linux-kernel - Re: [PATCH 6/7] mm/x86: Add missing pud helpers

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Message-ID: <59d518698f664e07c036a5098833d7b56b953305.camel@intel.com>
Date: Fri, 21 Jun 2024 22:06:38 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "peterx@...hat.com" <peterx@...hat.com>
CC: "hughd@...gle.com" <hughd@...gle.com>, "willy@...radead.org"
	<willy@...radead.org>, "Hansen, Dave" <dave.hansen@...el.com>,
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	<dan.j.williams@...el.com>
Subject: Re: [PATCH 6/7] mm/x86: Add missing pud helpers

On Fri, 2024-06-21 at 16:27 -0400, Peter Xu wrote:
> On Fri, Jun 21, 2024 at 07:36:30PM +0000, Edgecombe, Rick P wrote:
> > On Fri, 2024-06-21 at 07:51 -0700, Dave Hansen wrote:
> > > 
> > > But, still, what if you take a Dirty=1,Write=1 pud and pud_modify() it
> > > to make it Dirty=1,Write=0?  What prevents that from being
> > > misinterpreted by the hardware as being a valid 1G shadow stack mapping?
> > 
> > Hmm, it looks like we could use an arch_check_zapped_pud() that does a
> > warning
> > like arch_check_zapped_pte/pmd() too. Not that we had no use for one before
> > this.
> 
> I can definitely look into that, but this check only happens when zapping,
> and IIUC it means there can still be outliers floating around.  I wonder
> whether it should rely on page_table_check_pxx_set() from that regard.

Yes, it's not perfect. Hmm, it looks like the page_table_check would catch a lot
more cases, but it would have to be changed to plumb the vma in.