netdev - Re: [PATCH] bpf: Fix recursion check in trampoline

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Date:   Tue, 27 Apr 2021 18:10:32 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [PATCH] bpf: Fix recursion check in trampoline

On Tue, Apr 27, 2021 at 3:42 PM Jiri Olsa <jolsa@...nel.org> wrote:
>
> The recursion check in __bpf_prog_enter and __bpf_prog_exit leaves
> some (not inlined) functions unprotected:
>
> In __bpf_prog_enter:
>   - migrate_disable is called before prog->active is checked
>
> In __bpf_prog_exit:
>   - migrate_enable,rcu_read_unlock_strict are called after
>     prog->active is decreased
>
> When attaching trampoline to them we get panic like:
>
>   traps: PANIC: double fault, error_code: 0x0
>   double fault: 0000 [#1] SMP PTI
>   RIP: 0010:__bpf_prog_enter+0x4/0x50
>   ...
>   Call Trace:
>    <IRQ>
>    bpf_trampoline_6442466513_0+0x18/0x1000
>    migrate_disable+0x5/0x50
>    __bpf_prog_enter+0x9/0x50
>    bpf_trampoline_6442466513_0+0x18/0x1000
>    migrate_disable+0x5/0x50
>    __bpf_prog_enter+0x9/0x50
>    bpf_trampoline_6442466513_0+0x18/0x1000
>    migrate_disable+0x5/0x50
>    __bpf_prog_enter+0x9/0x50
>    bpf_trampoline_6442466513_0+0x18/0x1000
>    migrate_disable+0x5/0x50
>    ...
>
> Making the recursion check before the rest of the calls
> in __bpf_prog_enter and as last call in __bpf_prog_exit.
>
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
>  kernel/bpf/trampoline.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> index 4aa8b52adf25..301735f7e88e 100644
> --- a/kernel/bpf/trampoline.c
> +++ b/kernel/bpf/trampoline.c
> @@ -558,12 +558,12 @@ static void notrace inc_misses_counter(struct bpf_prog *prog)
>  u64 notrace __bpf_prog_enter(struct bpf_prog *prog)
>         __acquires(RCU)
>  {
> -       rcu_read_lock();
> -       migrate_disable();
>         if (unlikely(__this_cpu_inc_return(*(prog->active)) != 1)) {
>                 inc_misses_counter(prog);
>                 return 0;
>         }
> +       rcu_read_lock();
> +       migrate_disable();

That obviously doesn't work.
After cpu_inc the task can migrate and cpu_dec
will happen on a different cpu likely underflowing
the counter into negative.
We can either mark migrate_disable as nokprobe/notrace or have bpf
trampoline specific denylist.