linux-kernel - Re: [PATCH v3 3/5] x86/paravirt: Add paravirt_{read,write}

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Message-ID: <CALCETrWYmJdOeHqjR6-Uyqiyym35DOjFpsB1xhgTHO_JB==EMA@mail.gmail.com>
Date:	Mon, 14 Mar 2016 10:02:27 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	xen-devel <Xen-devel@...ts.xen.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	X86 ML <x86@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KVM list <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v3 3/5] x86/paravirt: Add paravirt_{read,write}_msr

On Mon, Mar 14, 2016 at 9:58 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Mar 14, 2016 9:53 AM, "Andy Lutomirski" <luto@...capital.net> wrote:
>>
>> Can you clarify?  KVM uses the native version, and the native version
>> only oopses with this series applied if panic_on_oops is set.
>
> Can we please remove that idiocy?
>
> There is no reason to panic whatsoever. Seriously. What's the upside of that
> logic?

I imagine that people who set panic_on_oops want their systems to stop
running user code if something happens that could corrupt the state or
if there's any sign that user code is trying some non-deterministic
exploit.  So I'm guessing that they'd want this type of "the kernel
screwed up -- abort" to actually result in a panic.

As a concrete, although somewhat silly, example, suppose that a write
to MSR_SYSENTER_STACK fails.  If that happened, then user code could
subsequently try to take over the kernel by evil manipulation of TF
and/or perf.

I'd be okay with removing this too, though, since arranging for MSR
access to fail seems unlikely as an exploit vector.

Borislav: SUSE actually uses panic_on_oops, right?  What's their goal?

--Andy