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

From: Valdis . Kletnieks
Date: Fri Jul 22 2016 - 00:44:29 EST

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....

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