From: Paolo Bonzini <pbonzini@redhat.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>,
Fenghua Yu <fenghua.yu@intel.com>, Ingo Molnar <mingo@redhat.com>,
Borislav Petkov <bp@alien8.de>, H Peter Anvin <hpa@zytor.com>,
Peter Zijlstra <peterz@infradead.org>,
Andrew Morton <akpm@linux-foundation.org>,
Dave Hansen <dave.hansen@intel.com>,
Radim Krcmar <rkrcmar@redhat.com>,
Ashok Raj <ashok.raj@intel.com>, Tony Luck <tony.luck@intel.com>,
Dan Williams <dan.j.williams@intel.com>,
Xiaoyao Li <xiaoyao.li@intel.com>,
Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>,
Ravi V Shankar <ravi.v.shankar@intel.com>,
linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>,
kvm@vger.kernel.org
Subject: Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split lock
Date: Wed, 16 Oct 2019 13:58:58 +0200 [thread overview]
Message-ID: <3a12810b-1196-b70a-aa2e-9fe17dc7341a@redhat.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1910161244060.2046@nanos.tec.linutronix.de>
On 16/10/19 13:49, Thomas Gleixner wrote:
> On Wed, 16 Oct 2019, Paolo Bonzini wrote:
>> Yes it does. But Sean's proposal, as I understand it, leads to the
>> guest receiving #AC when it wasn't expecting one. So for an old guest,
>> as soon as the guest kernel happens to do a split lock, it gets an
>> unexpected #AC and crashes and burns. And then, after much googling and
>> gnashing of teeth, people proceed to disable split lock detection.
>
> I don't think that this was what he suggested/intended.
Xiaoyao's reply suggests that he also understood it like that.
>> In all of these cases, the common final result is that split-lock
>> detection is disabled on the host. So might as well go with the
>> simplest one and not pretend to virtualize something that (without core
>> scheduling) is obviously not virtualizable.
>
> You are completely ignoring any argument here and just leave it behind your
> signature (instead of trimming your reply).
I am not ignoring them, I think there is no doubt that this is the
intended behavior. I disagree that Sean's patches achieve it, however.
>>> 1) Sane guest
>>>
>>> Guest kernel has #AC handler and you basically prevent it from
>>> detecting malicious user space and killing it. You also prevent #AC
>>> detection in the guest kernel which limits debugability.
>
> That's a perfectly fine situation. Host has #AC enabled and exposes the
> availability of #AC to the guest. Guest kernel has a proper handler and
> does the right thing. So the host _CAN_ forward #AC to the guest and let it
> deal with it. For that to work you need to expose the MSR so you know the
> guest state in the host.
>
> Your lazy 'solution' just renders #AC completely useless even for
> debugging.
>
>>> 2) Malicious guest
>>>
>>> Trigger #AC to disable the host detection and then carry out the DoS
>>> attack.
>
> With your proposal you render #AC useless even on hosts which have SMT
> disabled, which is just wrong. There are enough good reasons to disable
> SMT.
My lazy "solution" only applies to SMT enabled. When SMT is either not
supported, or disabled as in "nosmt=force", we can virtualize it like
the posted patches have done so far.
> I agree that with SMT enabled the situation is truly bad, but we surely can
> be smarter than just disabling it globally unconditionally and forever.
>
> Plus we want a knob which treats guests triggering #AC in the same way as
> we treat user space, i.e. kill them with SIGBUS.
Yes, that's a valid alternative. But if SMT is possible, I think the
only sane possibilities are global disable and SIGBUS. SIGBUS (or
better, a new KVM_RUN exit code) can be acceptable for debugging guests too.
Paolo
next prev parent reply other threads:[~2019-10-16 11:59 UTC|newest]
Thread overview: 85+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-18 22:41 [PATCH v9 00/17] x86/split_lock: Enable split lock detection Fenghua Yu
2019-06-18 22:41 ` [PATCH v9 01/17] x86/common: Align cpu_caps_cleared and cpu_caps_set to unsigned long Fenghua Yu
2019-06-18 22:41 ` [PATCH v9 02/17] drivers/net/b44: Align pwol_mask to unsigned long for better performance Fenghua Yu
2019-06-24 15:12 ` David Laight
2019-06-24 18:43 ` Paolo Bonzini
2019-06-18 22:41 ` [PATCH v9 03/17] x86/split_lock: Align x86_capability to unsigned long to avoid split locked access Fenghua Yu
2019-06-24 15:12 ` David Laight
2019-06-25 23:54 ` Fenghua Yu
2019-06-26 19:15 ` Thomas Gleixner
2019-06-18 22:41 ` [PATCH v9 04/17] x86/msr-index: Define MSR_IA32_CORE_CAP and split lock detection bit Fenghua Yu
2019-06-18 22:41 ` [PATCH v9 05/17] x86/cpufeatures: Enumerate MSR_IA32_CORE_CAP Fenghua Yu
2019-06-18 22:41 ` [PATCH v9 06/17] x86/split_lock: Enumerate split lock detection by MSR_IA32_CORE_CAP Fenghua Yu
2019-06-18 22:41 ` [PATCH v9 07/17] x86/split_lock: Enumerate split lock detection on Icelake mobile processor Fenghua Yu
2019-06-18 22:41 ` [PATCH v9 08/17] x86/split_lock: Define MSR TEST_CTL register Fenghua Yu
2019-06-18 22:41 ` [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split lock Fenghua Yu
2019-06-26 20:20 ` Thomas Gleixner
2019-06-26 20:36 ` Fenghua Yu
2019-06-26 21:47 ` Thomas Gleixner
2019-09-25 18:09 ` Sean Christopherson
2019-10-16 6:58 ` Xiaoyao Li
2019-10-16 9:29 ` Thomas Gleixner
2019-10-16 15:59 ` Sean Christopherson
2019-10-16 9:40 ` Paolo Bonzini
2019-10-16 9:47 ` Thomas Gleixner
2019-10-16 10:16 ` Paolo Bonzini
2019-10-16 11:23 ` Xiaoyao Li
2019-10-16 11:26 ` Paolo Bonzini
2019-10-16 13:13 ` Xiaoyao Li
2019-10-16 14:43 ` Thomas Gleixner
2019-10-16 15:37 ` Paolo Bonzini
2019-10-16 16:25 ` Xiaoyao Li
2019-10-16 16:38 ` Paolo Bonzini
2019-10-17 12:29 ` [RFD] x86/split_lock: Request to Intel Thomas Gleixner
2019-10-17 17:23 ` Sean Christopherson
2019-10-17 21:31 ` Thomas Gleixner
2019-10-17 23:38 ` Sean Christopherson
2019-10-17 23:28 ` Luck, Tony
2019-10-18 10:45 ` David Laight
2019-10-18 21:03 ` hpa
2019-10-18 2:36 ` Xiaoyao Li
2019-10-18 9:02 ` Thomas Gleixner
2019-10-18 10:20 ` Xiaoyao Li
2019-10-18 10:43 ` Peter Zijlstra
2019-10-16 11:49 ` [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split lock Thomas Gleixner
2019-10-16 11:58 ` Paolo Bonzini [this message]
2019-10-16 13:51 ` Xiaoyao Li
2019-10-16 14:08 ` Paolo Bonzini
2019-10-16 14:14 ` David Laight
2019-10-16 15:03 ` Thomas Gleixner
2019-10-16 15:41 ` Sean Christopherson
2019-10-16 15:43 ` Paolo Bonzini
2019-10-16 16:23 ` Sean Christopherson
2019-10-16 17:42 ` Sean Christopherson
2019-10-17 1:23 ` Xiaoyao Li
2019-10-21 13:06 ` Paolo Bonzini
2019-10-21 13:03 ` Paolo Bonzini
2019-10-21 13:02 ` Paolo Bonzini
2019-10-16 14:50 ` Thomas Gleixner
2019-06-18 22:41 ` [PATCH v9 10/17] kvm/x86: Emulate MSR IA32_CORE_CAPABILITY Fenghua Yu
2019-06-18 22:41 ` [PATCH v9 11/17] kvm/vmx: Emulate MSR TEST_CTL Fenghua Yu
2019-06-27 2:24 ` Xiaoyao Li
2019-06-27 7:12 ` Thomas Gleixner
2019-06-27 7:58 ` Xiaoyao Li
2019-06-27 12:11 ` Thomas Gleixner
2019-06-27 12:22 ` Xiaoyao Li
2019-06-18 22:41 ` [PATCH v9 12/17] x86/split_lock: Enable split lock detection by default Fenghua Yu
2019-06-18 22:41 ` [PATCH v9 13/17] x86/split_lock: Disable split lock detection by kernel parameter "nosplit_lock_detect" Fenghua Yu
2019-06-26 20:34 ` Thomas Gleixner
2019-06-26 20:37 ` Fenghua Yu
2019-06-18 22:41 ` [PATCH v9 14/17] x86/split_lock: Add a debugfs interface to enable/disable split lock detection during run time Fenghua Yu
2019-06-26 21:37 ` Thomas Gleixner
2019-06-18 22:41 ` [PATCH v9 15/17] x86/split_lock: Add documentation for split lock detection interface Fenghua Yu
2019-06-26 21:51 ` Thomas Gleixner
2019-06-18 22:41 ` [PATCH v9 16/17] x86/split_lock: Reorganize few header files in order to call WARN_ON_ONCE() in atomic bit ops Fenghua Yu
2019-06-18 22:41 ` [PATCH v9 17/17] x86/split_lock: Warn on unaligned address in atomic bit operations Fenghua Yu
2019-06-26 22:00 ` Thomas Gleixner
2019-09-16 22:39 ` [PATCH 0/3] Fix some 4-byte vs. 8-byte alignment issues Tony Luck
2019-09-16 22:39 ` [PATCH 1/3] x86/common: Align cpu_caps_cleared and cpu_caps_set to unsigned long Tony Luck
2019-11-15 19:26 ` [tip: x86/cpu] x86/cpu: " tip-bot2 for Fenghua Yu
2019-09-16 22:39 ` [PATCH 2/3] drivers/net/b44: Align pwol_mask to unsigned long for better performance Tony Luck
2019-09-16 22:39 ` [PATCH 3/3] x86/split_lock: Align the x86_capability array to size of unsigned long Tony Luck
2019-09-17 8:29 ` David Laight
2019-09-17 19:14 ` Luck, Tony
2019-09-18 8:54 ` David Laight
2019-11-15 19:26 ` [tip: x86/cpu] x86/cpu: " tip-bot2 for Fenghua Yu
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