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Date: Thu, 7 Sep 2023 09:08:32 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Toke Høiland-Jørgensen <toke@...nel.org>
Cc: Kumar Kartikeya Dwivedi <memxor@...il.com>, Hsin-Wei Hung <hsinweih@....edu>,
Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <kafai@...com>, Song Liu <songliubraving@...com>,
Yonghong Song <yhs@...com>, John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
Network Development <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: Possible deadlock in bpf queue map
On Thu, Sep 7, 2023 at 9:05 AM Toke Høiland-Jørgensen <toke@...nel.org> wrote:
>
> Alexei Starovoitov <alexei.starovoitov@...il.com> writes:
>
> > On Thu, Sep 7, 2023 at 6:04 AM Toke Høiland-Jørgensen <toke@...nel.org> wrote:
> >>
> >> Kumar Kartikeya Dwivedi <memxor@...il.com> writes:
> >>
> >> > On Thu, 7 Sept 2023 at 12:26, Toke Høiland-Jørgensen <toke@...nel.org> wrote:
> >> >>
> >> >> +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? :)
> >> >>
> >> >
> >> > The locking should probably be protected by a percpu integer counter,
> >> > incremented and decremented before and after the lock is taken,
> >> > respectively. If it is already non-zero, then -EBUSY should be
> >> > returned. It is similar to what htab_lock_bucket protects against in
> >> > hashtab.c.
> >>
> >> Ah, neat! Okay, seems straight-forward enough to replicate. Hsin, could
> >> you please check if the patch below gets rid of the splat?
> >
> > Instead of adding all this complexity for the map that is so rarely used
> > it's easier to disallow it perf_event prog types.
> > Or add a single if (in_nmi()) return -EBUSY.
>
> Heh, I was actually thinking the "nmi-safe lock" thing might be
> something that could be neatly encapsulated into the lock library
> helpers. But OK, so you mean just something like the below, then?
Yep.
In bpf timers we do:
if (in_nmi())
return -EOPNOTSUPP;
while in ringbuf we have:
if (in_nmi()) {
if (!spin_trylock_irqsave(&rb->spinlock, flags))
return NULL;
} else {
spin_lock_irqsave(&rb->spinlock, flags);
}
I think both options are fine to use with queue/stack map.
trylock approach is probably less drastic.
> I'll send a proper patch for that later if no one objects (or beats me
> to it) :)
>
> -Toke
>
> diff --git a/kernel/bpf/queue_stack_maps.c b/kernel/bpf/queue_stack_maps.c
> index 8d2ddcb7566b..138525424745 100644
> --- a/kernel/bpf/queue_stack_maps.c
> +++ b/kernel/bpf/queue_stack_maps.c
> @@ -98,6 +98,9 @@ static long __queue_map_get(struct bpf_map *map, void *value, bool delete)
> int err = 0;
> void *ptr;
>
> + if (in_nmi())
> + return -EBUSY;
> +
> raw_spin_lock_irqsave(&qs->lock, flags);
>
> if (queue_stack_map_is_empty(qs)) {
> @@ -128,6 +131,9 @@ static long __stack_map_get(struct bpf_map *map, void *value, bool delete)
> void *ptr;
> u32 index;
>
> + if (in_nmi())
> + return -EBUSY;
> +
> raw_spin_lock_irqsave(&qs->lock, flags);
>
> if (queue_stack_map_is_empty(qs)) {
> @@ -193,6 +199,9 @@ static long queue_stack_map_push_elem(struct bpf_map *map, void *value,
> if (flags & BPF_NOEXIST || flags > BPF_EXIST)
> return -EINVAL;
>
> + if (in_nmi())
> + return -EBUSY;
> +
> raw_spin_lock_irqsave(&qs->lock, irq_flags);
>
> if (queue_stack_map_is_full(qs)) {
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