Gold Linker Patch: Introduce the "retpoline" x86 mitigation technique for variant #2 of the speculative execution vulnerabilities disclosed today, specifically identified by CVE-2017-5715 and in some places called "spectre".

Sriraman Tallam via binutils binutils@sourceware.org
Mon Jan 8 18:04:00 GMT 2018


Hello HJ,

On Sun, Jan 7, 2018 at 10:10 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Fri, Jan 5, 2018 at 6:53 PM, Alan Modra <amodra@gmail.com> wrote:
>> On Fri, Jan 05, 2018 at 03:28:34PM -0800, Cary Coutant wrote:
>>> > It's also incompatible with shadow stack support, so the binary marker for
>>> > that needs to be removed.
>>>
>>> Ugh. But that marker shouldn't be set in the first place, since this
>>> linker option is useful only in conjunction with a corresponding
>>> compiler option.
>>>
>>> > I don't think this is the right approach at all.  What is this trying to
>>> > accomplish?  What kind of speculation barrier does this implement on current
>>> > CPUs?  Isn't this *extremely* costly?
>>>
>>> Supposedly, this strategy aims to disable branch prediction for all
>>> indirect branches in a piece of code, so that attackers cannot use
>>> branch predictor training to force the speculative execution of any
>>> available "gadgets" in the target code. I haven't yet seen any claims
>>> where branch predictor training by itself can be exploited -- it's
>>> simply one way to exploit the cache side channel vulnerabilities.
>>
>> I don't think it's just the victim code.  It seems to me that you'd
>> need to disable indirect branch prediction for all indirect branches
>> in the victim address space.  So it won't be sufficient to simply
>> relink the app with fancy PLT call code.  You'd need to relink *all*
>> libraries that make PLT calls, including libc.so, too.  (libc PLT
>> calls to __tls_get_addr, calloc and any ifunc come to mind as possible
>> attack vectors.)  And of course recompile everything to mitigate any
>> inline function pointer calls.
>>
>> Unless I'm missing something, this makes the fancy PLT mitigation
>> unworkable in practice.  You will definitely not want a slow shared
>> libc, libstdc++ etc. to be used by all applications.  So build a set
>> of hardened static libraries and link them into your hardened app.
>> No PLT calls involved, and thus no PLT mitigation needed.
>>
>
> Adding x86-64 psABI group.
>
> Also Florian pointed out, this doesn't work for shadow stack.  If you
> are really concerned about PLT, you should avoid PLT altogether as
> suggested by
>
> https://github.com/hjl-tools/x86-psABI/wiki/x86-64-psABI-secure.pdf
>
> This feature has been implemented in GCC + binutils.

Are you referring to the no-plt feature?   If that is the case, I
contributed to  the -fno-plt option in GCC and also in LLVM and the
gold patches to binutils,  and I did bring this up.  The problem is
there is still an indirect jmp directly from the callsite via the GOT
and hence it is still exposed to the attack.

Thanks
Sri

>
> --
> H.J.



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