Re: [PATCH] x86: Use entire page for the per-cpu GDT only if paravirt-enabled

From: Denys Vlasenko
Date: Mon Sep 28 2015 - 08:45:22 EST

On 09/28/2015 09:58 AM, Ingo Molnar wrote:
> * Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
>> On 09/26/2015 09:50 PM, H. Peter Anvin wrote:
>>> NAK. We really should map the GDT read-only on all 64 bit systems,
>>> since we can't hide the address from SLDT. Same with the IDT.
>> Sorry, I don't understand your point.
> So the problem is that right now the SGDT instruction (which is unprivileged)
> leaks the real address of the kernel image:
> fomalhaut:~> ./sgdt
> SGDT: ffff88303fd89000 / 007f
> that 'ffff88303fd89000' is a kernel address.

Thank you.
I do know that SGDT and friends are unprivileged on x86
and thus they allow userspace (and guest kernels in paravirt)
learn things they don't need to know.

I don't see how making GDT page-aligned and page-sized
changes anything in this regard. SGDT will still work,
and still leak GDT address.

> Your observation in the changelog and your patch:
>>>> It is page-sized because of paravirt. [...]
> ... conflicts with the intention to mark (remap) the primary GDT address read-only
> on native kernels as well.
> So what we should do instead is to use the page alignment properly and remap the
> GDT to a read-only location, and load that one.

If we'd have a small GDT (i.e. what my patch does), we still can remap the entire page
which contains small GDT, and simply don't care that some other data is also visible
through that RO page.

> This would have a couple of advantages:
> - This would give kernel address randomization more teeth on x86.
> - An additional advantage would be that rootkits overwriting the GDT would have
> a bit more work to do.
> - A third advantage would be that for NUMA systems we could 'mirror' the GDT into
> node-local memory and load those. This makes GDT load cache-misses a bit less
> expensive.

GDT is per-cpu. Isn't per-cpu memory already NUMA-local?

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