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SubjectKASAN: use-after-free Read in __proc_create

syzbot found the following issue on:

HEAD commit: 7c7ec322 Merge tag 'for-linus' of git://
git tree: upstream
console output:
kernel config:
dashboard link:
compiler: clang version 10.0.0 ( c2443155a0fb245c8f17f2c1c72b6ea391e86e81)

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

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

BUG: KASAN: use-after-free in strlen+0x4c/0x60 lib/string.c:544
Read of size 1 at addr ffff8880a29662c0 by task kworker/1:1/23

CPU: 1 PID: 23 Comm: kworker/1:1 Not tainted 5.9.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: afs afs_manage_cell
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x1d6/0x29e lib/dump_stack.c:118
print_address_description+0x66/0x620 mm/kasan/report.c:383
__kasan_report mm/kasan/report.c:513 [inline]
kasan_report+0x132/0x1d0 mm/kasan/report.c:530
strlen+0x4c/0x60 lib/string.c:544
__proc_create+0x28d/0xb60 fs/proc/generic.c:399
proc_mkdir_data+0x84/0x160 fs/proc/generic.c:482
proc_net_mkdir include/linux/proc_fs.h:201 [inline]
afs_proc_cell_setup+0xaf/0x180 fs/afs/proc.c:620
afs_activate_cell fs/afs/cell.c:615 [inline]
afs_manage_cell+0x7db/0x1540 fs/afs/cell.c:697
process_one_work+0x789/0xfc0 kernel/workqueue.c:2269
process_scheduled_works kernel/workqueue.c:2331 [inline]
worker_thread+0xde4/0x1460 kernel/workqueue.c:2417
kthread+0x37e/0x3a0 drivers/block/aoe/aoecmd.c:1234
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Allocated by task 8801:
kasan_save_stack mm/kasan/common.c:48 [inline]
kasan_set_track mm/kasan/common.c:56 [inline]
__kasan_kmalloc+0x100/0x130 mm/kasan/common.c:461
__do_kmalloc mm/slab.c:3655 [inline]
__kmalloc+0x205/0x300 mm/slab.c:3664
kmalloc include/linux/slab.h:559 [inline]
afs_alloc_cell fs/afs/cell.c:157 [inline]
afs_lookup_cell+0x4d2/0x14d0 fs/afs/cell.c:262
afs_parse_source fs/afs/super.c:290 [inline]
afs_parse_param+0x526/0x790 fs/afs/super.c:326
vfs_parse_fs_param+0x1e5/0x460 fs/fs_context.c:117
vfs_parse_fs_string fs/fs_context.c:161 [inline]
generic_parse_monolithic+0x230/0x350 fs/fs_context.c:201
do_new_mount fs/namespace.c:2871 [inline]
path_mount+0x176c/0x29e0 fs/namespace.c:3192
do_mount fs/namespace.c:3205 [inline]
__do_sys_mount fs/namespace.c:3413 [inline]
__se_sys_mount+0x126/0x180 fs/namespace.c:3390
do_syscall_64+0x31/0x70 arch/x86/entry/common.c:46

Freed by task 8668:
kasan_save_stack mm/kasan/common.c:48 [inline]
kasan_set_track+0x3d/0x70 mm/kasan/common.c:56
kasan_set_free_info+0x17/0x30 mm/kasan/generic.c:355
__kasan_slab_free+0xdd/0x110 mm/kasan/common.c:422
__cache_free mm/slab.c:3418 [inline]
kfree+0x113/0x200 mm/slab.c:3756
afs_cell_destroy+0x16b/0x240 fs/afs/cell.c:500
rcu_do_batch kernel/rcu/tree.c:2430 [inline]
rcu_core+0x79b/0x1130 kernel/rcu/tree.c:2658
__do_softirq+0x256/0x6d5 kernel/softirq.c:298

The buggy address belongs to the object at ffff8880a29662c0
which belongs to the cache kmalloc-32 of size 32
The buggy address is located 0 bytes inside of
32-byte region [ffff8880a29662c0, ffff8880a29662e0)
The buggy address belongs to the page:
page:00000000f15c0556 refcount:1 mapcount:0 mapping:0000000000000000 index:0xffff8880a2966fc1 pfn:0xa2966
flags: 0xfffe0000000200(slab)
raw: 00fffe0000000200 ffffea00028f7d48 ffffea0002a22248 ffff8880aa440100
raw: ffff8880a2966fc1 ffff8880a2966000 000000010000003f 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff8880a2966180: fa fb fb fb fc fc fc fc fa fb fb fb fc fc fc fc
ffff8880a2966200: fa fb fb fb fc fc fc fc fa fb fb fb fc fc fc fc
>ffff8880a2966280: 00 00 03 fc fc fc fc fc fa fb fb fb fc fc fc fc
ffff8880a2966300: 00 07 fc fc fc fc fc fc fa fb fb fb fc fc fc fc
ffff8880a2966380: fa fb fb fb fc fc fc fc 00 00 00 00 fc fc fc fc

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