linux-kernel - Re: [PATCH 12/16] x86/mm/pae: Populate the user page-table with user pgd's

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Date:   Tue, 16 Jan 2018 22:10:52 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Joerg Roedel <joro@...tes.org>
cc:     Ingo Molnar <mingo@...nel.org>, "H . Peter Anvin" <hpa@...or.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>, aliguori@...zon.com,
        daniel.gruss@...k.tugraz.at, hughd@...gle.com, keescook@...gle.com,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, jroedel@...e.de
Subject: Re: [PATCH 12/16] x86/mm/pae: Populate the user page-table with user
 pgd's

On Tue, 16 Jan 2018, 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;

I'd suggest to have:

static inline pteval_t supported_pgd_mask(void)
{
	if (IS_ENABLED(CONFIG_X86_64))
		return __supported_pte_mask;
	return __supported_pte_mask & ~_PAGE_NX);
}

and get rid of the ifdeffery completely.

Thanks,

	tglx