Re: [PATCH 07/35] x86/mm: Remove _PAGE_DIRTY from kernel RO pages

Edgecombe, Rick P
2022-02-08T22:53:38Z

On Mon, 2022-02-07 at 16:13 -0800, Dave Hansen wrote:
> On 1/30/22 13:18, Rick Edgecombe wrote:
> > The x86 family of processors do not directly create read-only and
> > Dirty
> > PTEs. These PTEs are created by software.
>
> That's not strictly correct.
>
> There's nothing in the architecture today to prevent the CPU from
> creating Write=0,Dirty=1 PTEs. In fact, some CPUs do this in weird
> situations. It wouldn't be wrong to say:
>
> Processors sometimes directly create read-only and Dirty PTEs.
>
> which is the opposite of what is written above. This is why the CET
> spec has the blurb about shadow-stack-supporting CPUs promise not to
> do
> this any more.

Yea, it's wrong. The whole point of the new assurance is that it could
before as you say.

>
> > One such case is that kernel
> > read-only pages are historically setup as Dirty.
>
> ^ set up
>
> > New processors that support Shadow Stack regard read-only and Dirty
> > PTEs as
> > shadow stack pages.
>
> This also isn't *quite* correct. It's not just having a new
> processor,
> it includes enabling shadow stacks.

Right.

>
> > This results in ambiguity between shadow stack and kernel read-only
> > pages. To resolve this, removed Dirty from kernel read- only
> > pages.
>
> One thing that's not clear from the spec: does this cause an *actual*
> problem? For instance, does setting:
>
> IA32_U_CET.SH_STK_EN=1
> but
> IA32_S_CET.SH_STK_EN=0
>
> means that shadow stacks are enforced in user *MODE* or on
> user-paging-permission (U=0) PTEs?
>
> I think it's modes, but it would be nice to be clear. *BUT*, if this
> is
> accurate, doesn't it also mean that this patch is not strictly
> necessary?
>
> Don't get me wrong, the patch is probably still a good idea, but
> let's
> make sure we get the exact reasoning clear.

Yea, I think this is just a tying up loose ends thing. It is not
functionally needed until there would be shadow stack mode for kernel.
I'll update the patch to make this clear.