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Message-Id: <20190410123258.37f182cf@mschwideX1> Date: Wed, 10 Apr 2019 12:32:58 +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 O'Donell" <carlos@...hat.com> Subject: Re: rseq/s390: choosing code signature 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 -- 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.
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