[kernel-hardening] Re: x86: PIE support and option to extend KASLR randomization

[Thomas Garnier <thgarnie@google.com>]

On Thu, Sep 21, 2017 at 9:24 PM, Markus Trippelsdorf
<markus@trippelsdorf.de> wrote:
> On 2017.09.21 at 14:21 -0700, Thomas Garnier wrote:
>> On Thu, Sep 21, 2017 at 9:10 AM, Ard Biesheuvel
>> <ard.biesheuvel@linaro.org> wrote:
>> >
>> > On 21 September 2017 at 08:59, Ingo Molnar <mingo@kernel.org> wrote:
>> > >
>> > > ( Sorry about the delay in answering this. I could blame the delay on the merge
>> > >   window, but in reality I've been procrastinating this is due to the permanent,
>> > >   non-trivial impact PIE has on generated C code. )
>> > >
>> > > * Thomas Garnier <thgarnie@google.com> wrote:
>> > >
>> > >> 1) PIE sometime needs two instructions to represent a single
>> > >> instruction on mcmodel=kernel.
>> > >
>> > > What again is the typical frequency of this occurring in an x86-64 defconfig
>> > > kernel, with the very latest GCC?
>> > >
>> > > Also, to make sure: which unwinder did you use for your measurements,
>> > > frame-pointers or ORC? Please use ORC only for future numbers, as
>> > > frame-pointers is obsolete from a performance measurement POV.
>> > >
>> > >> 2) GCC does not optimize switches in PIE in order to reduce relocations:
>> > >
>> > > Hopefully this can either be fixed in GCC or at least influenced via a compiler
>> > > switch in the future.
>> > >
>> >
>> > There are somewhat related concerns in the ARM world, so it would be
>> > good if we could work with the GCC developers to get a more high level
>> > and arch neutral command line option (-mkernel-pie? sounds yummy!)
>> > that stops the compiler from making inferences that only hold for
>> > shared libraries and/or other hosted executables (GOT indirections,
>> > avoiding text relocations etc). That way, we will also be able to drop
>> > the 'hidden' visibility override at some point, which we currently
>> > need to prevent the compiler from redirecting all global symbol
>> > references via entries in the GOT.
>>
>> My plan was to add a -mtls-reg=<fs|gs> to switch the default segment
>> register for stack cookies but I can see great benefits in having a
>> more general kernel flag that would allow to get rid of the GOT and
>> PLT when you are building position independent code for the kernel. It
>> could also include optimizations like folding switch tables etc...
>>
>> Should we start a separate discussion on that? Anyone that would be
>> more experienced than I to push that to gcc & clang upstream?
>
> Just open a gcc bug. See
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81708 as an example.

Make sense, I will look into this. Thanks Andy for the stack cookie bug!

>
> --
> Markus



-- 
Thomas