Re: KASAN: use-after-free Read in fscache_alloc_cookie

[syzbot]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
WARNING: proc registration bug in afs_manage_cell_work

------------[ cut here ]------------
proc_dir_entry 'afs/^]$[+%]0${' already registered
WARNING: CPU: 0 PID: 8309 at fs/proc/generic.c:371 proc_register+0x34c/0x700 fs/proc/generic.c:371
Kernel panic - not syncing: panic_on_warn set ...
CPU: 0 PID: 8309 Comm: kworker/0:4 Not tainted 5.9.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: afs afs_manage_cell_work
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x198/0x1fd lib/dump_stack.c:118
 panic+0x382/0x7fb kernel/panic.c:231
 __warn.cold+0x20/0x4b kernel/panic.c:600
 report_bug+0x1bd/0x210 lib/bug.c:198
 handle_bug+0x38/0x90 arch/x86/kernel/traps.c:234
 exc_invalid_op+0x14/0x40 arch/x86/kernel/traps.c:254
 asm_exc_invalid_op+0x12/0x20 arch/x86/include/asm/idtentry.h:536
RIP: 0010:proc_register+0x34c/0x700 fs/proc/generic.c:371
Code: df 48 89 f9 48 c1 e9 03 80 3c 01 00 0f 85 5d 03 00 00 48 8b 44 24 28 48 c7 c7 a0 62 9a 88 48 8b b0 d8 00 00 00 e8 46 03 5d ff <0f> 0b 48 c7 c7 e0 f6 1e 8a e8 36 df 3d 06 48 8b 4c 24 38 48 b8 00
RSP: 0018:ffffc90009dffac8 EFLAGS: 00010282
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: ffff8880923a4480 RSI: ffffffff815f5a55 RDI: fffff520013bff4b
RBP: ffff888088829b88 R08: 0000000000000001 R09: ffff8880ae4318e7
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8880904699d8
R13: ffff8880931d3340 R14: dffffc0000000000 R15: 000000000000000a
 proc_mkdir_data+0x140/0x1a0 fs/proc/generic.c:487
 proc_net_mkdir include/linux/proc_fs.h:201 [inline]
 afs_proc_cell_setup+0xb2/0x1f0 fs/afs/proc.c:619
 afs_activate_cell fs/afs/cell.c:684 [inline]
 afs_manage_cell fs/afs/cell.c:768 [inline]
 afs_manage_cell_work+0x5b7/0x11c0 fs/afs/cell.c:832
 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
Kernel Offset: disabled
Rebooting in 86400 seconds..


Tested on:

commit:         f8eb8d1c afs: Add tracing for cell refcount and active use..
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git
console output: https://syzkaller.appspot.com/x/log.txt?x=1414b0d8500000
kernel config:  https://syzkaller.appspot.com/x/.config?x=302928762dfb5528
dashboard link: https://syzkaller.appspot.com/bug?extid=2d0585e5efcd43d113c2
compiler:       gcc (GCC) 10.1.0-syz 20200507