rseq/x86: choosing rseq code signature

From: Mathieu Desnoyers
Date: Tue Apr 09 2019 - 15:32:21 EST


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.

Currently, tools/testing/selftests/rseq/rseq-x86.h defines RSEQ_SIG
as 0x53053053, and uses it as an immediate operand to the following
instruction opcodes (as suggested by Andy Lutomirski):

- .byte 0x0f, 0x1f, 0x05: nopl <sig>

- .byte 0x0f, 0x1f, 0x05: nopl <sig>(%rip)

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.

The main advantage of choosing a trap instruction over a no-op is to ensure the
program traps if the execution flow gets redirected to the signature by mistake
(makes it easier to debug). It's not a hard requirement, but it would be a bonus.

Are there trap instructions that take an uncommon 4-byte immediate
operand you would recommend on x86 32/64 ? Or is the current choice of
nopl confirmed to be right one ?

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########



Mathieu Desnoyers
EfficiOS Inc.