Re: [PATCH] x86/mce: Fix set_mce_nospec() to avoid #GP fault

From: Linus Torvalds
Date: Fri Aug 31 2018 - 00:27:56 EST

On Thu, Aug 30, 2018 at 6:49 PM Tony Luck <tony.luck@xxxxxxxxx> wrote:
> Just checking "do we have a non-canonical address" at the bottom of that
> call stack and flipping bit 63 back on again seems like a bad idea.

You could literally do something like

/* Make it canonical in case we flipped the high bit */
addr = (long)(addr<<1)>>1;

in the call to clflush and it magically does the right thing.

Pretty? No. But with a big comment about what is going on and why it's
done, I think it's prettier than your much bigger patch.

I dunno. It does strike me as a bit hacky, but I'd rather have a
*small* one-liner hack that generates two instructions, than add a
complex hack that modifies the page tables three times and has a
serializing instruction in it.

Both are subtle fixes for a subtle issue, but one seems pretty
harmless in comparison.


But I'll bow to the x86 maintainers.