linux-kernel - Re: [PATCH 2/3] x86, ras: Extend machine check recovery code to annotated ring0 areas

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Date:	Thu, 12 Nov 2015 11:55:52 -0800
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
	linux-edac@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH 2/3] x86, ras: Extend machine check recovery code to
 annotated ring0 areas

On Wed, Nov 11, 2015 at 08:19:35PM -0800, Andy Lutomirski wrote:
> >@@ -1132,9 +1133,15 @@ void do_machine_check(struct pt_regs *regs, long error_code)
> >  		if (no_way_out)
> >  			mce_panic("Fatal machine check on current CPU", &m, msg);
> >  		if (worst == MCE_AR_SEVERITY) {
> >-			recover_paddr = m.addr;
> >-			if (!(m.mcgstatus & MCG_STATUS_RIPV))
> >-				flags |= MF_MUST_KILL;
> >+			if ((m.cs & 3) == 3) {
> >+				recover_paddr = m.addr;
> >+				if (!(m.mcgstatus & MCG_STATUS_RIPV))
> >+					flags |= MF_MUST_KILL;
> >+			} else if (fixup_mcexception(regs)) {
> >+				regs->ax = BIT(63) | m.addr;
> >+			} else
> >+				mce_panic("Failed kernel mode recovery",
> >+					  &m, NULL);
> 
> Maybe I'm misunderstanding this, but presumably you shouldn't call
> fixup_mcexception unless you've first verified RIPV (i.e. that the ip you're
> looking up in the table is valid).

Good point. We can only arrive here with a AR_SEVERITY from some
kernel code if the code in mce_severity.c assigned that severity.
But it doesn't currently look at RIPV ... I should make it do that.
Actually I'll check for both RIPV and EIPV: we don't need to look for
a fixup entry for all the innocent bystander cpus that got dragged
into the exception handler because the exception was broadcast to
everyone.

> Also... I find the general flow of this code very hard to follow.  It's
> critical that an MCE hitting kernel mode not get as far as
> ist_begin_non_atomic.  It was already hard enough to tell that the code
> follows that rule, and now it's even harder.  Would it make sense to add
> clear assertions that m.cs == regs->cs and that user_mode(regs) when you get
> to the end?  Simplifying the control flow might also be nice.

Yes. This is a mess now. It works (because we only set recover_paddr
in the user mode case ... so we'll take the "goto done" for the kernel
case). But I agree that this is far from obvious.

> >  		} else if (kill_it) {
> >  			force_sig(SIGBUS, current);
> >  		}
> >
> 
> I would argue that this should happen in the non-atomic section.  It's
> probably okay as long as we came from user mode, but it's more obviously
> safe in the non-atomic section.

Will look at relocating this too when I restructure the tail of the
function.

Thanks for the review.

-Tony
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