[syzbot] [media?] [dri?] kernel BUG in sync_timeline_signal

[syzbot] [media?] [dri?] kernel BUG in sync_timeline_signal

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    6628e7d4c6b2 Linux 7.4-rc1
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
console output: https://syzkaller.appspot.com/x/log.txt?x=13fa63f1580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=1d2c97eac7d33533
dashboard link: https://syzkaller.appspot.com/bug?extid=cba1a1f459230395a076
compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=134d2959580000

AI-generated root cause summary:
In drivers/dma-buf/sw_sync.c, a race condition exists between dma_fence_put()
and sync_timeline_signal(). When the last reference to a sync_pt fence is
dropped, its refcount reaches 0 before dma_fence_release() ->
timeline_fence_release() acquires obj->lock to remove the sync_pt from
obj->pt_list. If sync_timeline_signal() runs concurrently under obj->lock, it
iterates obj->pt_list and calls dma_fence_get() on the fence whose refcount is
already 0, violating refcount invariants and causing a use-after-free / refcount
addition-on-0 kernel crash.

full analysis:  https://syzkaller.appspot.com/x/ai_root_cause?x=cba1a1f459230395a076

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+cba1a1f459230395a076@syzkaller.appspotmail.com

refcount_t: addition on 0; use-after-free.
------------[ cut here ]------------
kernel BUG at lib/refcount.c:34!
Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
CPU: 1 UID: 0 PID: 6018 Comm: syz-executor369 Tainted: G             L      syzkaller #0 PREEMPT(full) 
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/15/2026
RIP: 0010:refcount_warn_saturate.cold+0x12/0x14 lib/refcount.c:34
Code: 89 ea be 01 00 00 00 48 c7 c7 40 c1 c3 90 e8 0a 5a 01 00 e9 72 e3 02 04 e8 c0 54 d7 00 48 c7 c7 a0 4a 81 8c e8 04 85 eb ff 90 <0f> 0b e8 ac 54 d7 00 48 c7 c7 40 4b 81 8c e8 f0 84 eb ff 90 0f 0b
RSP: 0018:ffffc90003e57cb8 EFLAGS: 00010086
RAX: 000000000000002a RBX: ffff88802d271570 RCX: 0000000000000000
RDX: 000000000000002a RSI: ffffffff81ebf219 RDI: fffff520007caf88
RBP: 0000000000000002 R08: 0000000000000000 R09: 0000000000000000
R10: ffffc90003e57a7f R11: 0000000000000001 R12: 0000000000000000
R13: dffffc0000000000 R14: ffff88802d271300 R15: ffffc90003e57d18
FS:  00007f83e74d26c0(0000) GS:ffff88812257d000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f83e75a93c8 CR3: 00000000357c8000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 __refcount_add include/linux/refcount.h:289 [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:323 [inline]
 sync_timeline_signal+0x6d9/0x8a0 drivers/dma-buf/sw_sync.c:224
 sw_sync_ioctl_inc drivers/dma-buf/sw_sync.c:405 [inline]
 sw_sync_ioctl+0x7b4/0x13f0 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 fs/ioctl.c:583 [inline]
 __x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:583
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x123/0x790 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f83e7507d5b
Code: 00 48 89 44 24 18 31 c0 48 8d 44 24 60 c7 04 24 10 00 00 00 48 89 44 24 08 48 8d 44 24 20 48 89 44 24 10 b8 10 00 00 00 0f 05 <89> c2 3d 00 f0 ff ff 77 1c 48 8b 44 24 18 64 48 2b 04 25 28 00 00
RSP: 002b:00007f83e74d2100 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f83e7507d5b
RDX: 00007f83e74d2170 RSI: 0000000040045701 RDI: 0000000000000003
RBP: 00000000000003d0 R08: 0000000000000000 R09: 00007f83e74d26c0
R10: 0000000000000008 R11: 0000000000000246 R12: 00007f83e74d2170
R13: 00000000000000f3 R14: 00007ffcce841440 R15: 00007ffcce841528
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:refcount_warn_saturate.cold+0x12/0x14 lib/refcount.c:34
Code: 89 ea be 01 00 00 00 48 c7 c7 40 c1 c3 90 e8 0a 5a 01 00 e9 72 e3 02 04 e8 c0 54 d7 00 48 c7 c7 a0 4a 81 8c e8 04 85 eb ff 90 <0f> 0b e8 ac 54 d7 00 48 c7 c7 40 4b 81 8c e8 f0 84 eb ff 90 0f 0b
RSP: 0018:ffffc90003e57cb8 EFLAGS: 00010086
RAX: 000000000000002a RBX: ffff88802d271570 RCX: 0000000000000000
RDX: 000000000000002a RSI: ffffffff81ebf219 RDI: fffff520007caf88
RBP: 0000000000000002 R08: 0000000000000000 R09: 0000000000000000
R10: ffffc90003e57a7f R11: 0000000000000001 R12: 0000000000000000
R13: dffffc0000000000 R14: ffff88802d271300 R15: ffffc90003e57d18
FS:  00007f83e74d26c0(0000) GS:ffff88812257d000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f83e75a93c8 CR3: 00000000357c8000 CR4: 00000000003526f0


---
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