[PATCH v3 6/8] arm64: module-plts: Extend veneer to address 52-bit VAs

[Ard Biesheuvel]

On 11 May 2018 at 12:11, Steve Capper <steve.capper at arm.com> wrote:
> On Fri, May 11, 2018 at 12:01:05AM +0200, Ard Biesheuvel wrote:
>> On 10 May 2018 at 18:23, Steve Capper <steve.capper at arm.com> wrote:
>> > From: Ard Bieusheuval <ard.biesheuvel at linaro.org>
>> >
>> > In preparation for 52-bit VA support in the Linux kernel, we extend the
>> > plts veneer to support 52-bit addresses via an extra movk instruction.
>> >
>> > [Steve: code from Ard off-list, changed the #ifdef logic to inequality]
>> > Signed-off-by: Steve Capper <steve.capper at arm.com>
>> >
>> > ---
>> >
>> > New in V3 of the series.
>> >
>> > I'm not sure if this is strictly necessary as the VAs of the module
>> > space will fit within 48-bits of addressing even when a 52-bit VA space
>> > is enabled.
>>
>> What about the kernel text itself? Is that also guaranteed to have
>> bits [51:48] of its VAs equal 0xf, even under randomization?
>>
>> If so, I agree we don't need the patch.
>>
>
> Hi Ard,
> The kernel modules and text are guaranteed to have addresses greater
> than or equal to KASAN_SHADOW_END (same for both 48, 52-bit VAs) or
> _VA_START(VA_BITS_MIN) (same for both 48, 52-bit VAs). Also, IIUC, the
> KASLR displacemnt is always non-negative?
>

Correct.

> So I think we're safe in that modules and kernel text will be 48-bit
> addressable in 52-bit configurations.
>
> I'll have a think about a BUILD_BUG to capture any change to the above.
>

Yes please