linux-kernel - Re: [PATCH v2] x86/mm: fix kern_addr_valid to cope with existing but not present entries

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Date:   Wed, 8 Sep 2021 12:35:21 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Mike Rapoport <rppt@...nel.org>, x86@...nel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Oscar Salvador <osalvador@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] x86/mm: fix kern_addr_valid to cope with existing but
 not present entries

On Wed, Aug 25, 2021 at 11:47:10AM -0700, Dave Hansen wrote:
> On 8/19/21 6:27 AM, Mike Rapoport wrote:
> > Such PMDs are created when free_kernel_image_pages() frees regions larger
> > than 2Mb. In this case a part of the freed memory is mapped with PMDs and
> > the set_memory_np_noalias() -> ... -> __change_page_attr() sequence will
> > mark the PMD as not present rather than wipe it completely.
> > 
> > Make kern_addr_valid() to check whether higher level page table entries are
> > present before trying to dereference them to fix this issue and to avoid
> > similar issues in the future.
> > 
> > Reported-by: Jiri Olsa <jolsa@...hat.com>
> > Signed-off-by: Mike Rapoport <rppt@...ux.ibm.com>
> > Cc: <stable@...r.kernel.org>	# 4.4...
> >  	pmd = pmd_offset(pud, addr);
> > -	if (pmd_none(*pmd))
> > +	if (!pmd_present(*pmd))
> >  		return 0;
> 
> Yeah, that seems like the right fix.  The one kern_addr_valid() user is
> going to touch the memory so it *better* be present.  p*d_none() was
> definitely the wrong check.
> 
> Acked-by: Dave Hansen <dave.hansen@...el.com>

So I did stare at this for a while, trying to make sense of it and David
Hildenbrand asked for a Fixes: tag in v1 review and from doing a bit of
git archeology I think it should be:

c40a56a7818c ("x86/mm/init: Remove freed kernel image areas from alias mapping")

because that thing added the clearing of the Present bit for the high
kernel image mapping of those areas.

Right?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette