Re: [kernel-hardening] [PATCH v5 13/32] x86/mm/64: In vmalloc_fault(), use CR3 instead of current->active_mm

[Andy Lutomirski]

On Tue, Jul 12, 2016 at 10:51 AM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> On 07/11/2016 01:53 PM, Andy Lutomirski wrote:
>> If we get a vmalloc fault while current->active_mm->pgd doesn't
>> match CR3, we'll crash without this change.  I've seen this failure
>> mode on heavily instrumented kernels with virtually mapped stacks.
>
> When does this happen, btw?  Crossing page boundaries on the stack
> between the time we swap mm's and the time we switch stacks?

This can happen for any vmalloc fault between the mm swap and writing
to current or current->active_mm.  I hit it when playing with KASAN
during the first clone.  (KASAN has other issues, but this was one of
them AFAICT.)

--Andy