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Date: Tue, 26 Mar 2024 21:37:43 -0700
From: "Paul E. McKenney" <paulmck@kernel.org>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: syzbot <syzbot+1fa663a2100308ab6eab@syzkaller.appspotmail.com>,
	Andrii Nakryiko <andrii@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>, bpf <bpf@vger.kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>, Eddy Z <eddyz87@gmail.com>,
	Hao Luo <haoluo@google.com>,
	John Fastabend <john.fastabend@gmail.com>,
	Jiri Olsa <jolsa@kernel.org>, KP Singh <kpsingh@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Martin KaFai Lau <martin.lau@linux.dev>,
	Stanislav Fomichev <sdf@google.com>, Song Liu <song@kernel.org>,
	syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
	Yonghong Song <yonghong.song@linux.dev>, urezki@gmail.com,
	rcu@vger.kernel.org
Subject: Re: false positive deadlock? Was: [syzbot] [bpf?] possible deadlock
 in kvfree_call_rcu
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On Tue, Mar 26, 2024 at 12:53:35PM -0700, Alexei Starovoitov wrote:
> Hi Paul,
> 
> syzbot found an interesting false positive deadlock.
> See below.
> My understanding is the following:
> 
> cpu 2:
>   grabs timer_base lock
>     spins on bpf_lpm lock
> 
> cpu 1:
>   grab rcu krcp lock
>     spins on timer_base lock
> 
> cpu 0:
>   grab bpf_lpm lock
>     spins on rcu krcp lock
> 
> bpf_lpm lock can be the same.
> timer_base lock can also be the same due to timer migration.
> 
> but rcu krcp lock is always per-cpu, so it cannot be the same lock.
> Hence it's a false positive, but still interesting.
> 
> I don't think rcu can tell lockdep that these are different locks.

It might be possible.  I will play with this tomorrow, modeling after
the use of lockdep_set_class_and_name() in rcu_init_one().  I am a bit
concerned about systems with thousands of CPUs, but it just might be OK.

> Few ideas/questions on how to address this:
> 
> 1. in kernel/rcu/tree.c:
>         if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING)
>                 schedule_delayed_monitor_work(krcp);
> 
> unlock_return:
>         krc_this_cpu_unlock(krcp, flags);
> 
> moving schedule_delayed_monitor_work() after unlock will not work, right?

If telling lockdep that these are different locks works, that should
be easier.  Though maybe Uladzislau can assure me that moving this
schedule_delayed_monitor_work() is OK.

							Thanx, Paul

> 2. if not, we can move 4 kfree_rcu-s in kernel/bpf/lpm_trie.c
>  to a place after unlock of lpm_trie
> 
> 3. move bpf_lpm_trie to bpf_mem_alloc.
> 
> The 2 or 3 will address this false positive.
> 
> Other ideas?
> 
> On Tue, Mar 26, 2024 at 12:00 PM syzbot
> <syzbot+1fa663a2100308ab6eab@syzkaller.appspotmail.com> wrote:
> >
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    fe46a7dd189e Merge tag 'sound-6.9-rc1' of git://git.kernel..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=11547a65180000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=4d90a36f0cab495a
> > dashboard link: https://syzkaller.appspot.com/bug?extid=1fa663a2100308ab6eab
> > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/f6c04726a2ae/disk-fe46a7dd.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/09c26ce901ea/vmlinux-fe46a7dd.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/134acf7f5322/bzImage-fe46a7dd.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+1fa663a2100308ab6eab@syzkaller.appspotmail.com
> >
> > ======================================================
> > WARNING: possible circular locking dependency detected
> > 6.8.0-syzkaller-08951-gfe46a7dd189e #0 Not tainted
> > ------------------------------------------------------
> > syz-executor.3/6590 is trying to acquire lock:
> > ffff8880b9529470 (krc.lock){..-.}-{2:2}, at: krc_this_cpu_lock kernel/rcu/tree.c:2960 [inline]
> > ffff8880b9529470 (krc.lock){..-.}-{2:2}, at: add_ptr_to_bulk_krc_lock kernel/rcu/tree.c:3359 [inline]
> > ffff8880b9529470 (krc.lock){..-.}-{2:2}, at: kvfree_call_rcu+0x18a/0x790 kernel/rcu/tree.c:3444
> >
> > but task is already holding lock:
> > ffff888021a271f8 (&trie->lock){..-.}-{2:2}, at: trie_update_elem+0xcb/0xc10 kernel/bpf/lpm_trie.c:324
> >
> > which lock already depends on the new lock.
> >
> >
> > the existing dependency chain (in reverse order) is:
> >
> > -> #2 (&trie->lock){..-.}-{2:2}:
> >        lock_acquire+0x1e4/0x530 kernel/locking/lockdep.c:5754
> >        __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
> >        _raw_spin_lock_irqsave+0xd5/0x120 kernel/locking/spinlock.c:162
> >        trie_delete_elem+0x96/0x6a0 kernel/bpf/lpm_trie.c:451
> >        bpf_prog_510c7248c5f60c92+0x2e/0x46
> >        bpf_dispatcher_nop_func include/linux/bpf.h:1234 [inline]
> >        __bpf_prog_run include/linux/filter.h:657 [inline]
> >        bpf_prog_run include/linux/filter.h:664 [inline]
> >        __bpf_trace_run kernel/trace/bpf_trace.c:2381 [inline]
> >        bpf_trace_run2+0x204/0x420 kernel/trace/bpf_trace.c:2420
> >        trace_timer_start include/trace/events/timer.h:52 [inline]
> >        enqueue_timer+0x396/0x550 kernel/time/timer.c:663
> >        internal_add_timer kernel/time/timer.c:688 [inline]
> >        __mod_timer+0xa0e/0xeb0 kernel/time/timer.c:1183
> >        call_timer_fn+0x17e/0x600 kernel/time/timer.c:1792
> >        expire_timers kernel/time/timer.c:1843 [inline]
> >        __run_timers kernel/time/timer.c:2408 [inline]
> >        __run_timer_base+0x66a/0x8e0 kernel/time/timer.c:2419
> >        run_timer_base kernel/time/timer.c:2428 [inline]
> >        run_timer_softirq+0xb7/0x170 kernel/time/timer.c:2438
> >        __do_softirq+0x2bc/0x943 kernel/softirq.c:554
> >        invoke_softirq kernel/softirq.c:428 [inline]
> >        __irq_exit_rcu+0xf2/0x1c0 kernel/softirq.c:633
> >        irq_exit_rcu+0x9/0x30 kernel/softirq.c:645
> >        instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1043 [inline]
> >        sysvec_apic_timer_interrupt+0xa6/0xc0 arch/x86/kernel/apic/apic.c:1043
> >        asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:702
> >        memory_is_poisoned_n mm/kasan/generic.c:130 [inline]
> >        memory_is_poisoned mm/kasan/generic.c:161 [inline]
> >        check_region_inline mm/kasan/generic.c:180 [inline]
> >        kasan_check_range+0x4f/0x290 mm/kasan/generic.c:189
> >        instrument_atomic_read_write include/linux/instrumented.h:96 [inline]
> >        atomic_try_cmpxchg_acquire include/linux/atomic/atomic-instrumented.h:1301 [inline]
> >        queued_spin_lock include/asm-generic/qspinlock.h:111 [inline]
> >        do_raw_spin_lock+0x14f/0x370 kernel/locking/spinlock_debug.c:116
> >        spin_lock include/linux/spinlock.h:351 [inline]
> >        lockref_get+0x15/0x60 lib/lockref.c:50
> >        dget include/linux/dcache.h:333 [inline]
> >        __traverse_mounts+0x3b4/0x580 fs/namei.c:1401
> >        traverse_mounts fs/namei.c:1442 [inline]
> >        handle_mounts fs/namei.c:1545 [inline]
> >        step_into+0x5e5/0x1080 fs/namei.c:1842
> >        walk_component fs/namei.c:2010 [inline]
> >        link_path_walk+0x748/0xea0 fs/namei.c:2331
> >        path_lookupat+0xa9/0x450 fs/namei.c:2484
> >        filename_lookup+0x256/0x610 fs/namei.c:2514
> >        user_path_at_empty+0x42/0x60 fs/namei.c:2921
> >        do_readlinkat+0x118/0x3b0 fs/stat.c:499
> >        __do_sys_readlink fs/stat.c:532 [inline]
> >        __se_sys_readlink fs/stat.c:529 [inline]
> >        __x64_sys_readlink+0x7f/0x90 fs/stat.c:529
> >        do_syscall_64+0xfb/0x240
> >        entry_SYSCALL_64_after_hwframe+0x6d/0x75
> >
> > -> #1 (&base->lock){-.-.}-{2:2}:
> >        lock_acquire+0x1e4/0x530 kernel/locking/lockdep.c:5754
> >        __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
> >        _raw_spin_lock_irqsave+0xd5/0x120 kernel/locking/spinlock.c:162
> >        lock_timer_base+0x112/0x240 kernel/time/timer.c:1051
> >        __mod_timer+0x1ca/0xeb0 kernel/time/timer.c:1132
> >        queue_delayed_work_on+0x15a/0x260 kernel/workqueue.c:2595
> >        kvfree_call_rcu+0x47f/0x790 kernel/rcu/tree.c:3472
> >        rtnl_register_internal+0x482/0x590 net/core/rtnetlink.c:265
> >        rtnl_register+0x36/0x80 net/core/rtnetlink.c:315
> >        ip_rt_init+0x2f5/0x3a0 net/ipv4/route.c:3719
> >        ip_init+0xe/0x20 net/ipv4/ip_output.c:1664
> >        inet_init+0x3d8/0x580 net/ipv4/af_inet.c:2022
> >        do_one_initcall+0x238/0x830 init/main.c:1241
> >        do_initcall_level+0x157/0x210 init/main.c:1303
> >        do_initcalls+0x3f/0x80 init/main.c:1319
> >        kernel_init_freeable+0x435/0x5d0 init/main.c:1550
> >        kernel_init+0x1d/0x2a0 init/main.c:1439
> >        ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147
> >        ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:243
> >
> > -> #0 (krc.lock){..-.}-{2:2}:
> >        check_prev_add kernel/locking/lockdep.c:3134 [inline]
> >        check_prevs_add kernel/locking/lockdep.c:3253 [inline]
> >        validate_chain+0x18cb/0x58e0 kernel/locking/lockdep.c:3869
> >        __lock_acquire+0x1346/0x1fd0 kernel/locking/lockdep.c:5137
> >        lock_acquire+0x1e4/0x530 kernel/locking/lockdep.c:5754
> >        __raw_spin_lock include/linux/spinlock_api_smp.h:133 [inline]
> >        _raw_spin_lock+0x2e/0x40 kernel/locking/spinlock.c:154
> >        krc_this_cpu_lock kernel/rcu/tree.c:2960 [inline]
> >        add_ptr_to_bulk_krc_lock kernel/rcu/tree.c:3359 [inline]
> >        kvfree_call_rcu+0x18a/0x790 kernel/rcu/tree.c:3444
> >        trie_update_elem+0x819/0xc10 kernel/bpf/lpm_trie.c:385
> >        bpf_map_update_value+0x4d3/0x540 kernel/bpf/syscall.c:203
> >        generic_map_update_batch+0x60d/0x900 kernel/bpf/syscall.c:1876
> >        bpf_map_do_batch+0x3e0/0x690 kernel/bpf/syscall.c:5145
> >        __sys_bpf+0x377/0x810
> >        __do_sys_bpf kernel/bpf/syscall.c:5738 [inline]
> >        __se_sys_bpf kernel/bpf/syscall.c:5736 [inline]
> >        __x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:5736
> >        do_syscall_64+0xfb/0x240
> >        entry_SYSCALL_64_after_hwframe+0x6d/0x75
> >
> > other info that might help us debug this:
> >
> > Chain exists of:
> >   krc.lock --> &base->lock --> &trie->lock
> >
> >  Possible unsafe locking scenario:
> >
> >        CPU0                    CPU1
> >        ----                    ----
> >   lock(&trie->lock);
> >                                lock(&base->lock);
> >                                lock(&trie->lock);
> >   lock(krc.lock);
> >
> >  *** DEADLOCK ***
> >
> > 2 locks held by syz-executor.3/6590:
> >  #0: ffffffff8e132020 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire include/linux/rcupdate.h:298 [inline]
> >  #0: ffffffff8e132020 (rcu_read_lock){....}-{1:2}, at: rcu_read_lock include/linux/rcupdate.h:750 [inline]
> >  #0: ffffffff8e132020 (rcu_read_lock){....}-{1:2}, at: bpf_map_update_value+0x3c4/0x540 kernel/bpf/syscall.c:202
> >  #1: ffff888021a271f8 (&trie->lock){..-.}-{2:2}, at: trie_update_elem+0xcb/0xc10 kernel/bpf/lpm_trie.c:324
> >
> > stack backtrace:
> > CPU: 1 PID: 6590 Comm: syz-executor.3 Not tainted 6.8.0-syzkaller-08951-gfe46a7dd189e #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/29/2024
> > Call Trace:
> >  <TASK>
> >  __dump_stack lib/dump_stack.c:88 [inline]
> >  dump_stack_lvl+0x241/0x360 lib/dump_stack.c:114
> >  check_noncircular+0x36a/0x4a0 kernel/locking/lockdep.c:2187
> >  check_prev_add kernel/locking/lockdep.c:3134 [inline]
> >  check_prevs_add kernel/locking/lockdep.c:3253 [inline]
> >  validate_chain+0x18cb/0x58e0 kernel/locking/lockdep.c:3869
> >  __lock_acquire+0x1346/0x1fd0 kernel/locking/lockdep.c:5137
> >  lock_acquire+0x1e4/0x530 kernel/locking/lockdep.c:5754
> >  __raw_spin_lock include/linux/spinlock_api_smp.h:133 [inline]
> >  _raw_spin_lock+0x2e/0x40 kernel/locking/spinlock.c:154
> >  krc_this_cpu_lock kernel/rcu/tree.c:2960 [inline]
> >  add_ptr_to_bulk_krc_lock kernel/rcu/tree.c:3359 [inline]
> >  kvfree_call_rcu+0x18a/0x790 kernel/rcu/tree.c:3444
> >  trie_update_elem+0x819/0xc10 kernel/bpf/lpm_trie.c:385
> >  bpf_map_update_value+0x4d3/0x540 kernel/bpf/syscall.c:203
> >  generic_map_update_batch+0x60d/0x900 kernel/bpf/syscall.c:1876
> >  bpf_map_do_batch+0x3e0/0x690 kernel/bpf/syscall.c:5145
> >  __sys_bpf+0x377/0x810
> >  __do_sys_bpf kernel/bpf/syscall.c:5738 [inline]
> >  __se_sys_bpf kernel/bpf/syscall.c:5736 [inline]
> >  __x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:5736
> >  do_syscall_64+0xfb/0x240
> >  entry_SYSCALL_64_after_hwframe+0x6d/0x75
> > RIP: 0033:0x7fe5f987dda9
> > Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 e1 20 00 00 90 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 b0 ff ff ff f7 d8 64 89 01 48
> > RSP: 002b:00007fe5fa6000c8 EFLAGS: 00000246 ORIG_RAX: 0000000000000141
> > RAX: ffffffffffffffda RBX: 00007fe5f99abf80 RCX: 00007fe5f987dda9
> > RDX: 0000000000000038 RSI: 0000000020000240 RDI: 000000000000001a
> > RBP: 00007fe5f98ca47a R08: 0000000000000000 R09: 0000000000000000
> > R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> > R13: 000000000000000b R14: 00007fe5f99abf80 R15: 00007ffe908076c8
> >  </TASK>
> >
> >
> > ---
> > 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 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
> >