[PATCH bpf-next] bpf, doc: Update bpf_jit_enable limitation for CONFIG_BPF_JIT_ALWAYS_ON

[PATCH bpf-next] bpf, doc: Update bpf_jit_enable limitation for CONFIG_BPF_JIT_ALWAYS_ON

[Leo Yan]

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@linaro.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.
+
 In the kernel source tree under tools/bpf/, there's bpf_jit_disasm for
 generating disassembly out of the kernel log's hexdump:
 
-- 
1.9.1

Re: [PATCH bpf-next] bpf, doc: Update bpf_jit_enable limitation for CONFIG_BPF_JIT_ALWAYS_ON

[Daniel Borkmann]

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@linaro.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

Re: [PATCH bpf-next] bpf, doc: Update bpf_jit_enable limitation for CONFIG_BPF_JIT_ALWAYS_ON

[Leo Yan]

On Fri, Apr 27, 2018 at 11:44:44AM +0200, Daniel Borkmann wrote:
> 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@linaro.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.

Yeah, your rephrasing is more clear and better.  Will do this and send
new patch soon.  Thanks for your helping.

> Thanks,
> Daniel

Re: [PATCH bpf-next] bpf, doc: Update bpf_jit_enable limitation for CONFIG_BPF_JIT_ALWAYS_ON

[Daniel Borkmann]

On 04/27/2018 11:49 AM, Leo Yan wrote:
> On Fri, Apr 27, 2018 at 11:44:44AM +0200, Daniel Borkmann wrote:
>> 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@linaro.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.
> 
> Yeah, your rephrasing is more clear and better.  Will do this and send
> new patch soon.  Thanks for your helping.

Awesome, thank you!