Subject: Re: [PATCH 12/16] x86/mm/pae: Populate the user page-table with user
 pgd's
To: Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>
References: <1516120619-1159-1-git-send-email-joro@8bytes.org>
 <1516120619-1159-13-git-send-email-joro@8bytes.org>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Juergen Gross <jgross@suse.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bp@alien8.de>, Jiri Kosina <jkosina@suse.cz>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Brian Gerst <brgerst@gmail.com>,
        David Laight <David.Laight@aculab.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Eduardo Valentin <eduval@amazon.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Will Deacon <will.deacon@arm.com>, aliguori@amazon.com,
        daniel.gruss@iaik.tugraz.at, hughd@google.com, keescook@google.com,
        Andrea Arcangeli <aarcange@redhat.com>,
        Waiman Long <llong@redhat.com>, jroedel@suse.de
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <df637ada-c2f6-c137-0287-0964e29fc11f@intel.com>
Date: Tue, 16 Jan 2018 10:11:14 -0800
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.5.0
MIME-Version: 1.0
In-Reply-To: <1516120619-1159-13-git-send-email-joro@8bytes.org>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org

On 01/16/2018 08:36 AM, Joerg Roedel wrote:
> +#ifdef CONFIG_X86_64
>  	/*
>  	 * If this is normal user memory, make it NX in the kernel
>  	 * pagetables so that, if we somehow screw up and return to
> @@ -134,10 +135,16 @@ pgd_t __pti_set_user_pgd(pgd_t *pgdp, pgd_t pgd)
>  	 *     may execute from it
>  	 *  - we don't have NX support
>  	 *  - we're clearing the PGD (i.e. the new pgd is not present).
> +	 *  - We run on a 32 bit kernel. 2-level paging doesn't support NX at
> +	 *    all and PAE paging does not support it on the PGD level. We can
> +	 *    set it in the PMD level there in the future, but that means we
> +	 *    need to unshare the PMDs between the kernel and the user
> +	 *    page-tables.
>  	 */
>  	if ((pgd.pgd & (_PAGE_USER|_PAGE_PRESENT)) == (_PAGE_USER|_PAGE_PRESENT) &&
>  	    (__supported_pte_mask & _PAGE_NX))
>  		pgd.pgd |= _PAGE_NX;
> +#endif

Ugh.  The ghosts of PAE have come back to haunt us.

Could we do:

static inline bool pgd_supports_nx(unsigned long)
{
#ifdef CONFIG_X86_64
	return (__supported_pte_mask & _PAGE_NX);
#else
	/* No 32-bit page tables support NX at PGD level */
	return 0;
#endif
}

Nobody will ever spot the #ifdef the way you laid it out.