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SubjectRe: rseq/s390: choosing code signature
----- On Apr 10, 2019, at 6:32 AM, schwidefsky schwidefsky@de.ibm.com wrote:

> On Tue, 9 Apr 2019 15:32:22 -0400 (EDT)
> Mathieu Desnoyers <mathieu.desnoyers@efficios.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@de.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@de.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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