Subject: [syzbot] [net?] [v9fs?] WARNING: refcount bug in p9_req_put (3)

From: syzbot <[email-masked]>
Date: Fri, 26 Jan 2024 01:05:28 -0800

Hello,

syzbot found the following issue on:

HEAD commit:    4fbbed787267 Merge tag 'timers-core-2024-01-21' of git://g..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11bfbdc7e80000
kernel config:  https://syzkaller.appspot.com/x/.config?x=4059ab9bf06b6ceb
dashboard link: https://syzkaller.appspot.com/bug?extid=d99d2414db66171fccbb
compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
userspace arch: i386

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

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/7bc7510fe41f/non_bootable_disk-4fbbed78.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/13a98041382d/vmlinux-4fbbed78.xz
kernel image: https://storage.googleapis.com/syzbot-assets/0a02086862ee/bzImage-4fbbed78.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: [email-masked]

9p: Unknown Cache mode or invalid value fsca�he
9pnet: Tag 65535 still in use
------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 3 PID: 30609 at lib/refcount.c:28 refcount_warn_saturate+0x14a/0x210 lib/refcount.c:28
Modules linked in:
CPU: 3 PID: 30609 Comm: syz-executor.1 Not tainted 6.7.0-syzkaller-13004-g4fbbed787267 #0
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
RIP: 0010:refcount_warn_saturate+0x14a/0x210 lib/refcount.c:28
Code: ff 89 de e8 e8 af 1e fd 84 db 0f 85 66 ff ff ff e8 ab b4 1e fd c6 05 1b 87 bb 0a 01 90 48 c7 c7 c0 70 2f 8b e8 97 a9 e4 fc 90 <0f> 0b 90 90 e9 43 ff ff ff e8 88 b4 1e fd 0f b6 1d f6 86 bb 0a 31
RSP: 0018:ffffc9000345f9b0 EFLAGS: 00010286
RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffc90006349000
RDX: 0000000000040000 RSI: ffffffff814e1906 RDI: 0000000000000001
RBP: ffff8880687d59a0 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000001 R12: ffffc9000345fa48
R13: ffff8880687d59a0 R14: ffff888026a9d800 R15: 1ffff9200068bf41
FS:  0000000000000000(0000) GS:ffff88802c900000(0063) knlGS:00000000f7ff1b40
CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
CR2: 0000000020003000 CR3: 000000006eb7c000 CR4: 0000000000350ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 __refcount_sub_and_test include/linux/refcount.h:272 [inline]
 __refcount_dec_and_test include/linux/refcount.h:304 [inline]
 refcount_dec_and_test include/linux/refcount.h:322 [inline]
 p9_req_put+0x1f0/0x250 net/9p/client.c:401
 p9_tag_cleanup net/9p/client.c:428 [inline]
 p9_client_destroy+0x226/0x480 net/9p/client.c:1073
 v9fs_session_init+0xba5/0x1a80 fs/9p/v9fs.c:490
 v9fs_mount+0xc6/0xcd0 fs/9p/vfs_super.c:123
 legacy_get_tree+0x109/0x220 fs/fs_context.c:662
 vfs_get_tree+0x8c/0x370 fs/super.c:1784
 do_new_mount fs/namespace.c:3352 [inline]
 path_mount+0x14e6/0x1f20 fs/namespace.c:3679
 do_mount fs/namespace.c:3692 [inline]
 __do_sys_mount fs/namespace.c:3898 [inline]
 __se_sys_mount fs/namespace.c:3875 [inline]
 __ia32_sys_mount+0x291/0x310 fs/namespace.c:3875
 do_syscall_32_irqs_on arch/x86/entry/common.c:165 [inline]
 __do_fast_syscall_32+0x79/0x110 arch/x86/entry/common.c:321
 do_fast_syscall_32+0x33/0x70 arch/x86/entry/common.c:346
 entry_SYSENTER_compat_after_hwframe+0x70/0x7a
RIP: 0023:0xf7ff6579
Code: b8 01 10 06 03 74 b4 01 10 07 03 74 b0 01 10 08 03 74 d8 01 00 00 00 00 00 00 00 00 00 00 00 00 00 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90 90 90 90 8d b4 26 00 00 00 00 8d b4 26 00 00 00 00
RSP: 002b:00000000f7ff15ac EFLAGS: 00000292 ORIG_RAX: 0000000000000015
RAX: ffffffffffffffda RBX: 00000000200001c0 RCX: 0000000020000480
RDX: 00000000200004c0 RSI: 0000000000000404 RDI: 0000000020001080
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000292 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
 </TASK>
----------------
Code disassembly (best guess), 2 bytes skipped:
   0:	10 06                	adc    %al,(%rsi)
   2:	03 74 b4 01          	add    0x1(%rsp,%rsi,4),%esi
   6:	10 07                	adc    %al,(%rdi)
   8:	03 74 b0 01          	add    0x1(%rax,%rsi,4),%esi
   c:	10 08                	adc    %cl,(%rax)
   e:	03 74 d8 01          	add    0x1(%rax,%rbx,8),%esi
  1e:	00 51 52             	add    %dl,0x52(%rcx)
  21:	55                   	push   %rbp
  22:	89 e5                	mov    %esp,%ebp
  24:	0f 34                	sysenter
  26:	cd 80                	int    $0x80
* 28:	5d                   	pop    %rbp <-- trapping instruction
  29:	5a                   	pop    %rdx
  2a:	59                   	pop    %rcx
  2b:	c3                   	ret
  2c:	90                   	nop
  2d:	90                   	nop
  2e:	90                   	nop
  2f:	90                   	nop
  30:	8d b4 26 00 00 00 00 	lea    0x0(%rsi,%riz,1),%esi
  37:	8d b4 26 00 00 00 00 	lea    0x0(%rsi,%riz,1),%esi

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