linux-kernel - Re: [PATCH bpf-next] bpf, doc: Update bpf_jit_enable limitation for CONFIG_BPF_JIT_ALWAYS

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Message-ID: <275e03a2-b74e-8f60-4ffe-26c9a79fae9d@iogearbox.net>
Date:   Fri, 27 Apr 2018 11:44:44 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Leo Yan <leo.yan@...aro.org>, Alexei Starovoitov <ast@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jonathan Corbet <corbet@....net>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH bpf-next] bpf, doc: Update bpf_jit_enable limitation for
 CONFIG_BPF_JIT_ALWAYS_ON

On 04/26/2018 04:26 AM, Leo Yan wrote:
> When CONFIG_BPF_JIT_ALWAYS_ON is enabled, kernel has limitation for
> bpf_jit_enable, so it has fixed value 1 and we cannot set it to 2
> for JIT opcode dumping; this patch is to update the doc for it.
> 
> Signed-off-by: Leo Yan <leo.yan@...aro.org>
> ---
>  Documentation/networking/filter.txt | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/Documentation/networking/filter.txt b/Documentation/networking/filter.txt
> index fd55c7d..feddab9 100644
> --- a/Documentation/networking/filter.txt
> +++ b/Documentation/networking/filter.txt
> @@ -483,6 +483,12 @@ Example output from dmesg:
>  [ 3389.935851] JIT code: 00000030: 00 e8 28 94 ff e0 83 f8 01 75 07 b8 ff ff 00 00
>  [ 3389.935852] JIT code: 00000040: eb 02 31 c0 c9 c3
>  
> +When CONFIG_BPF_JIT_ALWAYS_ON is enabled, bpf_jit_enable is set to 1 by default
> +and it returns failure if change to any other value from proc node; this is
> +for security consideration to avoid leaking info to unprivileged users. In this
> +case, we can't directly dump JIT opcode image from kernel log, alternatively we
> +need to use bpf tool for the dumping.
> +

Could you change this doc text a bit, I think it's slightly misleading. From the first
sentence one could also interpret that value 0 would leaking info to unprivileged users
whereas here we're only talking about the case of value 2. Maybe something roughly like
this to make it more clear:

  When CONFIG_BPF_JIT_ALWAYS_ON is enabled, bpf_jit_enable is permanently set to 1 and
  setting any other value than that will return in failure. This is even the case for
  setting bpf_jit_enable to 2, since dumping the final JIT image into the kernel log
  is discouraged and introspection through bpftool (under tools/bpf/bpftool/) is the
  generally recommended approach instead.

Thanks,
Daniel