netdev - Re: [PATCH bpf] bpf: fix sanitation rewrite in case of non-pointers

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Date:   Fri, 1 Mar 2019 15:18:23 -0800
From:   Song Liu <liu.song.a23@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Alexei Starovoitov <ast@...com>, afabre@...udflare.com,
        Marek Majkowski <marek@...udflare.com>, bpf@...r.kernel.org,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf] bpf: fix sanitation rewrite in case of non-pointers

On Fri, Mar 1, 2019 at 1:06 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> Marek reported that he saw an issue with the below snippet in that
> timing measurements where off when loaded as unpriv while results
> were reasonable when loaded as privileged:
>
>     [...]
>     uint64_t a = bpf_ktime_get_ns();
>     uint64_t b = bpf_ktime_get_ns();
>     uint64_t delta = b - a;
>     if ((int64_t)delta > 0) {
>     [...]
>
> Turns out there is a bug where a corner case is missing in the fix
> d3bd7413e0ca ("bpf: fix sanitation of alu op with pointer / scalar
> type from different paths"), namely fixup_bpf_calls() only checks
> whether aux has a non-zero alu_state, but it also needs to test for
> the case of BPF_ALU_NON_POINTER since in both occasions we need to
> skip the masking rewrite (as there is nothing to mask).
>
> Fixes: d3bd7413e0ca ("bpf: fix sanitation of alu op with pointer / scalar type from different paths")
> Reported-by: Marek Majkowski <marek@...udflare.com>
> Reported-by: Arthur Fabre <afabre@...udflare.com>
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> Link: https://lore.kernel.org/netdev/CAJPywTJqP34cK20iLM5YmUMz9KXQOdu1-+BZrGMAGgLuBWz7fg@mail.gmail.com/T/
> ---
>  [ Test case will be routed via bpf-next to avoid useless merge churn
>    due to test_verifier rework in bpf-next. ]
>
>  kernel/bpf/verifier.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 8f295b790297..5fcce2f4209d 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -6920,7 +6920,8 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env)
>                         u32 off_reg;
>
>                         aux = &env->insn_aux_data[i + delta];
> -                       if (!aux->alu_state)
> +                       if (!aux->alu_state ||
> +                           aux->alu_state == BPF_ALU_NON_POINTER)

alu_state is a bitmap. Shall we check "aux->alu_state &
BPF_ALU_NON_POINTER" here?

Thanks,
Song

>                                 continue;
>
>                         isneg = aux->alu_state & BPF_ALU_NEG_VALUE;
> --
> 2.17.1
>