On Wed, 10 Apr 2019 11:57:36 -0400 (EDT) Mathieu Desnoyers <mathieu.desnoy...@efficios.com> wrote:
> ----- On Apr 10, 2019, at 11:52 AM, schwidefsky schwidef...@de.ibm.com wrote: > > > 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. > > 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.
