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

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Message-ID: <YTic90lqv0HbuYOI@kernel.org>
Date:   Wed, 8 Sep 2021 14:22:31 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Dave Hansen <dave.hansen@...el.com>, 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, Sep 08, 2021 at 12:52:45PM +0200, Borislav Petkov wrote:
> On Wed, Sep 08, 2021 at 12:35:21PM +0200, Borislav Petkov wrote:
> > 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?

Yes, in a sense. 
As the only user of kern_addr_valid() is kcore and it only uses this check
for high kernel mappings, there should be no problem before 4.19.

But...


> Hmm, but that commit is in v4.19. Mike has added
> 
> Cc: <stable@...r.kernel.org>    # 4.4+
> 
> Mike, why 4.4 and newer?

kern_addr_valid() wrongly uses pxy_none() rather than pxy_present() because
according to 9a14aefc1d28 ("x86: cpa, fix lookup_address") there could be
cases when page table entries exist but they are not valid.
So a call to kern_addr_valid() for an address in the direct map would oops.

I've stopped digging at 9a14aefc1d28 (which is in v2.6.26) and added the
oldest stable we still support (4.4).

I agree that before 4.19 it's more of a theoretical bug, but you know,
things happen...
 
> Hmmm.

-- 
Sincerely yours,
Mike.