Re: Possible deadlock in bpf queue map

["Toke Høiland-Jørgensen" <toke@kernel.org>]

Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:

> On Thu, Sep 7, 2023 at 6:04 AM Toke Høiland-Jørgensen <toke@kernel.org> wrote:
>>
>> Kumar Kartikeya Dwivedi <memxor@gmail.com> writes:
>>
>> > On Thu, 7 Sept 2023 at 12:26, Toke Høiland-Jørgensen <toke@kernel.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?

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)) {