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

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Date:	Sat, 18 Jun 2016 03:29:01 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-arch <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>,
	Matt Fleming <matt@...eblueprint.co.uk>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>
Subject: Re: [PATCH v2 03/13] x86/cpa: Warn if kernel_unmap_pages_in_pgd is
 used inappropriately

On Fri, Jun 17, 2016 at 1:30 PM, Borislav Petkov <bp@...en8.de> wrote:
> 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;

I'll do one better: the only function that calls this function is
unused.  I'll just delete it.

--Andy