Time to enable partial relro

[Warner Losh]

On Fri, Aug 26, 2016 at 9:06 AM, Pedro Giffuni <pfg at freebsd.org> wrote:
>
>
> On 08/26/16 10:01, Warner Losh wrote:
>>
>> On Fri, Aug 26, 2016 at 8:36 AM, Ed Maste <emaste at freebsd.org> wrote:
>>>
>>> On 26 August 2016 at 10:18, Warner Losh <imp at bsdimp.com> wrote:
>>>>
>>>>
>>>> So what's the summary of why we'd want to do that? What benefit does it
>>>> bring?
>>>> Sure, other folks do it, but why?
>>>
>>>
>>> It's a relatively low cost technique to mitigate certain
>>> vulnerabilities. rtld needs to write to some sections during load but
>>> they don't need to be writeable after starting the program. relro
>>> reorders the output sections so that they are grouped together, and
>>> rtld remaps them read-only on start. This is often called "partial
>>> relro." I don't know of any real downside to enabling it, other than
>>> it could possibly break some strangely built third party software.
>>> It's been enabled on other platforms for quite some time though and I
>>> doubt we'd run into new issues.
>>>
>>> It doesn't bring a huge benefit by itself though; the PLT is still
>>> writeable. Adding "-z now" to the linker invocation produces "full
>>> relro" which makes the PLT read-only too. It has a negative impact on
>>> process start-up time though.
>>
>>
>> Sounds like this has implications for all the RTLD on all our
>> architectures. Has this been tested across all of them?
>>
>
> It affects anything ELF yes, but AFAICT the change is platform independent.

That's a different answer than 'it's been tested on all platforms and
it's fine.'

Warner