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

Dave Hansen
2022-02-08T01:06:23Z

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.

> 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.

> 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.