linux-kernel - Re: [PATCH v2 1/3] x86/mce: Avoid infinite loop for copy from user recovery

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Date:   Thu, 14 Jan 2021 13:05:08 -0800
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     x86@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        linux-kernel@...r.kernel.org, linux-edac@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v2 1/3] x86/mce: Avoid infinite loop for copy from user
 recovery

On Thu, Jan 14, 2021 at 09:22:13PM +0100, Borislav Petkov wrote:
> On Mon, Jan 11, 2021 at 01:44:50PM -0800, Tony Luck wrote:
> > @@ -1431,8 +1433,11 @@ noinstr void do_machine_check(struct pt_regs *regs)
> >  				mce_panic("Failed kernel mode recovery", &m, msg);
> >  		}
> >  
> > -		if (m.kflags & MCE_IN_KERNEL_COPYIN)
> > +		if (m.kflags & MCE_IN_KERNEL_COPYIN) {
> > +			if (current->mce_busy)
> > +				mce_panic("Multiple copyin", &m, msg);
> 
> So this: we're currently busy handling the first MCE, why do we must
> panic?
> 
> Can we simply ignore all follow-up MCEs to that page?

If we s/all/some/ you are saying the same as Andy:
> So I tend to think that the machine check code should arrange to
> survive some reasonable number of duplicate machine checks.

> I.e., the page will get poisoned eventually and that poisoning is
> currently executing so all following MCEs are simply nothing new and we
> can ignore them.
> 
> It's not like we're going to corrupt more data - we already are
> "corrupting" whole 4K.
> 
> Am I making sense?
> 
> Because if we do this, we won't have to pay attention to any get_user()
> callers and whatnot - we simply ignore and the solution is simple and
> you won't have to touch any get_user() callers...

Changing get_user() is a can of worms. I don't think its a very big can.
Perhaps two or three dozen places where code needs to change to account
for the -ENXIO return ... but touching a bunch of different subsystems
it is likley to take a while to get everyone in agreement.

I'll try out this new approach, and if it works, I'll post a v3 patch.

Thanks

-Tony