linux-kernel - Re: [GIT PULL] x86/shstk for 6.4

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Message-ID: <27770379-5e65-d231-f7ee-dff3975eeeda@intel.com>
Date:   Mon, 8 May 2023 17:07:26 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] x86/shstk for 6.4

On 5/8/23 16:31, Linus Torvalds wrote:
> On Mon, May 8, 2023 at 3:57 PM Dave Hansen <dave.hansen@...el.com> wrote:
...
>> This behavior is gone on shadow stack CPUs
> 
> Ok, so Intel has actually tightened up the rules on setting dirty, and
> now guarantees that it will set dirty only if the pte is actually
> writable?

Yep:

	Specifically, a processor that supports CET will never set the
	dirty flag in a paging-structure entry in which the R/W flag is
	clear.

and this was _absolutely_ one of the things the hardware folks did for
the benefit of software.

As for the mm->users==1 optimization, seems like something sane to
explore.  I can't think of any ways off the top of my head that it would
break, but I'll go take a closer look.