[moderation] [media?] [dri?] WARNING: refcount bug in sync_timeline_signal

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 3d5670d672ae Add linux-next specific files for 20260626
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=16ff46fa580000
kernel config: https://syzkaller.appspot.com/x/.config?x=fa604225b96364d9
dashboard link: https://syzkaller.appspot.com/bug?extid=ef4c2b1ce5c227d197d4
compiler: Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
CC: [christia...@amd.com dri-...@lists.freedesktop.org linaro...@lists.linaro.org linux-...@vger.kernel.org linux...@vger.kernel.org sumit....@linaro.org]

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/8743cd651f07/disk-3d5670d6.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/93f3e8326cd9/vmlinux-3d5670d6.xz
kernel image: https://storage.googleapis.com/syzbot-assets/cc1b7036179e/bzImage-3d5670d6.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+ef4c2b...@syzkaller.appspotmail.com

------------[ cut here ]------------
refcount_t: addition on 0; use-after-free.
WARNING: lib/refcount.c:25 at refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25, CPU#1: syz.0.371/6839
Modules linked in:
CPU: 1 UID: 0 PID: 6839 Comm: syz.0.371 Tainted: G L syzkaller #0 PREEMPT_{RT,(full)}
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
RIP: 0010:refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25
Code: eb 66 85 db 74 3e 83 fb 01 75 4c e8 2b 4e 13 fd 48 8d 3d 84 9e e5 0a 67 48 0f b9 3a eb 4a e8 18 4e 13 fd 48 8d 3d 81 9e e5 0a <67> 48 0f b9 3a eb 37 e8 05 4e 13 fd 48 8d 3d 7e 9e e5 0a 67 48 0f
RSP: 0018:ffffc9000d94fc48 EFLAGS: 00010287
RAX: ffffffff84b1cbb8 RBX: 0000000000000002 RCX: 0000000000080000
RDX: ffffc9000ebf5000 RSI: 000000000000075b RDI: ffffffff8f976a40
RBP: ffffc9000d94fd30 R08: ffff888020fe8000 R09: 0000000000000005
R10: 0000000000000100 R11: 0000000000000004 R12: dffffc0000000000
R13: ffff88803fb198b8 R14: ffff88803fb198b0 R15: ffff88803fb19800
FS: 00007f643fc5d6c0(0000) GS:ffff888125fca000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000bd0030 CR3: 000000003f844000 CR4: 00000000003526f0
Call Trace:
<TASK>
__refcount_add include/linux/refcount.h:-1 [inline]
__refcount_inc include/linux/refcount.h:366 [inline]
refcount_inc include/linux/refcount.h:383 [inline]
kref_get include/linux/kref.h:45 [inline]
dma_fence_get include/linux/dma-fence.h:317 [inline]
sync_timeline_signal+0x472/0x810 drivers/dma-buf/sw_sync.c:224
sw_sync_ioctl_inc drivers/dma-buf/sw_sync.c:405 [inline]
sw_sync_ioctl+0x522/0x1110 drivers/dma-buf/sw_sync.c:470
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xff/0x170 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f6441a2ce59
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f643fc5d028 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f6441ca6090 RCX: 00007f6441a2ce59
RDX: 00002000000002c0 RSI: 0000000040045701 RDI: 0000000000000003
RBP: 00007f6441ac2e6f R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f6441ca6128 R14: 00007f6441ca6090 R15: 00007ffe849b15e8
</TASK>
----------------
Code disassembly (best guess):
0: eb 66 jmp 0x68
2: 85 db test %ebx,%ebx
4: 74 3e je 0x44
6: 83 fb 01 cmp $0x1,%ebx
9: 75 4c jne 0x57
b: e8 2b 4e 13 fd call 0xfd134e3b
10: 48 8d 3d 84 9e e5 0a lea 0xae59e84(%rip),%rdi # 0xae59e9b
17: 67 48 0f b9 3a ud1 (%edx),%rdi
1c: eb 4a jmp 0x68
1e: e8 18 4e 13 fd call 0xfd134e3b
23: 48 8d 3d 81 9e e5 0a lea 0xae59e81(%rip),%rdi # 0xae59eab
* 2a: 67 48 0f b9 3a ud1 (%edx),%rdi <-- trapping instruction
2f: eb 37 jmp 0x68
31: e8 05 4e 13 fd call 0xfd134e3b
36: 48 8d 3d 7e 9e e5 0a lea 0xae59e7e(%rip),%rdi # 0xae59ebb
3d: 67 addr32
3e: 48 rex.W
3f: 0f .byte 0xf


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