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Re: rseq/x86: choosing rseq code signature
- From: Mathieu Desnoyers <mathieu dot desnoyers at efficios dot com>
- To: Andy Lutomirski <luto at kernel dot org>
- Cc: Zack Weinberg <zackw at panix dot com>, Thomas Gleixner <tglx at linutronix dot de>, Peter Zijlstra <peterz at infradead dot org>, "H. Peter Anvin" <hpa at zytor dot com>, Andi Kleen <andi at firstfloor dot org>, Ingo Molnar <mingo at redhat dot com>, Borislav Petkov <bp at alien8 dot de>, libc-alpha <libc-alpha at sourceware dot org>, linux-kernel <linux-kernel at vger dot kernel dot org>, carlos <carlos at redhat dot com>, x86 <x86 at kernel dot org>
- Date: Wed, 10 Apr 2019 00:19:11 -0400 (EDT)
- Subject: Re: rseq/x86: choosing rseq code signature
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----- On Apr 9, 2019, at 9:57 PM, Andy Lutomirski firstname.lastname@example.org wrote:
> On Tue, Apr 9, 2019 at 5:51 PM Zack Weinberg <email@example.com> wrote:
>> On Tue, Apr 9, 2019 at 4:43 PM Mathieu Desnoyers
>> <firstname.lastname@example.org> wrote:
>> > ----- On Apr 9, 2019, at 3:32 PM, Mathieu Desnoyers
>> > email@example.com wrote:
>> > >
>> > > 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.
>> > 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 ?
>> > +/*
>> > + * 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
>> On x86, you have to worry about what happens if control flow gets
>> redirected to an arbitrary byte address. The proposed sequence `0f 01
>> 3d 53 30 05 53` is a trap instruction if control lands seven bytes
>> before the beginning of the abort handler, but if it lands anywhere
>> _else_ within the marker sequence, you get one of these instruction
>> sequences, none of which trap, all but one of which will corrupt the
>> process state, and three of which will consume three bytes from the
>> beginning of the abort handler's code, continuing execution with a
>> misaligned PC:
>> 01 3d 53 30 05 53 add %edi,0x53053053(%rip)
>> 3d 53 30 05 53 cmp $0x53053053,%eax
>> 53 30 05 53 XX XX XX push %rbx; xor %al,0xXXXXXX78(%rip)
>> 30 05 53 XX XX XX xor %al,0xXXXXXX78(%rip)
>> 05 53 XX XX XX add $0xXXXXXX53,%eax
>> 53 push %rbx
>> So I'm going to suggest instead the four-byte sequence CD CF CD CF.
>> That's INT $0xCF if control lands either two or four bytes before the
>> beginning of the abort handler, and IRET if it lands one or three
>> bytes before. I believe both of these possibilities are currently
>> also forbidden in user mode. It doesn't need to be longer, does it?
> IRET works in user mode just fine. Why are you concerned about
> landing in the middle of the signature? A misaligned jump into code
> is screwy pretty much no matter what. It does seem genuinely useful
> to trap if you accidentally fall through to the beginning of the
> signature, but I don't see the point of worrying about jumping to the
> There's some argument that, for consistency with CET, the last couple
> bytes of the signature should match ENDBR.
> Mathieu, how many bytes do we have for the signature?
The signature is 4 bytes. Those 4 bytes need to be uncommon.
You can have a longer instruction than that, but then the
additional bytes at the beginning of the instruction will
not be part of the signature per se.