linux-kernel - Re: [PATCH 07/35] x86/mm: Remove _PAGE

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Date:   Tue, 8 Feb 2022 22:52:29 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "bsingharora@...il.com" <bsingharora@...il.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "Syromiatnikov, Eugene" <esyr@...hat.com>,
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        "rdunlap@...radead.org" <rdunlap@...radead.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Eranian, Stephane" <eranian@...gle.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "fweimer@...hat.com" <fweimer@...hat.com>,
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        "Hansen, Dave" <dave.hansen@...el.com>,
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CC:     "hch@....de" <hch@....de>, "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Subject: Re: [PATCH 07/35] x86/mm: Remove _PAGE_DIRTY from kernel RO pages

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.