linux-kernel - Re: [PATCH] x86/mce: Fix set_mce

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Date:   Fri, 31 Aug 2018 17:35:30 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Tony Luck <tony.luck@...el.com>, Ingo Molnar <mingo@...hat.com>,
        Peter Anvin <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        linux-edac <linux-edac@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Jiang <dave.jiang@...el.com>
Subject: Re: [PATCH] x86/mce: Fix set_mce_nospec() to avoid #GP fault

On Thu, 30 Aug 2018, Linus Torvalds wrote:
> On Thu, Aug 30, 2018 at 6:49 PM Tony Luck <tony.luck@...el.com> 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.
> 
> Hmm?
> 
> But I'll bow to the x86 maintainers.

The above is fugly, but it has the charm of simplicity and I assume it's
going to be useful for other places as well. With a big fat comment WHY we
are doing it it's not that horrible. We have all the other L1TF places
where we fiddle with bits in non-obvious ways, so having another instance
of magic bit fiddling is not that big of a problem.

Thanks,

	tglx