From: Xiaoyao Li <xiaoyao.li@intel.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>,
Paolo Bonzini <pbonzini@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
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>,
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: Thu, 17 Oct 2019 09:23:53 +0800 [thread overview]
Message-ID: <54cba514-23bb-5a96-f5f7-10520d1f0df2@intel.com> (raw)
In-Reply-To: <20191016174200.GF5866@linux.intel.com>
On 10/17/2019 1:42 AM, Sean Christopherson wrote:
> On Wed, Oct 16, 2019 at 09:23:37AM -0700, Sean Christopherson wrote:
>> On Wed, Oct 16, 2019 at 05:43:53PM +0200, Paolo Bonzini wrote:
>>> On 16/10/19 17:41, Sean Christopherson wrote:
>>>> On Wed, Oct 16, 2019 at 04:08:14PM +0200, Paolo Bonzini wrote:
>>>>> SIGBUS (actually a new KVM_EXIT_INTERNAL_ERROR result from KVM_RUN is
>>>>> better, but that's the idea) is for when you're debugging guests.
>>>>> Global disable (or alternatively, disable SMT) is for production use.
>>>>
>>>> Alternatively, for guests without split-lock #AC enabled, what if KVM were
>>>> to emulate the faulting instruction with split-lock detection temporarily
>>>> disabled?
>>>
>>> Yes we can get fancy, but remember that KVM is not yet supporting
>>> emulation of locked instructions. Adding it is possible but shouldn't
>>> be in the critical path for the whole feature.
>>
>> Ah, didn't realize that. I'm surprised emulating all locks with cmpxchg
>> doesn't cause problems (or am I misreading the code?). Assuming I'm
>> reading the code correctly, the #AC path could kick all other vCPUS on
>> emulation failure and then retry emulation to "guarantee" success. Though
>> that's starting to build quite the house of cards.
>
> Ugh, doesn't the existing emulation behavior create another KVM issue?
> KVM uses a locked cmpxchg in emulator_cmpxchg_emulated() and the address
> is guest controlled, e.g. a guest could coerce the host into disabling
> split-lock detection via the host's #AC handler by triggering emulation
> and inducing an #AC in the emulator.
>
Exactly right.
I have tested with force_emulation_prefix. It did go into the #AC
handler and disable the split-lock detection in host.
However, without force_emulation_prefix enabled, I'm not sure whether
malicious guest can create the case causing the emulation with a lock
prefix and going to the emulator_cmpxchg_emulated().
I found it impossible without force_emulation_prefix enabled and I'm not
familiar with emulation at all. If I missed something, please let me know.
>>> How would you disable split-lock detection temporarily? Just tweak
>>> MSR_TEST_CTRL for the time of running the one instruction, and cross
>>> fingers that the sibling doesn't notice?
>>
>> Tweak MSR_TEST_CTRL, with logic to handle the scenario where split-lock
>> detection is globally disable during emulation (so KVM doesn't
>> inadvertantly re-enable it).
>>
>> There isn't much for the sibling to notice. The kernel would temporarily
>> allow split-locks on the sibling, but that's a performance issue and isn't
>> directly fatal. A missed #AC in the host kernel would only delay the
>> inevitable global disabling of split-lock. A missed #AC in userspace would
>> again just delay the inevitable SIGBUS.
next prev parent reply other threads:[~2019-10-17 1:24 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
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 [this message]
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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