Re: rseq/s390: choosing code signature

[Mathieu Desnoyers]

----- On Apr 10, 2019, at 11:52 AM, schwidefsky schwidefsky@xxxxxxxxxx wrote:

> On Wed, 10 Apr 2019 11:50:39 -0400 (EDT)
> Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> 
>> ----- On Apr 10, 2019, at 6:32 AM, schwidefsky schwidefsky@xxxxxxxxxx wrote:
>> 
>> > On Tue, 9 Apr 2019 15:32:22 -0400 (EDT)
>> > Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>> >   
>> >> Hi,
>> >> 
>> >> We are about to include the code signature required prior to restartable
>> >> sequences abort handlers into glibc, which will make this ABI choice final.
>> >> We need architecture maintainer input on that signature value.
>> >> 
>> >> That code signature is placed before each abort handler, so the kernel can
>> >> validate that it is indeed jumping to an abort handler (and not some
>> >> arbitrary attacker-chosen code). The signature is never executed.
>> >> 
>> >> The current discussion thread on the glibc mailing list leads us towards
>> >> using a trap with uncommon immediate operand, which simplifies integration
>> >> with disassemblers, emulators, makes it easier to debug if the control
>> >> flow gets redirected there by mistake, and is nicer for some architecture's
>> >> speculative execution.
>> >> 
>> >> We can have different signatures for each sub-architecture, as long as they
>> >> don't have to co-exist within the same process. We can special-case with
>> >> #ifdef for each sub-architecture and endianness if need be. If the architecture
>> >> has instruction set extensions that can co-exist with the architecture
>> >> instruction set within the same process, we need to take into account to which
>> >> instruction the chosen signature value would map (and possibly decide if we
>> >> need to extend rseq to support many signatures).
>> >> 
>> >> Here is an example of rseq signature definition template:
>> >> 
>> >> /*
>> >>  * TODO: document trap instruction objdump output on each sub-architecture
>> >>  * instruction sets, as well as instruction set extensions.
>> >>  */
>> >> #define RSEQ_SIG 0x########
>> >> 
>> >> Ideally we'd need a patch on top of the Linux kernel
>> >> tools/testing/selftests/rseq/rseq-s390.h file that updates
>> >> the signature value, so I can then pick it up for the glibc
>> >> patchset.
>> > 
>> > The trap4 instruction is a suitable one. The patch would look like this
>> 
>> Great! I'm picking it up into my rseq tree if that's OK with you.
> 
> Just added the patch to s390/linux:features for the next merge window as well.

Sounds good! I'll carry it in my tree to have a comprehensive up-to-date list of
rseq signatures for all architectures in a single tree. Worse-case the exact same
change will be pulled from both architecture and rseq trees, which I don't think
should be an issue, right ?

Thanks,

Mathieu

> 
> --
> blue skies,
>   Martin.
> 
> "Reality continues to ruin my life." - Calvin.

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com