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

[Andy Lutomirski]

This is a multi-part message in MIME format.
On 07/21/2016 09:43 PM, Valdis.Kletnieks@xxxxxx 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@xxxxxxxxxx>
> > ---
> >  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