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

From: Andy Lutomirski
Date: Fri Jul 22 2016 - 01:34:43 EST

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

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.)