Re: [PATCH v2] x86/nmi: Fix some races in NMI uaccess

[Andy Lutomirski]

On Tue, Aug 28, 2018 at 10:56 AM, Rik van Riel <riel@xxxxxxxxxxx> wrote:
> On Mon, 27 Aug 2018 16:04:16 -0700
> Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>
>> The 0day bot is still chewing on this, but I've tested it a bit locally
>> and it seems to do the right thing.
>
> Hi Andy,
>
> the version of the patch below should fix the bug we talked about
> in email yesterday.  It should automatically cover kernel threads
> in lazy TLB mode, because current->mm will be NULL, while the
> cpu_tlbstate.loaded_mm should never be NULL.
>

That's better than mine.  I tweaked it a bit and added some debugging,
and I got this:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/fixes&id=dd956eba16646fd0b15c3c0741269dfd84452dac

I made the loaded_mm handling a little more conservative to make it
more obvious that switch_mm_irqs_off() is safe regardless of exactly
when it gets called relative to switching current.