

commit: b7a6cee59259 autofs - fix autofs_sbi()
git tree:
git://git.kernel.org/pub/scm/linux/kernel/git/raven/syzbot-bug.git/syzbot
kernel config: https://syzkaller.appspot.com/x/.config?x=d51a00c7aa060a55

compiler: gcc (GCC) 8.0.1 20180413 (experimental)

Note: testing is done by a robot and is best-effort only.