Date:   Wed, 10 Apr 2019 11:50:39 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     schwidefsky <schwidefsky@...ibm.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 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.

Thanks,

Mathieu


> --
> commit 2ee28f6d1de968a71f074ab150384b90b4121216
> Author: Martin Schwidefsky <schwidefsky@...ibm.com>
> Date:   Wed Apr 10 12:28:41 2019 +0200
> 
>    s390/rseq: use trap4 for RSEQ_SIG
>    
>    Use trap4 as the guard instruction for the restartable sequence abort
>    handler.
>    
>    Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>
> 
> diff --git a/tools/testing/selftests/rseq/rseq-s390.h
> b/tools/testing/selftests/rseq/rseq-s390.h
> index 1069e85258ce..d4c8e1147d86 100644
> --- a/tools/testing/selftests/rseq/rseq-s390.h
> +++ b/tools/testing/selftests/rseq/rseq-s390.h
> @@ -1,6 +1,13 @@
> /* SPDX-License-Identifier: LGPL-2.1 OR MIT */
> 
> -#define RSEQ_SIG	0x53053053
> +/*
> + * RSEQ_SIG uses the trap4 instruction. As Linux does not make use of the
> + * access-register mode nor the linkage stack this instruction will always
> + * cause a special-operation exception (the trap-enabled bit in the DUCT
> + * is and will stay 0). The instruction pattern is
> + *	b2 ff 0f ff	trap4   4095(%r0)
> + */
> +#define RSEQ_SIG	0xB2FF0FFF
> 
> #define rseq_smp_mb()	__asm__ __volatile__ ("bcr 15,0" ::: "memory")
> #define rseq_smp_rmb()	rseq_smp_mb()
> --
> blue skies,
>   Martin.
> 
> "Reality continues to ruin my life." - Calvin.

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

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