Re: [syzbot] [v9fs?] WARNING: refcount bug in p9_req_put (4)

[syzbot]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
WARNING: refcount bug in p9_req_put

------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: lib/refcount.c:28 at refcount_warn_saturate+0xf4/0x130 lib/refcount.c:28, CPU#3: swapper/3/0
Modules linked in:
CPU: 3 UID: 0 PID: 0 Comm: swapper/3 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:refcount_warn_saturate+0xf4/0x130 lib/refcount.c:28
Code: cc e8 10 53 0f fd 48 8d 3d 89 4c e8 0b 67 48 0f b9 3a e8 ff 52 0f fd 5b 5d e9 d8 5f 93 06 e8 f3 52 0f fd 48 8d 3d 7c 4c e8 0b <67> 48 0f b9 3a e8 e2 52 0f fd 5b 5d c3 cc cc cc cc e8 d6 52 0f fd
RSP: 0018:ffffc900006f8d90 EFLAGS: 00010046
RAX: 0000000080010002 RBX: ffff8880341eb658 RCX: ffffffff84f95c3b
RDX: ffff88801eac2540 RSI: ffffffff84f95ccd RDI: ffffffff90e1a950
RBP: 0000000000000003 R08: 0000000000000005 R09: 0000000000000004
R10: 0000000000000003 R11: 0000000000000001 R12: ffff8880341eb658
R13: ffff8880287ccc00 R14: 000000000000000b R15: 1ffff1100465880c
FS:  0000000000000000(0000) GS:ffff8880d6687000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00001e1e2cc78000 CR3: 00000000389b6000 CR4: 0000000000352ef0
Call Trace:
 <IRQ>
 __refcount_sub_and_test include/linux/refcount.h:400 [inline]
 __refcount_dec_and_test include/linux/refcount.h:432 [inline]
 refcount_dec_and_test include/linux/refcount.h:450 [inline]
 p9_req_put+0x1ec/0x250 net/9p/client.c:276
 req_done+0x1dc/0x2e0 net/9p/trans_virtio.c:147
 vring_interrupt+0x2ef/0x650 drivers/virtio/virtio_ring.c:3254
 __handle_irq_event_percpu+0x235/0x8c0 kernel/irq/handle.c:209
 handle_irq_event_percpu kernel/irq/handle.c:246 [inline]
 handle_irq_event+0xab/0x1e0 kernel/irq/handle.c:263
 handle_edge_irq+0x35e/0x960 kernel/irq/chip.c:857
 generic_handle_irq_desc include/linux/irqdesc.h:186 [inline]
 handle_irq arch/x86/kernel/irq.c:262 [inline]
 call_irq_handler arch/x86/kernel/irq.c:318 [inline]
 __common_interrupt+0xd8/0x2f0 arch/x86/kernel/irq.c:333
 common_interrupt+0xb9/0xe0 arch/x86/kernel/irq.c:326
 </IRQ>
 <TASK>
 asm_common_interrupt+0x26/0x40 arch/x86/include/asm/idtentry.h:688
RIP: 0010:pv_native_safe_halt+0xf/0x20 arch/x86/kernel/paravirt.c:63
Code: a6 92 02 e9 43 44 03 00 0f 1f 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa eb 07 0f 00 2d 23 02 22 00 fb f4 <c3> cc cc cc cc 66 2e 0f 1f 84 00 00 00 00 00 66 90 90 90 90 90 90
RSP: 0018:ffffc90000197df0 EFLAGS: 00000202
RAX: 0000000000162557 RBX: ffff88801eac2540 RCX: ffffffff8b899225
RDX: 0000000000000000 RSI: ffffffff8df0fbdb RDI: ffffffff8c1c4680
RBP: 0000000000000000 R08: 0000000000000001 R09: ffffed100d4e67b5
R10: ffff88806a733dab R11: 0000000000000000 R12: 0000000000000003
R13: ffffed1003d584a8 R14: 0000000000000003 R15: ffffffff90d71950
 arch_safe_halt arch/x86/include/asm/paravirt.h:62 [inline]
 default_idle+0x9/0x10 arch/x86/kernel/process.c:767
 default_idle_call+0x6c/0xb0 kernel/sched/idle.c:122
 cpuidle_idle_call kernel/sched/idle.c:199 [inline]
 do_idle+0x464/0x590 kernel/sched/idle.c:352
 cpu_startup_entry+0x4f/0x60 kernel/sched/idle.c:451
 start_secondary+0x21d/0x2d0 arch/x86/kernel/smpboot.c:312
 common_startup_64+0x13e/0x148
 </TASK>
----------------
Code disassembly (best guess):
   0:	cc                   	int3
   1:	e8 10 53 0f fd       	call   0xfd0f5316
   6:	48 8d 3d 89 4c e8 0b 	lea    0xbe84c89(%rip),%rdi        # 0xbe84c96
   d:	67 48 0f b9 3a       	ud1    (%edx),%rdi
  12:	e8 ff 52 0f fd       	call   0xfd0f5316
  17:	5b                   	pop    %rbx
  18:	5d                   	pop    %rbp
  19:	e9 d8 5f 93 06       	jmp    0x6935ff6
  1e:	e8 f3 52 0f fd       	call   0xfd0f5316
  23:	48 8d 3d 7c 4c e8 0b 	lea    0xbe84c7c(%rip),%rdi        # 0xbe84ca6
* 2a:	67 48 0f b9 3a       	ud1    (%edx),%rdi <-- trapping instruction
  2f:	e8 e2 52 0f fd       	call   0xfd0f5316
  34:	5b                   	pop    %rbx
  35:	5d                   	pop    %rbp
  36:	c3                   	ret
  37:	cc                   	int3
  38:	cc                   	int3
  39:	cc                   	int3
  3a:	cc                   	int3
  3b:	e8 d6 52 0f fd       	call   0xfd0f5316


Tested on:

commit:         8e65320d Merge tag 'drm-fixes-2026-06-06' of https://g..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11f57166580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=8118209836970b54
dashboard link: https://syzkaller.appspot.com/bug?extid=f6fdb3bdd2c1d419e263
compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
patch:          https://syzkaller.appspot.com/x/patch.diff?x=1705eb7e580000