linux-kernel - Re: [PATCH v4 04/29] x86/cpa: In populate_pgd, don't set the pgd entry until it's populated

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Message-ID: <CALCETrV5aNcfdkNwJna-sOm84zzFccTBqv2KBn-p16U63rYUcQ@mail.gmail.com>
Date:	Tue, 28 Jun 2016 12:07:38 -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>
Subject: Re: [PATCH v4 04/29] x86/cpa: In populate_pgd, don't set the pgd
 entry until it's populated

On Tue, Jun 28, 2016 at 11:48 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Sun, Jun 26, 2016 at 02:55:26PM -0700, Andy Lutomirski wrote:
>> This avoids pointless races in which another CPU or task might see a
>> partially populated global pgd entry.  These races should normally
>> be harmless, but, if another CPU propagates the entry via
>> vmalloc_fault and then populate_pgd fails (due to memory allocation
>> failure, for example), this prevents a use-after-free of the pgd
>> entry.
>>
>> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>> ---
>>  arch/x86/mm/pageattr.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
>> index 7a1f7bbf4105..6a8026918bf6 100644
>> --- a/arch/x86/mm/pageattr.c
>> +++ b/arch/x86/mm/pageattr.c
>> @@ -1113,7 +1113,9 @@ static int populate_pgd(struct cpa_data *cpa, unsigned long addr)
>>
>>       ret = populate_pud(cpa, addr, pgd_entry, pgprot);
>>       if (ret < 0) {
>> -             unmap_pgd_range(cpa->pgd, addr,
>> +             if (pud)
>> +                     free_page((unsigned long)pud);
>> +             unmap_pud_range(pgd_entry, addr,
>>                               addr + (cpa->numpages << PAGE_SHIFT));
>>               return ret;
>>       }
>> --
>
> So something's amiss here. Subject says:
>
> "x86/cpa: In populate_pgd, don't set the pgd entry until it's populated"
>
> but you haven't moved
>
>         set_pgd(pgd_entry, __pgd(__pa(pud) | _KERNPG_TABLE));
>
> after populate_pud() succeeds... Which is a good catch but your patch
> should do it too. :-)

Good catch.  I'll fix this in the next version.

--Andy