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

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Date:	Fri, 01 Jul 2016 13:12:55 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Dave Hansen <dave@...1.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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

Dave Hansen <dave@...1.net> writes:

> 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?

Given that __pte_to_swp_entry on x86_64 is just __pte_val or pte.pte it
does no filtering.  Similarly __swp_type(arch_entry) is a >> and
swp_entry(type, ...) is a << of what appears to be same amount
for the swap type.

So any corruption in the upper bits of the pte will be preserved as a
swap type.

In fact I strongly suspect that the compiler can optimize out all of the
work done by "swp_entry(__swp_type(arch_entry), _swp_offset(arch_entry))".

Eric