Messages in this thread Patch in this message | | | Date | Tue, 9 Apr 2019 16:43:42 -0400 (EDT) | From | Mathieu Desnoyers <> | Subject | Re: rseq/x86: choosing rseq code signature |
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----- On Apr 9, 2019, at 3:32 PM, 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. > > 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): > > x86-32: > - .byte 0x0f, 0x1f, 0x05: nopl <sig> > > x86-64: > - .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########
Peter Zijlstra suggested to use "invlpg" in user-space, which should generate a trap. The only concern would be emulators, but ideally they would not try to decode an instruction that is never executed. This would lead to the following patch. Any objections/ack ?
diff --git a/tools/testing/selftests/rseq/rseq-x86.h b/tools/testing/selftests/rseq/rseq-x86.h index 2d4887b5d3f0..e9c8a9879e18 100644 --- a/tools/testing/selftests/rseq/rseq-x86.h +++ b/tools/testing/selftests/rseq/rseq-x86.h @@ -7,6 +7,11 @@ #include <stdint.h> +/* + * RSEQ_SIG is used with the following privileged instructions, which trap in user-space: + * x86-32: 0f 01 3d 53 30 05 53 invlpg 0x53053053 + * x86-64: 0f 01 3d 53 30 05 53 invlpg 0x53053053(%rip) + */ #define RSEQ_SIG 0x53053053 #ifdef __x86_64__ @@ -78,8 +83,8 @@ do { \ #define RSEQ_ASM_DEFINE_ABORT(label, teardown, abort_label) \ ".pushsection __rseq_failure, \"ax\"\n\t" \ - /* Disassembler-friendly signature: nopl <sig>(%rip). */\ - ".byte 0x0f, 0x1f, 0x05\n\t" \ + /* Disassembler-friendly signature: invlpg <sig>(%rip). */\ + ".byte 0x0f, 0x01, 0x3d\n\t" \ ".long " __rseq_str(RSEQ_SIG) "\n\t" \ __rseq_str(label) ":\n\t" \ teardown \ @@ -605,8 +610,8 @@ do { \ #define RSEQ_ASM_DEFINE_ABORT(label, teardown, abort_label) \ ".pushsection __rseq_failure, \"ax\"\n\t" \ - /* Disassembler-friendly signature: nopl <sig>. */ \ - ".byte 0x0f, 0x1f, 0x05\n\t" \ + /* Disassembler-friendly signature: invlpg <sig>. */ \ + ".byte 0x0f, 0x1f, 0x3d\n\t" \ ".long " __rseq_str(RSEQ_SIG) "\n\t" \ __rseq_str(label) ":\n\t" \ teardown \
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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