Re: rseq/arm32: choosing rseq code signature

From: Mathieu Desnoyers
Date: Wed Apr 10 2019 - 16:29:23 EST


----- On Apr 9, 2019, at 3:32 PM, Mathieu Desnoyers mathieu.desnoyers@xxxxxxxxxxxx wrote:

> Hi Will,
>
> 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 (e.g. thumb for arm), 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-arm.h file that updates
> the signature value, so I can then pick it up for the glibc
> patchset.

Would the following diff work for you ? If so, can I get your
acked-by ?

diff --git a/tools/testing/selftests/rseq/rseq-arm.h b/tools/testing/selftests/rseq/rseq-arm.h
index 5f262c54364f..1f261ad2ac1b 100644
--- a/tools/testing/selftests/rseq/rseq-arm.h
+++ b/tools/testing/selftests/rseq/rseq-arm.h
@@ -5,7 +5,17 @@
* (C) Copyright 2016-2018 - Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
*/

-#define RSEQ_SIG 0x53053053
+/*
+ * RSEQ_SIG uses the udf A32 instruction with an uncommon immediate operand
+ * value 0x5305. This traps if user-space reaches this instruction by mistake,
+ * and the uncommon operand ensures the kernel does not move the instruction
+ * pointer to attacker-controlled code on rseq abort.
+ *
+ * The instruction pattern is:
+ *
+ * e7f530f5 udf #21253 ; 0x5305
+ */
+#define RSEQ_SIG 0xe7f530f5

#define rseq_smp_mb() __asm__ __volatile__ ("dmb" ::: "memory", "cc")
#define rseq_smp_rmb() __asm__ __volatile__ ("dmb" ::: "memory", "cc")
@@ -78,7 +88,8 @@ do { \
__rseq_str(table_label) ":\n\t" \
".word " __rseq_str(version) ", " __rseq_str(flags) "\n\t" \
".word " __rseq_str(start_ip) ", 0x0, " __rseq_str(post_commit_offset) ", 0x0, " __rseq_str(abort_ip) ", 0x0\n\t" \
- ".word " __rseq_str(RSEQ_SIG) "\n\t" \
+ ".arm\n\t" \
+ ".inst " __rseq_str(RSEQ_SIG) "\n\t" \
__rseq_str(label) ":\n\t" \
teardown \
"b %l[" __rseq_str(abort_label) "]\n\t"


>
> Thanks!
>
> Mathieu
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com

--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com