Re: rseq/s390: choosing code signature

[Martin Schwidefsky <schwidefsky@de.ibm.com>]

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
--
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.