linux-kernel - Re: [kernel-hardening] [PATCH v5 03/32] x86/cpa: In populate_pgd, don't set the pgd entry until it's populated

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Date:	Thu, 21 Jul 2016 22:34:33 -0700
From:	Andy Lutomirski <luto@...nel.org>
To:	Valdis.Kletnieks@...edu, kernel-hardening@...ts.openwall.com
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
	Nadav Amit <nadav.amit@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.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: [kernel-hardening] [PATCH v5 03/32] x86/cpa: In populate_pgd,
 don't set the pgd entry until it's populated

On 07/21/2016 09:43 PM, Valdis.Kletnieks@...edu wrote:
> On Mon, 11 Jul 2016 13:53:36 -0700, Andy Lutomirski said:
>> 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 | 9 ++++++---
>>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> I just bisected a failure to boot down to this patch.  On my Dell Latitude
> laptop, it results in the kernel being loaded and then just basically sitting
> there dead in the water - as far as I can tell, it dies before the kernel
> ever gets going far enough to do any console I/O (even with ignore_loglevel).
> Nothing in /sys/fs/pstore either.  I admit not understanding the VM code
> at all, so I don't have a clue *why* this causes indigestion...
>
> CPU is an Intel Core i5-3340M in case that matters....
>

How much memory do you have and what's your config?  My code is 
obviously buggy, but I'm wondering why neither I nor the 0day bot caught 
this.

The attached patch is compile-tested only.  (Even Thunderbird doesn't 
want to send non-flowed text right now, sigh.)

--Andy

View attachment "0001-x86-mm-Fix-populate_pgd.patch" of type "text/x-patch" (3464 bytes)