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 13:34:18 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Mike Rapoport <rppt@...nel.org>
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 02:22:31PM +0300, Mike Rapoport wrote:
> 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...

Hmmkay, I guess I should add the gist of that to the commit message so
that it is explained why 4.4.

I'm assuming the pxy_present() check is more strict than pxy_none() so
that backporting to all stable kernels should not introduce any risks...

-- 
Regards/Gruss,
    Boris.

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