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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: __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 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 --- This report is generated by a bot. It may contain errors. See https://goo.gl/tpsmEJ for more information about syzbot. syzbot engineers can be reached at syzkaller@googlegroups.com. syzbot will keep track of this issue. See: https://goo.gl/tpsmEJ#status for how to communicate with syzbot. If the report is already addressed, let syzbot know by replying with: #syz fix: exact-commit-title If you want syzbot to run the reproducer, reply with: #syz test: git://repo/address.git branch-or-commit-hash If you attach or paste a git patch, syzbot will apply it before testing. If you want to overwrite report's subsystems, reply with: #syz set subsystems: new-subsystem (See the list of subsystem names on the web dashboard) If the report is a duplicate of another one, reply with: #syz dup: exact-subject-of-another-report If you want to undo deduplication, reply with: #syz undup