[PATCH v5sub2 1/8] arm64: add support for module PLTs

[Ard Biesheuvel]

On 5 February 2016 at 17:46, Catalin Marinas <catalin.marinas at arm.com> wrote:
> On Fri, Feb 05, 2016 at 05:20:14PM +0100, Ard Biesheuvel wrote:
>> On 5 February 2016 at 17:00, Catalin Marinas <catalin.marinas at arm.com> wrote:
>> > On Fri, Feb 05, 2016 at 04:53:10PM +0100, Ard Biesheuvel wrote:
>> >> On 5 February 2016 at 16:42, Catalin Marinas <catalin.marinas at arm.com> wrote:
>> >> > On Thu, Feb 04, 2016 at 04:31:59PM +0100, Ard Biesheuvel wrote:
>> >> >> On 4 February 2016 at 16:13, Catalin Marinas <catalin.marinas at arm.com> wrote:
>> >> >> > On Mon, Feb 01, 2016 at 02:09:31PM +0100, Ard Biesheuvel wrote:
>> >> >> >> This adds support for emitting PLTs at module load time for relative
>> >> >> >> branches that are out of range. This is a prerequisite for KASLR, which
>> >> >> >> may place the kernel and the modules anywhere in the vmalloc area,
>> >> >> >> making it more likely that branch target offsets exceed the maximum
>> >> >> >> range of +/- 128 MB.
>> >> >> >
>> >> >> > Any downside to trying to keep the kernel+modules coupled together so
>> >> >> > that we avoid the PLT?
>> >> >>
>> >> >> First of all, note that it is unlikely that the PLTs are ever required
>> >> >> in practice, considering that either
>> >> >> a) the kernel is loaded at the default location right at the base of
>> >> >> the vmalloc range, and in this case, the module space is reserved for
>> >> >> modules only, or
>> >> >> b) the kernel is loaded at some random offset in the 240+ GB vmalloc
>> >> >> space, and it is unlikely that all VMA space around the kernel happens
>> >> >> to be given out to non-randomized vmalloc/ioremap allocations
>> >> >
>> >> > My worry is that we merge some code that's rarely tested.
>> >>
>> >> I understand. But unfortunately, having corner cases that are unlikely
>> >> but not impossible comes with the territory of randomization.
>> >>
>> >> Alternatively, we could take the performance hit if KASLR is in effect
>> >> and allocate each module completely randomly as well. This way, the
>> >> code is always exercised (for now), and we can always backpedal later
>> >> if the performance is measurably worse.
>> >
>> > I'm fine with this. You can post it as a separate patch that we can
>> > easily revert/modify later (like turning it into a config option).
>>
>> OK, I will hack something up
>
> If it's simpler, you can just add a config option but defaulting to the
> full vmalloc space for modules.
>

What would be the simplest is to randomize the 128 MB module region as
a whole, and either put it close to the kernel (which is what I am
doing now), or put it at a random offset inside the vmalloc space, in
which case all branches will be resolved via PLTs. My suggestion to
randomize each module_alloc() call separately is actually not that
straight-forward.

So what I propose now is to keep a single module_load_offset that
randomizes the base of the region, and add a default-n config option
that shrinks the interval it is chosen from so that PLTs are usually
not needed.

-- 
Ard.