KASAN: use-after-free Read in css_task_iter_advance

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 56b697c6 Add linux-next specific files for 20190604
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1310e25aa00000
kernel config: https://syzkaller.appspot.com/x/.config?x=4248d6bc70076f7d
dashboard link: https://syzkaller.appspot.com/bug?extid=678796542f88f534c79e
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:
Reported-by: syzbot+678796...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: use-after-free in css_task_iter_advance+0x49b/0x540
kernel/cgroup/cgroup.c:4507
Read of size 4 at addr ffff888096f20fe4 by task syz-executor.4/24150

CPU: 1 PID: 24150 Comm: syz-executor.4 Not tainted 5.2.0-rc3-next-20190604
#8
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+0x172/0x1f0 lib/dump_stack.c:113
print_address_description.cold+0xd4/0x306 mm/kasan/report.c:351
__kasan_report.cold+0x1b/0x36 mm/kasan/report.c:482
kasan_report+0x12/0x20 mm/kasan/common.c:614
__asan_report_load4_noabort+0x14/0x20 mm/kasan/generic_report.c:131
css_task_iter_advance+0x49b/0x540 kernel/cgroup/cgroup.c:4507
css_task_iter_start+0x18b/0x230 kernel/cgroup/cgroup.c:4543
__cgroup_procs_start.isra.0+0x32f/0x400 kernel/cgroup/cgroup.c:4634
cgroup_threads_start+0x23/0x30 kernel/cgroup/cgroup.c:4741
cgroup_seqfile_start+0xa4/0xd0 kernel/cgroup/cgroup.c:3752
kernfs_seq_start+0xdc/0x190 fs/kernfs/file.c:118
traverse fs/seq_file.c:108 [inline]
traverse+0x165/0x740 fs/seq_file.c:91
seq_read+0x8dd/0x1110 fs/seq_file.c:188
kernfs_fop_read+0xed/0x560 fs/kernfs/file.c:252
do_loop_readv_writev fs/read_write.c:714 [inline]
do_loop_readv_writev fs/read_write.c:701 [inline]
do_iter_read+0x4a4/0x660 fs/read_write.c:935
vfs_readv+0xf0/0x160 fs/read_write.c:997
do_preadv+0x1c4/0x280 fs/read_write.c:1089
__do_sys_preadv fs/read_write.c:1139 [inline]
__se_sys_preadv fs/read_write.c:1134 [inline]
__x64_sys_preadv+0x9a/0xf0 fs/read_write.c:1134
do_syscall_64+0xfd/0x680 arch/x86/entry/common.c:301
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x459279
Code: fd 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 cb b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f55b7e66c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000127
RAX: ffffffffffffffda RBX: 0000000000000004 RCX: 0000000000459279
RDX: 0000000000000001 RSI: 0000000020000b40 RDI: 0000000000000004
RBP: 000000000075bf20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000002 R11: 0000000000000246 R12: 00007f55b7e676d4
R13: 00000000004c614c R14: 00000000004da9a8 R15: 00000000ffffffff

Allocated by task 8993:
save_stack+0x23/0x90 mm/kasan/common.c:71
set_track mm/kasan/common.c:79 [inline]
__kasan_kmalloc mm/kasan/common.c:489 [inline]
__kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:462
kasan_slab_alloc+0xf/0x20 mm/kasan/common.c:497
slab_post_alloc_hook mm/slab.h:444 [inline]
slab_alloc mm/slab.c:3320 [inline]
kmem_cache_alloc+0x11a/0x6f0 mm/slab.c:3482
getname_flags fs/namei.c:138 [inline]
getname_flags+0xd6/0x5b0 fs/namei.c:128
getname+0x1a/0x20 fs/namei.c:209
do_sys_open+0x2c9/0x5d0 fs/open.c:1064
__do_sys_open fs/open.c:1088 [inline]
__se_sys_open fs/open.c:1083 [inline]
__x64_sys_open+0x7e/0xc0 fs/open.c:1083
do_syscall_64+0xfd/0x680 arch/x86/entry/common.c:301
entry_SYSCALL_64_after_hwframe+0x49/0xbe

Freed by task 8993:
save_stack+0x23/0x90 mm/kasan/common.c:71
set_track mm/kasan/common.c:79 [inline]
__kasan_slab_free+0x102/0x150 mm/kasan/common.c:451
kasan_slab_free+0xe/0x10 mm/kasan/common.c:459
__cache_free mm/slab.c:3426 [inline]
kmem_cache_free+0x86/0x320 mm/slab.c:3692
putname+0xef/0x130 fs/namei.c:259
do_sys_open+0x318/0x5d0 fs/open.c:1079
__do_sys_open fs/open.c:1088 [inline]
__se_sys_open fs/open.c:1083 [inline]
__x64_sys_open+0x7e/0xc0 fs/open.c:1083
do_syscall_64+0xfd/0x680 arch/x86/entry/common.c:301
entry_SYSCALL_64_after_hwframe+0x49/0xbe

The buggy address belongs to the object at ffff888096f20d40
which belongs to the cache names_cache of size 4096
The buggy address is located 676 bytes inside of
4096-byte region [ffff888096f20d40, ffff888096f21d40)
The buggy address belongs to the page:
page:ffffea00025bc800 refcount:1 mapcount:0 mapping:ffff8880aa593ac0
index:0x0 compound_mapcount: 0
flags: 0x1fffc0000010200(slab|head)
raw: 01fffc0000010200 ffffea0002a01388 ffffea0002315988 ffff8880aa593ac0
raw: 0000000000000000 ffff888096f20d40 0000000100000001 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff888096f20e80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff888096f20f00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ffff888096f20f80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff888096f21000: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff888096f21080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================


---
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#syz fix: cgroup: Fix css_task_iter_advance_css_set() cset skip condition