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: <35f4a263-1001-5ba5-7b6c-3fcc5f93cc30@intel.com>
Date:   Wed, 25 Aug 2021 11:47:10 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Mike Rapoport <rppt@...nel.org>, x86@...nel.org
Cc:     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>,
        Borislav Petkov <bp@...en8.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 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>