Re: rseq/x86: choosing rseq code signature

[Mathieu Desnoyers]

----- On Apr 9, 2019, at 9:57 PM, Andy Lutomirski luto@xxxxxxxxxx wrote:

> On Tue, Apr 9, 2019 at 5:51 PM Zack Weinberg <zackw@xxxxxxxxx> wrote:
>>
>> On Tue, Apr 9, 2019 at 4:43 PM Mathieu Desnoyers
>> <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>> > ----- On Apr 9, 2019, at 3:32 PM, Mathieu Desnoyers
>> > mathieu.desnoyers@xxxxxxxxxxxx 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
> middle.
> 
> 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.

Thanks,

Mathieu


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