Subject: [syzbot] WARNING: refcount bug in p9_req_put (2)

From: syzbot <[email-masked]>
Date: Sun, 18 Sep 2022 21:54:35 -0700

Hello,

syzbot found the following issue on:

HEAD commit:    80e78fcce86d Linux 6.0-rc5
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=10397700880000
kernel config:  https://syzkaller.appspot.com/x/.config?x=d4cc03e6a78af26d
dashboard link: https://syzkaller.appspot.com/bug?extid=3ba8f2097df93bc26d2f
compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2

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:
Reported-by: [email-masked]

9p: Unknown Cache mode f��*ux�lI��cache
9pnet: Tag 65535 still in use
------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 1 PID: 11668 at lib/refcount.c:28 refcount_warn_saturate+0x1d1/0x1e0 lib/refcount.c:28
Modules linked in:
CPU: 1 PID: 11668 Comm: syz-executor.3 Not tainted 6.0.0-rc5-syzkaller #0
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-2 04/01/2014
RIP: 0010:refcount_warn_saturate+0x1d1/0x1e0 lib/refcount.c:28
Code: e9 db fe ff ff 48 89 df e8 1c a6 c9 fd e9 8a fe ff ff e8 62 07 7e fd 48 c7 c7 e0 a0 48 8a c6 05 81 f7 cb 09 01 e8 41 36 3d 05 <0f> 0b e9 af fe ff ff 0f 1f 84 00 00 00 00 00 41 56 41 55 41 54 55
RSP: 0018:ffffc90006bff9f0 EFLAGS: 00010282
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000040000 RSI: ffffffff81611e08 RDI: fffff52000d7ff30
RBP: 0000000000000003 R08: 0000000000000005 R09: 0000000000000000
R10: 0000000080000000 R11: 0000000000000001 R12: 0000000000000000
R13: ffff8880706a4808 R14: ffff888025838400 R15: dffffc0000000000
FS:  00007f2496ffe700(0000) GS:ffff88802c900000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020003000 CR3: 000000006f38b000 CR4: 0000000000150ee0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 __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_req_put+0x1f2/0x250 net/9p/client.c:374
 p9_tag_cleanup net/9p/client.c:401 [inline]
 p9_client_destroy.cold+0x20/0xcc net/9p/client.c:1027
 v9fs_session_init+0x1003/0x1810 fs/9p/v9fs.c:488
 v9fs_mount+0xba/0xc90 fs/9p/vfs_super.c:126
 legacy_get_tree+0x105/0x220 fs/fs_context.c:610
 vfs_get_tree+0x89/0x2f0 fs/super.c:1530
 do_new_mount fs/namespace.c:3040 [inline]
 path_mount+0x1326/0x1e20 fs/namespace.c:3370
 do_mount fs/namespace.c:3383 [inline]
 __do_sys_mount fs/namespace.c:3591 [inline]
 __se_sys_mount fs/namespace.c:3568 [inline]
 __x64_sys_mount+0x27f/0x300 fs/namespace.c:3568
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd
RIP: 0033:0x7f24980893c9
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f2496ffe168 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 00007f249819c050 RCX: 00007f24980893c9
RDX: 0000000020000580 RSI: 0000000020000540 RDI: 0000000020000240
RBP: 00007f24980e433f R08: 0000000020000640 R09: 0000000000000000
R10: 0000000000004000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ffcf7d7d3df R14: 00007f2496ffe300 R15: 0000000000022000
 </TASK>

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