[PATCH] arm64: mark kernel text segment as MEMBLOCK_NOMAP

[Ard Biesheuvel]

On 15 February 2016 at 12:45, Catalin Marinas <catalin.marinas at arm.com> wrote:
> On Mon, Feb 15, 2016 at 10:28:32AM +0100, Ard Biesheuvel wrote:
>> Commit 752af28bd711 ("arm64: move kernel image to base of vmalloc area")
>> moves the mapping of the kernel text and data segments out of the linear
>> region, and takes care not to create a writable alias of the read-only
>> kernel text segment by checking each memblock against overlap when the
>> memblocks are mapped into the linear mapping.
>>
>> However, it is more correct, and much simpler, to mark the [_stext, _etext]
>> interval as MEMBLOCK_NOMAP. This will also prevent the interval from being
>> omitted from the linear region, but this fact will now also be reflected
>> in the output of pfn_valid(), and so code that expects any pfn_valid()
>> page to be mapped and accessible (which is a reasonable assumption) does
>> not get surprised by the text segment being inaccessible via the linear
>> mapping.
>>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>> ---
>>
>> This should hopefully address the issue reported by James, but I suppose
>> more work is required on the hibernate side to ensure the unmapped text
>> region is preserved, since it is no longer covered by the linear mapping.
>
> Ard, I'm about to drop the kernel memory map patches from -next. There
> are several issues like KASAN (which may as well be a KASAN bug but I
> haven't got to the bottom of it yet), initrd (you have patches but
> require additional acks).
>
> I'll keep your stuff on the for-next/kernmap branch and do further
> debugging. If it stabilises in the next 1-2 weeks, I'll merge it into
> for-next/core for 4.6, otherwise, it will have to wait. I would really
> like these patches merged but it looks like they need wider testing.
>

Agreed.

> I have another branch with your kaslr patches but I didn't dare to merge
> them into for-next/core until we sort out the first sub-series.
>

Yes, I noticed. As I pointed out in its cover letter, I expected the
first subseries to be the most problematic in terms of fallout in
other areas, and I turned out to be right, unfortunately. The coverage
so far has been quite useful tbh, especially since I apparently never
tested initrd.

If you are going to drop it for now anyway, I can do a v6sub1 with
this issue and the initrd issue addressed, but also an issue with the
KVM ksym ref patch with GCC 4.8. Since these issues affect
bisectability, I'd prefer to rebase entirely, and fold all the fixes
rather than apply them on top.