From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Xiaoyao Li <xiaoyao.li@intel.com>, Ingo Molnar <mingo@redhat.com>,
Borislav Petkov <bp@alien8.de>,
Andy Lutomirski <luto@amacapital.net>,
x86@kernel.org, kvm@vger.kernel.org,
linux-kernel@vger.kernel.org,
David Laight <David.Laight@aculab.com>
Subject: Re: [PATCH v2 3/6] kvm: x86: Emulate split-lock access as a write
Date: Wed, 26 Feb 2020 16:11:17 -0800 [thread overview]
Message-ID: <20200227001117.GX9940@linux.intel.com> (raw)
In-Reply-To: <d690c2e3-e9ef-a504-ede3-d0059ec1e0f6@redhat.com>
On Tue, Feb 11, 2020 at 02:34:18PM +0100, Paolo Bonzini wrote:
> On 11/02/20 14:22, Thomas Gleixner wrote:
> > Paolo Bonzini <pbonzini@redhat.com> writes:
> >> On 03/02/20 16:16, Xiaoyao Li wrote:
> >>> A sane guest should never tigger emulation on a split-lock access, but
> >>> it cannot prevent malicous guest from doing this. So just emulating the
> >>> access as a write if it's a split-lock access to avoid malicous guest
> >>> polluting the kernel log.
> >>
> >> Saying that anything doing a split lock access is malicious makes little
> >> sense.
> >
> > Correct, but we also have to accept, that split lock access can be used
> > in a malicious way, aka. DoS.
>
> Indeed, a more accurate emulation such as temporarily disabling
> split-lock detection in the emulator would allow the guest to use split
> lock access as a vehicle for DoS, but that's not what the commit message
> says. If it were only about polluting the kernel log, there's
> printk_ratelimited for that. (In fact, if we went for incorrect
> emulation as in this patch, a rate-limited pr_warn would be a good idea).
>
> It is much more convincing to say that since this is pretty much a
> theoretical case, we can assume that it is only done with the purpose of
> DoS-ing the host or something like that, and therefore we kill the guest.
The problem with "kill the guest", and the reason I'd prefer to emulate the
split-lock as a write, is that killing the guest in this case is annoyingly
difficult.
Returning X86EMUL_UNHANDLEABLE / EMULATION_FAILED gets KVM to
handle_emulation_failure(), but handle_emulation_failure() will only "kill"
the guest if emulation failed in L1 CPL==0. For all other modes, it will
inject a #UD and resume the guest. KVM also injects a #UD for L1 CPL==0,
but that's the least annoying thing.
Adding a new emulation type isn't an option because this code can be
triggered through normal emulation. A new return type could be added for
split-lock, but that's code I'd really not add, both from an Intel
perspective and a KVM maintenance perspective. And, we'd still have the
conundrum of what to do if/when split-lock #AC is exposed to L1, e.g. in
that case, KVM should inject an #AC into L1, not kill the guest. Again,
totally doable, but ugly and IMO an unnecessary maintenance burden.
I completely agree that poorly emulating the instruction from the (likely)
malicious guest is a hack, but it's a simple and easy to maintain hack.
> >> Split lock detection is essentially a debugging feature, there's a
> >> reason why the MSR is called "TEST_CTL". So you don't want to make the
> >
> > The fact that it ended up in MSR_TEST_CTL does not say anything. That's
> > where they it ended up to be as it was hastily cobbled together for
> > whatever reason.
>
> Or perhaps it was there all the time in test silicon or something like
> that... That would be a very plausible reason for all the quirks behind it.
>
> Paolo
>
next prev parent reply other threads:[~2020-02-27 0:11 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-03 15:16 [PATCH v2 0/6] kvm/split_lock: Add feature split lock detection support in kvm Xiaoyao Li
2020-02-03 15:16 ` [PATCH v2 1/6] x86/split_lock: Add and export get_split_lock_detect_state() Xiaoyao Li
2020-02-03 21:45 ` Sean Christopherson
2020-02-03 15:16 ` [PATCH v2 2/6] x86/split_lock: Add and export split_lock_detect_set() Xiaoyao Li
2020-02-03 15:16 ` [PATCH v2 3/6] kvm: x86: Emulate split-lock access as a write Xiaoyao Li
2020-02-03 20:54 ` Sean Christopherson
2020-02-04 2:55 ` Xiaoyao Li
2020-02-11 12:20 ` Paolo Bonzini
2020-02-11 13:22 ` Thomas Gleixner
2020-02-11 13:34 ` Paolo Bonzini
2020-02-11 14:02 ` Xiaoyao Li
2020-02-11 14:34 ` David Laight
2020-02-27 0:11 ` Sean Christopherson [this message]
2020-03-12 11:42 ` Xiaoyao Li
2020-03-12 15:00 ` Paolo Bonzini
2020-02-03 15:16 ` [PATCH v2 4/6] kvm: vmx: Extend VMX's #AC handding for split lock in guest Xiaoyao Li
2020-02-03 21:14 ` Sean Christopherson
2020-02-04 6:46 ` Xiaoyao Li
2020-02-10 21:30 ` Sean Christopherson
2020-02-03 15:16 ` [PATCH v2 5/6] kvm: x86: Emulate MSR IA32_CORE_CAPABILITIES Xiaoyao Li
2020-02-03 21:43 ` Sean Christopherson
2020-02-04 9:19 ` Xiaoyao Li
2020-02-04 9:37 ` Peter Zijlstra
2020-02-11 3:52 ` Andy Lutomirski
2020-02-11 12:38 ` Peter Zijlstra
2020-02-03 15:16 ` [PATCH v2 6/6] x86: vmx: virtualize split lock detection Xiaoyao Li
2020-02-03 15:58 ` Xiaoyao Li
2020-02-03 18:52 ` Andy Lutomirski
2020-02-03 21:42 ` Sean Christopherson
2020-02-04 2:52 ` Xiaoyao Li
2020-02-04 5:35 ` Sean Christopherson
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