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Date: Mon, 6 Aug 2018 18:45:01 +0100
From: Robin Murphy <robin.murphy@....com>
To: Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc: Kernel Hardening <kernel-hardening@...ts.openwall.com>,
 Mark Rutland <mark.rutland@....com>, Kees Cook <keescook@...omium.org>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon
 <will.deacon@....com>, Christoffer Dall <christoffer.dall@....com>,
 linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
 Laura Abbott <labbott@...oraproject.org>,
 Julien Thierry <julien.thierry@....com>
Subject: Re: [RFC/PoC PATCH 0/3] arm64: basic ROP mitigation



On 06/08/18 17:04, Ard Biesheuvel wrote:
> On 6 August 2018 at 17:50, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
>> On 6 August 2018 at 17:38, Robin Murphy <robin.murphy@....com> wrote:
>>> On 06/08/18 15:04, Ard Biesheuvel wrote:
>>>>
>>>> On 6 August 2018 at 15:55, Robin Murphy <robin.murphy@....com> wrote:
>>>>>
>>>>> On 02/08/18 14:21, Ard Biesheuvel wrote:
>>>>>>
>>>>>>
>>>>>> This is a proof of concept I cooked up, primarily to trigger a
>>>>>> discussion
>>>>>> about whether there is a point to doing anything like this, and if there
>>>>>> is, what the pitfalls are. Also, while I am not aware of any similar
>>>>>> implementations, the idea is so simple that I would be surprised if
>>>>>> nobody
>>>>>> else thought of the same thing way before I did.
>>>>>
>>>>>
>>>>>
>>>>> So, "TTBR0 PAN: Pointer Auth edition"? :P
>>>>>
>>>>>> The idea is that we can significantly limit the kernel's attack surface
>>>>>> for ROP based attacks by clearing the stack pointer's sign bit before
>>>>>> returning from a function, and setting it again right after proceeding
>>>>>> from the [expected] return address. This should make it much more
>>>>>> difficult
>>>>>> to return to arbitrary gadgets, given that they rely on being chained to
>>>>>> the next via a return address popped off the stack, and this is
>>>>>> difficult
>>>>>> when the stack pointer is invalid.
>>>>>>
>>>>>> Of course, 4 additional instructions per function return is not exactly
>>>>>> for free, but they are just movs and adds, and leaf functions are
>>>>>> disregarded unless they allocate a stack frame (this comes for free
>>>>>> because simple_return insns are disregarded by the plugin)
>>>>>>
>>>>>> Please shoot, preferably with better ideas ...
>>>>>
>>>>>
>>>>>
>>>>> Actually, on the subject of PAN, shouldn't this at least have a very hard
>>>>> dependency on that? AFAICS without PAN clearing bit 55 of SP is
>>>>> effectively
>>>>> giving userspace direct control of the kernel stack (thanks to TBI).
>>>>> Ouch.
>>>>>
>>>>
>>>> How's that? Bits 52 .. 54 will still be set, so SP will never contain
>>>> a valid userland address in any case. Or am I missing something?
>>>
>>>
>>> Ah, yes, I'd managed to forget about the address hole, but I think that only
>>> makes it a bit trickier, rather than totally safe - it feels like you just
>>> need to chain one or two returns through "valid" targets until you can hit
>>> an epilogue with a "mov sp, x29" (at first glance there are a fair few of
>>> those in my vmlinux), after which we're back to the bit 55 scheme alone
>>> giving no protection against retargeting the stack to a valid TTBR0 address.
>>>
>>
>> Wouldn't such an epilogue clear the SP bit before returning again?
>>
> 
> ... or are you saying you can play tricks and clear bits 52 .. 54 ? If
> so, you can already do that, right? And apply it to bit 55 as well?

Indeed, in this scenario clearing bit 55 immediately before the final 
ret does nothing because the "valid" return beforehand loaded x29 with 
an arbitrary userspace address from a doctored stack frame, so the rest 
of that epilogue beyond that first mov already ran off the fake stack.

Admittedly you might have to retain control of the "real" kernel stack 
and go through much the same dance if the gadget chain ever needs to 
pass through a real return target (to mitigate bit 55 being 
unconditionally set again to make an invalid TTBR1 address). Working 
around the mitigations certainly makes the exploit more difficult, but 
still seemingly far from impossible. And yes, AFAICS an attacker could 
indeed use the same SP-hijacking trick today (in the same absence of 
PAN), it's just that without any mitigations to prevent using the kernel 
stack alone I can't imagine it would be worth the extra complication.

I guess what I'm getting at is that if the protection mechanism is 
"always return with SP outside TTBR1", there seems little point in going 
through the motions if SP in TTBR0 could still be valid and allow an 
attack to succeed anyway; this is basically just me working through a 
justification for saying the proposed scheme needs "depends on ARM64_PAN 
|| ARM64_SW_TTBR0_PAN", making it that much uglier for v8.0 CPUs...

Robin.

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