linux-kernel - Re: [PATCHv3 07/17] x86/mm: Preserve KeyID on pte_modify() and pgprot

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Date:   Wed, 13 Jun 2018 11:13:03 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Tom Lendacky <thomas.lendacky@....com>
Cc:     Kai Huang <kai.huang@...ux.intel.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv3 07/17] x86/mm: Preserve KeyID on pte_modify() and
 pgprot_modify()

On 06/12/2018 07:39 AM, Kirill A. Shutemov wrote:
> Encrypted VMA will have KeyID stored in vma->vm_page_prot. This way we

"An encrypted VMA..."

> don't need to do anything special to setup encrypted page table entries
> and don't need to reserve space for KeyID in a VMA.
> 
> This patch changes _PAGE_CHG_MASK to include KeyID bits. Otherwise they
> are going to be stripped from vm_page_prot on the first pgprot_modify().
> 
> Define PTE_PFN_MASK_MAX similar to PTE_PFN_MASK but based on
> __PHYSICAL_MASK_SHIFT. This way we include whole range of bits
> architecturally available for PFN without referencing physical_mask and
> mktme_keyid_mask variables.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>  arch/x86/include/asm/pgtable_types.h | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index 1e5a40673953..e8ebe760b88d 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -121,8 +121,13 @@
>   * protection key is treated like _PAGE_RW, for
>   * instance, and is *not* included in this mask since
>   * pte_modify() does modify it.
> + *
> + * It includes full range of PFN bits regardless if they were claimed for KeyID
> + * or not: we want to preserve KeyID on pte_modify() and pgprot_modify().
>   */
> -#define _PAGE_CHG_MASK	(PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT |		\
> +#define PTE_PFN_MASK_MAX \
> +	(((signed long)PAGE_MASK) & ((1ULL << __PHYSICAL_MASK_SHIFT) - 1))

"signed long" is really unusual to see.  Was that intentional?

> +#define _PAGE_CHG_MASK	(PTE_PFN_MASK_MAX | _PAGE_PCD | _PAGE_PWT |		\
>  			 _PAGE_SPECIAL | _PAGE_ACCESSED | _PAGE_DIRTY |	\
>  			 _PAGE_SOFT_DIRTY)
>  #define _HPAGE_CHG_MASK (_PAGE_CHG_MASK | _PAGE_PSE)

This makes me a bit nervous.  We have some places (here) where we
pretend that the KeyID is part of the paddr and then other places like
pte_pfn() where it's not.

Seems like something that will come back to bite us.