Subject: Re: Possible deadlock in bpf queue map

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Thu, 07 Sep 2023 12:26:27 +0200

+Arnaldo

> Hi,
>
> Our bpf fuzzer, a customized Syzkaller, triggered a lockdep warning in
> the bpf queue map in v5.15. Since queue_stack_maps.c has no major changes
> since v5.15, we think this should still exist in the latest kernel.
> The bug can be occasionally triggered, and we suspect one of the
> eBPF programs involved to be the following one. We also attached the lockdep
> warning at the end.
>
> #define DEFINE_BPF_MAP_NO_KEY(the_map, TypeOfMap, MapFlags,
> TypeOfValue, MaxEntries) \
>         struct {                                                        \
>             __uint(type, TypeOfMap);                                    \
>             __uint(map_flags, (MapFlags));                              \
>             __uint(max_entries, (MaxEntries));                          \
>             __type(value, TypeOfValue);                                 \
>         } the_map SEC(".maps");
>
> DEFINE_BPF_MAP_NO_KEY(map_0, BPF_MAP_TYPE_QUEUE, 0 | BPF_F_WRONLY,
> struct_0, 162);
> SEC("perf_event")
> int func(struct bpf_perf_event_data *ctx) {
>         char v0[96] = {};
>         uint64_t v1 = 0;
>         v1 = bpf_map_pop_elem(&map_0, v0);
>         return 163819661;
> }
>
>
> The program is attached to the following perf event.
>
> struct perf_event_attr attr_type_hw = {
>         .type = PERF_TYPE_HARDWARE,
>         .config = PERF_COUNT_HW_CPU_CYCLES,
>         .sample_freq = 50,
>         .inherit = 1,
>         .freq = 1,
> };
>
> ================================WARNING: inconsistent lock state
> 5.15.26+ #2 Not tainted
> --------------------------------
> inconsistent {INITIAL USE} -> {IN-NMI} usage.
> syz-executor.5/19749 [HC1[1]:SC0[0]:HE0:SE1] takes:
> ffff88804c9fc198 (&qs->lock){..-.}-{2:2}, at: __queue_map_get+0x31/0x250
> {INITIAL USE} state was registered at:
>   lock_acquire+0x1a3/0x4b0
>   _raw_spin_lock_irqsave+0x48/0x60
>   __queue_map_get+0x31/0x250
>   bpf_prog_577904e86c81dead_func+0x12/0x4b4
>   trace_call_bpf+0x262/0x5d0
>   perf_trace_run_bpf_submit+0x91/0x1c0
>   perf_trace_sched_switch+0x46c/0x700
>   __schedule+0x11b5/0x24a0
>   schedule+0xd4/0x270
>   futex_wait_queue_me+0x25f/0x520
>   futex_wait+0x1e0/0x5f0
>   do_futex+0x395/0x1890
>   __x64_sys_futex+0x1cb/0x480
>   do_syscall_64+0x3b/0xc0
>   entry_SYSCALL_64_after_hwframe+0x44/0xae
> irq event stamp: 13640
> hardirqs last  enabled at (13639): [<ffffffff95eb2bf4>]
> _raw_spin_unlock_irq+0x24/0x40
> hardirqs last disabled at (13640): [<ffffffff95eb2d4d>]
> _raw_spin_lock_irqsave+0x5d/0x60
> softirqs last  enabled at (13464): [<ffffffff93e26de5>] __sys_bpf+0x3e15/0x4e80
> softirqs last disabled at (13462): [<ffffffff93e26da3>] __sys_bpf+0x3dd3/0x4e80
>
> other info that might help us debug this:
>  Possible unsafe locking scenario:
>
>        CPU0
>        ----
>   lock(&qs->lock);
>   <Interrupt>
>     lock(&qs->lock);

Hmm, so that lock() uses raw_spin_lock_irqsave(), which *should* be
disabling interrupts entirely for the critical section. But I guess a
Perf hardware event can still trigger? Which seems like it would
potentially wreak havoc with lots of things, not just this queue map
function?

No idea how to protect against this, though. Hoping Arnaldo knows? :)

-Toke