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Date:   Wed, 10 Apr 2019 16:29:19 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Will Deacon <will.deacon@arm.com>
Cc:     libc-alpha <libc-alpha@sourceware.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        carlos <carlos@redhat.com>
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Subject: Re: rseq/arm32: choosing rseq code signature
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Thread-Topic: rseq/arm32: choosing rseq code signature
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----- On Apr 9, 2019, at 3:32 PM, Mathieu Desnoyers mathieu.desnoyers@efficios.com 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@efficios.com>
  */
 
-#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