false positive deadlock? Was: [syzbot] [bpf?] possible deadlock in kvfree_call_rcu

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

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.

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?

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>
>
>
> ---
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