Message-Id: <20190410180441.69e1e85d@mschwideX1>
Date:   Wed, 10 Apr 2019 18:04:41 +0200
From:   Martin Schwidefsky <schwidefsky@...ibm.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     heiko carstens <heiko.carstens@...ibm.com>,
        gor <gor@...ux.ibm.com>, libc-alpha <libc-alpha@...rceware.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        carlos <carlos@...hat.com>
Subject: Re: rseq/s390: choosing code signature

On Wed, 10 Apr 2019 11:57:36 -0400 (EDT)
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> ----- On Apr 10, 2019, at 11:52 AM, schwidefsky schwidefsky@...ibm.com wrote:
> 
> > On Wed, 10 Apr 2019 11:50:39 -0400 (EDT)
> > Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> >   
> >> ----- On Apr 10, 2019, at 6:32 AM, schwidefsky schwidefsky@...ibm.com wrote:
> >>   
> >> > On Tue, 9 Apr 2019 15:32:22 -0400 (EDT)
> >> > Mathieu Desnoyers <mathieu.desnoyers@...icios.com> 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 ?

Should be fine, the worst that can happen is a minor merge conflict.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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