KASAN: use-after-free Read in fscache_alloc_cookie

Status: fixed on 2020/11/16 12:12
Subsystems: fs
[Documentation on labels]
Fix commit: 1d0e850a49a5 afs: Fix cell removal
First crash: 1365d, last: 1365d
Cause bisection: introduced by (bisect log) :
commit c99c2171fc61476afac0dfb59fb2c447a01fb1e0
Author: David Howells <>
Date: Thu Nov 1 23:07:27 2018 +0000

  afs: Use fs_context to pass parameters over automount

Crash: KASAN: use-after-free Write in afs_manage_cell (log)
Repro: syz .config
Discussions (3)
Title Replies (including bot) Last reply
[PATCH 5.9 000/757] 5.9.2-rc1 review 766 (766) 2020/10/30 08:32
[PATCH 5.8 000/633] 5.8.17-rc1 review 638 (638) 2020/10/28 22:08
KASAN: use-after-free Read in fscache_alloc_cookie 1 (4) 2020/10/13 23:22
Last patch testing requests (1)
Created Duration User Patch Repo Result
2020/10/13 20:13 13m git:// f8eb8d1c6a853f617ca9ee233bb2d230401c5bdc report log

Sample crash report:
BUG: KASAN: use-after-free in memcpy include/linux/string.h:406 [inline]
BUG: KASAN: use-after-free in fscache_set_key fs/fscache/cookie.c:93 [inline]
BUG: KASAN: use-after-free in fscache_alloc_cookie+0xff/0x730 fs/fscache/cookie.c:153
Read of size 10 at addr ffff8880a6c28200 by task kworker/1:6/8760

CPU: 1 PID: 8760 Comm: kworker/1:6 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+0x198/0x1fd lib/dump_stack.c:118
 print_address_description.constprop.0.cold+0xae/0x497 mm/kasan/report.c:383
 __kasan_report mm/kasan/report.c:513 [inline]
 kasan_report.cold+0x1f/0x37 mm/kasan/report.c:530
 check_memory_region_inline mm/kasan/generic.c:186 [inline]
 check_memory_region+0x13d/0x180 mm/kasan/generic.c:192
 memcpy+0x20/0x60 mm/kasan/common.c:105
 memcpy include/linux/string.h:406 [inline]
 fscache_set_key fs/fscache/cookie.c:93 [inline]
 fscache_alloc_cookie+0xff/0x730 fs/fscache/cookie.c:153
 __fscache_acquire_cookie+0x16c/0x610 fs/fscache/cookie.c:288
 fscache_acquire_cookie include/linux/fscache.h:334 [inline]
 afs_activate_cell fs/afs/cell.c:609 [inline]
 afs_manage_cell+0x4fa/0x11c0 fs/afs/cell.c:697
 process_one_work+0x94c/0x1670 kernel/workqueue.c:2269
 worker_thread+0x64c/0x1120 kernel/workqueue.c:2415
 kthread+0x3b5/0x4a0 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Allocated by task 10197:
 kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
 kasan_set_track mm/kasan/common.c:56 [inline]
 __kasan_kmalloc.constprop.0+0xbf/0xd0 mm/kasan/common.c:461
 __do_kmalloc mm/slab.c:3655 [inline]
 __kmalloc+0x1b0/0x360 mm/slab.c:3664
 kmalloc include/linux/slab.h:559 [inline]
 afs_alloc_cell fs/afs/cell.c:157 [inline]
 afs_lookup_cell+0x5e9/0x1440 fs/afs/cell.c:262
 afs_parse_source fs/afs/super.c:290 [inline]
 afs_parse_param+0x404/0x8c0 fs/afs/super.c:326
 vfs_parse_fs_param fs/fs_context.c:117 [inline]
 vfs_parse_fs_param+0x203/0x550 fs/fs_context.c:98
 vfs_parse_fs_string+0xe6/0x150 fs/fs_context.c:161
 generic_parse_monolithic+0x16f/0x1f0 fs/fs_context.c:201
 do_new_mount fs/namespace.c:2871 [inline]
 path_mount+0x133f/0x20a0 fs/namespace.c:3192
 do_mount fs/namespace.c:3205 [inline]
 __do_sys_mount fs/namespace.c:3413 [inline]
 __se_sys_mount fs/namespace.c:3390 [inline]
 __x64_sys_mount+0x27f/0x300 fs/namespace.c:3390
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46

Freed by task 16:
 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+0xd8/0x120 mm/kasan/common.c:422
 __cache_free mm/slab.c:3418 [inline]
 kfree+0x10e/0x2b0 mm/slab.c:3756
 afs_cell_destroy+0x1b0/0x240 fs/afs/cell.c:500
 rcu_do_batch kernel/rcu/tree.c:2428 [inline]
 rcu_core+0x5ca/0x1130 kernel/rcu/tree.c:2656
 __do_softirq+0x1f8/0xb23 kernel/softirq.c:298

The buggy address belongs to the object at ffff8880a6c28200
 which belongs to the cache kmalloc-32 of size 32
The buggy address is located 0 bytes inside of
 32-byte region [ffff8880a6c28200, ffff8880a6c28220)
The buggy address belongs to the page:
page:00000000c2f50dc7 refcount:1 mapcount:0 mapping:0000000000000000 index:0xffff8880a6c28fc1 pfn:0xa6c28
flags: 0xfffe0000000200(slab)
raw: 00fffe0000000200 ffffea00028969c8 ffffea00025ca748 ffff8880aa040100
raw: ffff8880a6c28fc1 ffff8880a6c28000 000000010000003f 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8880a6c28100: fa fb fb fb fc fc fc fc fa fb fb fb fc fc fc fc
 ffff8880a6c28180: fa fb fb fb fc fc fc fc fa fb fb fb fc fc fc fc
>ffff8880a6c28200: fa fb fb fb fc fc fc fc fa fb fb fb fc fc fc fc
 ffff8880a6c28280: 00 00 00 00 fc fc fc fc 00 fc fc fc fc fc fc fc
 ffff8880a6c28300: fa fb fb fb fc fc fc fc fa fb fb fb fc fc fc fc

Crashes (1):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2020/09/25 03:45 upstream 171d4ff79f96 54289b08 .config console log report syz ci-upstream-kasan-gce-root
* Struck through repros no longer work on HEAD.