[syzbot] WARNING: refcount bug in p9_client_walk

[syzbot <syzbot+2600f43a81c05675a9ae@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    9500fc6e9e60 Merge branch 'for-next/core' into for-kernelci
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci
console output: https://syzkaller.appspot.com/x/log.txt?x=1519526e880000
kernel config:  https://syzkaller.appspot.com/x/.config?x=b25c9f218686dd5e
dashboard link: https://syzkaller.appspot.com/bug?extid=2600f43a81c05675a9ae
compiler:       Debian clang version 13.0.1-++20220126092033+75e33f71c2da-1~exp1~20220126212112.63, GNU ld (GNU Binutils for Debian) 2.35.2
userspace arch: arm64
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15ff2aed880000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13b38365880000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/1363e60652f7/disk-9500fc6e.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/fcc4da811bb6/vmlinux-9500fc6e.xz
kernel image: https://storage.googleapis.com/syzbot-assets/0b554298f1fa/Image-9500fc6e.gz.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+2600f43a81c05675a9ae@syzkaller.appspotmail.com

------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 1 PID: 3083 at lib/refcount.c:28 refcount_warn_saturate+0x1a0/0x1c8 lib/refcount.c:28
Modules linked in:
CPU: 1 PID: 3083 Comm: syz-executor181 Not tainted 6.1.0-rc5-syzkaller-32269-g9500fc6e9e60 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/30/2022
pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : refcount_warn_saturate+0x1a0/0x1c8 lib/refcount.c:28
lr : refcount_warn_saturate+0x1a0/0x1c8 lib/refcount.c:28
sp : ffff800012e7b9b0
x29: ffff800012e7b9b0 x28: ffff0000c6b51a40 x27: 0000000020000040
x26: 0000000000010002 x25: 0000000000000000 x24: ffff0000cd574088
x23: 0000000000000000 x22: 0000000000000000 x21: ffff0000ca9ffc0c
x20: 0000000000000003 x19: ffff80000d98f000 x18: 00000000000001cc
x17: 0000000000000000 x16: ffff80000dc18158 x15: ffff0000c6b51a40
x14: 0000000000000000 x13: 00000000ffffffff x12: ffff0000c6b51a40
x11: ff808000081c6510 x10: 0000000000000000 x9 : 2060ebe174811d00
x8 : 2060ebe174811d00 x7 : ffff800008165f54 x6 : 0000000000000000
x5 : 0000000000000080 x4 : 0000000000000001 x3 : 0000000000000000
x2 : ffff0001fefddcc8 x1 : 0000000100000000 x0 : 0000000000000026
Call trace:
 refcount_warn_saturate+0x1a0/0x1c8 lib/refcount.c:28
 __refcount_sub_and_test include/linux/refcount.h:283 [inline]
 __refcount_dec_and_test include/linux/refcount.h:315 [inline]
 refcount_dec_and_test include/linux/refcount.h:333 [inline]
 p9_fid_put include/net/9p/client.h:275 [inline]
 p9_client_walk+0x2a4/0x2e8 net/9p/client.c:1190
 v9fs_vfs_lookup+0xa0/0x37c fs/9p/vfs_inode.c:777
 __lookup_slow+0x14c/0x204 fs/namei.c:1685
 lookup_slow+0x44/0x68 fs/namei.c:1702
 walk_component+0x178/0x1b0 fs/namei.c:1993
 lookup_last fs/namei.c:2450 [inline]
 path_lookupat+0xc4/0x208 fs/namei.c:2474
 filename_lookup+0xf8/0x264 fs/namei.c:2503
 user_path_at_empty+0x5c/0x114 fs/namei.c:2876
 user_path_at include/linux/namei.h:57 [inline]
 do_mount fs/namespace.c:3380 [inline]
 __do_sys_mount fs/namespace.c:3591 [inline]
 __se_sys_mount fs/namespace.c:3568 [inline]
 __arm64_sys_mount+0x28c/0x3c4 fs/namespace.c:3568
 __invoke_syscall arch/arm64/kernel/syscall.c:38 [inline]
 invoke_syscall arch/arm64/kernel/syscall.c:52 [inline]
 el0_svc_common+0x138/0x220 arch/arm64/kernel/syscall.c:142
 do_el0_svc+0x48/0x164 arch/arm64/kernel/syscall.c:206
 el0_svc+0x58/0x150 arch/arm64/kernel/entry-common.c:637
 el0t_64_sync_handler+0x84/0xf0 arch/arm64/kernel/entry-common.c:655
 el0t_64_sync+0x190/0x194 arch/arm64/kernel/entry.S:584
irq event stamp: 1078
hardirqs last  enabled at (1077): [<ffff800008165fe4>] raw_spin_rq_unlock_irq kernel/sched/sched.h:1366 [inline]
hardirqs last  enabled at (1077): [<ffff800008165fe4>] finish_lock_switch+0x94/0xe8 kernel/sched/core.c:4950
hardirqs last disabled at (1078): [<ffff80000c0a4f34>] el1_dbg+0x24/0x80 arch/arm64/kernel/entry-common.c:405
softirqs last  enabled at (1070): [<ffff8000080102e4>] _stext+0x2e4/0x37c
softirqs last disabled at (1059): [<ffff800008017c88>] ____do_softirq+0x14/0x20 arch/arm64/kernel/irq.c:79
---[ end trace 0000000000000000 ]---


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