linux-kernel - Re: [PATCH 6/6] x86: Fix stray A/D bit setting into non-present PTEs

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Message-ID: <57769188.9060708@sr71.net>
Date:	Fri, 1 Jul 2016 08:51:36 -0700
From:	Dave Hansen <dave@...1.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Andi Kleen <ak@...ux.intel.com>,
	Michal Hocko <mhocko@...e.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 6/6] x86: Fix stray A/D bit setting into non-present PTEs

On 07/01/2016 07:25 AM, Eric W. Biederman wrote:
> Linus Torvalds <torvalds@...ux-foundation.org> writes:
>> > On Thu, Jun 30, 2016 at 9:39 PM, Dave Hansen <dave@...1.net> wrote:
>>> >>
>>> >> I think what you suggest will work if we don't consider A/D in
>>> >> pte_none().  I think there are a bunch of code path where assume that
>>> >> !pte_present() && !pte_none() means swap.
>> >
>> > Yeah, we would need to change pte_none() to mask off D/A, but I think
>> > that might be the only real change needed (other than making sure that
>> > we don't use the bits in the swap entries, I didn't look at that part
>> > at all)
> It looks like __pte_to_swp_entry also needs to be changed to mask out
> those bits when the swap code reads pte entries.  For all of the same
> reasons as pte_none.

I guess that would be nice, but isn't it redundant?

static inline swp_entry_t pte_to_swp_entry(pte_t pte)
{
	...
        arch_entry = __pte_to_swp_entry(pte);
	return swp_entry(__swp_type(arch_entry), __swp_offset(arch_entry));
}

As long as __swp_type() and __swp_offset() don't let A/D through, then
we should be OK.  This site is the only call to __pte_to_swp_entry()
that I can find in the entire codebase.

Or am I missing something?