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

From: Mike Rapoport <rppt@kernel.org>
To: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@intel.com>,
	x86@kernel.org, Andrew Morton <akpm@linux-foundation.org>,
	Andy Lutomirski <luto@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	David Hildenbrand <david@redhat.com>,
	Ingo Molnar <mingo@redhat.com>, Jiri Olsa <jolsa@redhat.com>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Oscar Salvador <osalvador@suse.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	stable@vger.kernel.org
Subject: Re: [PATCH v2] x86/mm: fix kern_addr_valid to cope with existing but not present entries
Date: Wed, 8 Sep 2021 14:22:31 +0300	[thread overview]
Message-ID: <YTic90lqv0HbuYOI@kernel.org> (raw)
In-Reply-To: <YTiV/Sdm/T/jnsHC@zn.tnic>

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@vger.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.