linux-kernel - Re: [Xen-devel] [PATCH] x86/xen: Fix 64bit kernel pagetable setup of PV guests

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Date:	Mon, 1 Sep 2014 18:34:07 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Stefan Bader <stefan.bader@...onical.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
CC:	David Vrabel <david.vrabel@...rix.com>,
	Kees Cook <keescook@...omium.org>
Subject: Re: [Xen-devel] [PATCH] x86/xen: Fix 64bit kernel pagetable setup
 of PV guests

On 29/08/14 16:17, Stefan Bader wrote:
> 
> This change might not be the fully correct approach as it basically
> removes the pre-set page table entry for the fixmap that is compile
> time set (level2_fixmap_pgt[506]->level1_fixmap_pgt). For one the
> level1 page table is not yet declared in C headers (that might be
> fixed). But also with the current bug, it was removed, too. Since
> the Xen mappings for level2_kernel_pgt only covered kernel + initrd
> and some Xen data this did never reach that far. And still, something
> does create entries at level2_fixmap_pgt[506..507]. So it should be
> ok. At least I was able to successfully boot a kernel with 1G kernel
> image size without any vmalloc whinings.
[...]
> --- a/arch/x86/xen/mmu.c
> +++ b/arch/x86/xen/mmu.c
> @@ -1902,8 +1902,22 @@ void __init xen_setup_kernel_pagetable(pgd_t *pgd, unsigned long max_pfn)
>  		/* L3_i[0] -> level2_ident_pgt */
>  		convert_pfn_mfn(level3_ident_pgt);
>  		/* L3_k[510] -> level2_kernel_pgt
> -		 * L3_i[511] -> level2_fixmap_pgt */
> +		 * L3_k[511] -> level2_fixmap_pgt */
>  		convert_pfn_mfn(level3_kernel_pgt);
> +
> +		/* level2_fixmap_pgt contains a single entry for the
> +		 * fixmap area at offset 506. The correct way would
> +		 * be to convert level2_fixmap_pgt to mfn and set the
> +		 * level1_fixmap_pgt (which is completely empty) to RO,
> +		 * too. But currently this page table is not declared,
> +		 * so it would be a bit of voodoo to get its address.
> +		 * And also the fixmap entry was never set due to using
> +		 * the wrong l2 when getting Xen's tables. So let's just
> +		 * just nuke it.
> +		 * This orphans level1_fixmap_pgt, but that was basically
> +		 * done before the change as well.
> +		 */
> +		memset(level2_fixmap_pgt, 0, 512*sizeof(long));

level2_fixmap_pgt etc. are defined for the benefit of Xen only so I
think you should add an extern for level1_fixmap_pgt and fix this up
properly.

It might not matter now, but it might in the future...

David
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