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Date: Mon, 6 Aug 2018 17:50:16 +0200
From: Ard Biesheuvel <ard.biesheuvel@...aro.org>
To: Robin Murphy <robin.murphy@....com>
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 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?

>>> I wonder if there's a little  more mileage in using "{add,sub} sp, sp,
>>> #1"
>>> sequences to rely on stack alignment exceptions instead, with the added
>>> bonus that that's about as low as the instruction-level overhead can get.
>>>
>>
>> Good point. I did consider that, but couldn't convince myself that it
>> isn't easier to defeat: loads via x29 occur reasonably often, and you
>> can simply offset your doctored stack frame by a single byte.
>
>
> True; in theory there are 3072 possible unaligned offsets to choose from,
> but compile-time randomisation doesn't seem much use, and hotpatching just
> about every function call in the kernel isn't a nice thought either.
>
> Robin.

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