linux-kernel - Re: [PATCH 4.9] x86/pti/efi: broken conversion from efi to kernel page table

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Date:   Sat, 13 Jan 2018 18:12:53 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>
Cc:     steven.sistare@...cle.com, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        x86@...nel.org, jkosina@...e.cz, hughd@...gle.com,
        dave.hansen@...ux.intel.com, luto@...nel.org,
        torvalds@...ux-foundation.org
Subject: Re: [PATCH 4.9] x86/pti/efi: broken conversion from efi to kernel
 page table

On Thu, Jan 11, 2018 at 04:58:20PM -0500, Pavel Tatashin wrote:
> The page table order must be increased for EFI table in order to avoid a
> bug where NMI tries to change the page table to kernel page table, while
> efi page table is active.
> 
> For more disccussion about this bug, see this thread:
> http://lkml.iu.edu/hypermail/linux/kernel/1801.1/00951.html
> 
> Signed-off-by: Pavel Tatashin <pasha.tatashin@...cle.com>
> Reviewed-by: Steven Sistare <steven.sistare@...cle.com>
> Acked-by: Jiri Kosina <jkosina@...e.cz>
> ---
>  arch/x86/include/asm/pgalloc.h | 11 +++++++++++
>  arch/x86/platform/efi/efi_64.c |  2 +-
>  2 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/pgalloc.h b/arch/x86/include/asm/pgalloc.h
> index b6d425999f99..1178a51b77f3 100644
> --- a/arch/x86/include/asm/pgalloc.h
> +++ b/arch/x86/include/asm/pgalloc.h
> @@ -27,6 +27,17 @@ static inline void paravirt_release_pud(unsigned long pfn) {}
>   */
>  extern gfp_t __userpte_alloc_gfp;
>  
> +#ifdef CONFIG_PAGE_TABLE_ISOLATION
> +/*
> + * Instead of one PGD, we acquire two PGDs.  Being order-1, it is
> + * both 8k in size and 8k-aligned.  That lets us just flip bit 12
> + * in a pointer to swap between the two 4k halves.
> + */
> +#define PGD_ALLOCATION_ORDER 1
> +#else
> +#define PGD_ALLOCATION_ORDER 0
> +#endif

This conflicts with the definition of PGD_ALLOCATION_ORDER in
arch/x86/mm/pgtable.c that says:

/*
 * Instead of one pgd, Kaiser acquires two pgds.  Being order-1, it is
 * both 8k in size and 8k-aligned.  That lets us just flip bit 12
 * in a pointer to swap between the two 4k halves.
 */
#define PGD_ALLOCATION_ORDER    kaiser_enabled

So, which is it?

I'm going to go drop this from the 4.9 stable queue because of this.

thanks,

greg k-h