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Date: Fri, 18 Sep 2015 09:14:04 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: x86@kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>, KVM list <kvm@vger.kernel.org>,
        Arjan van de Ven <arjan@linux.intel.com>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 1/2] x86/msr: Carry on after a non-"safe" MSR access
 fails without !panic_on_oops
Message-ID: <20150918071403.GA1172@gmail.com>
References: <cover.1442523997.git.luto@kernel.org>
 <203bb8a52efae1781281fb70ccd45c3e164fbce2.1442523997.git.luto@kernel.org>
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* Andy Lutomirski <luto@kernel.org> wrote:

> This demotes an OOPS and likely panic due to a failed non-"safe" MSR
> access to a WARN_ON_ONCE and a return of poisoned values (in the
> RDMSR case).  We still write a pr_info entry unconditionally for
> debugging.
> 
> To be clear, this type of failure should *not* happen.  This patch
> exists to minimize the chance of nasty undebuggable failures due on
> systems that used to work due to a now-fixed CONFIG_PARAVIRT=y bug.

> +	if (opcode == 0x320f) {
> +		/* RDMSR */
> +		pr_info("bad kernel RDMSR from non-existent MSR 0x%x",
> +			(unsigned int)regs->cx);
> +		if (!panic_on_oops) {
> +			WARN_ON_ONCE(true);
> +
> +			/* Patch it up with deterministic poison. */
> +			regs->ax = 0x5aadc0de;
> +			regs->dx = 0x8badf00d;
> +			regs->ip += 2;
> +			return true;

IMHO this should really not poison the result, but use zero as the result.

The poison might randomly indicate 'present' feature in various registers that 
might be accessed in a buggy way. Don't send the code further down into la-la-land 
by giving it a 'success'.

And yes, zero can mean success too, but we have to pick a side here ...

The warning will be enough to fix these ups, people (and in particular distro 
testing people) will be watching out for them.

Thanks,

	Ingo