On Wed, 10 Apr 2019 11:50:39 -0400 (EDT) Mathieu Desnoyers <mathieu.desnoy...@efficios.com> wrote:
> ----- On Apr 10, 2019, at 6:32 AM, schwidefsky schwidef...@de.ibm.com wrote: > > > On Tue, 9 Apr 2019 15:32:22 -0400 (EDT) > > Mathieu Desnoyers <mathieu.desnoy...@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. Just added the patch to s390/linux:features for the next merge window as well. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin.