linux-kernel - Re: [PATCH v2 03/13] x86/cpa: Warn if kernel_unmap_pages_in

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Message-ID: <20160617203045.GB3842@pd.tnic>
Date:	Fri, 17 Jun 2016 22:30:45 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, Nadav Amit <nadav.amit@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Jann Horn <jann@...jh.net>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH v2 03/13] x86/cpa: Warn if kernel_unmap_pages_in_pgd is
 used inappropriately

On Fri, Jun 17, 2016 at 01:00:39PM -0700, Andy Lutomirski wrote:
> It's currently only used in the EFI code, which is safe AFAICT.

"It is basically useful for a pagetable hierarchy which is not init_mm."

> Warn if anyone tries to use it on the normal kernel pgd.
> 
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
>  arch/x86/mm/pageattr.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
> index 6a8026918bf6..e9b9c5cedbb8 100644
> --- a/arch/x86/mm/pageattr.c
> +++ b/arch/x86/mm/pageattr.c
> @@ -1996,6 +1996,9 @@ out:
>  void kernel_unmap_pages_in_pgd(pgd_t *root, unsigned long address,
>  			       unsigned numpages)
>  {
> +	/* Unmapping kernel entries from init_mm's pgd is not allowed. */
> +	WARN_ON(root == init_mm.pgd);

We can also return and not do the unmapping:

	if (WARN_ON(root == init_mm.pgd))
		return;

-- 
Regards/Gruss,
    Boris.

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