Re: [PATCH v2] i386: Add ratelimit for bus locks acquired in guest

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 4/21/2021 10:12 PM, Eduardo Habkost wrote:
> On Wed, Apr 21, 2021 at 02:26:42PM +0800, Chenyi Qiang wrote:
>> Hi, Eduardo, thanks for your comments!
>>
>>
>> On 4/21/2021 12:34 AM, Eduardo Habkost wrote:
>>> Hello,
>>>
>>> Thanks for the patch.  Comments below:
>>>
>>> On Tue, Apr 20, 2021 at 05:37:36PM +0800, Chenyi Qiang wrote:
>>>> Virtual Machines can exploit bus locks to degrade the performance of
>>>> system. To address this kind of performance DOS attack, bus lock VM exit
>>>> is introduced in KVM and it will report the bus locks detected in guest,
>>>> which can help userspace to enforce throttling policies.
>>>>
>>>
>>> Is there anything today that would protect the system from
>>> similar attacks from userspace with access to /dev/kvm?
>>>
>>
>> I can't fully understand your meaning for "similar attack with access to
>> /dev/kvm". But there are some similar associated detection features on bare
>> metal.
> 
> What I mean is: you say guests can make a performance DoS attack
> on the host, and your patch mitigates that.
> 
> What would be the available methods to prevent untrusted
> userspace running on the host with access to /dev/kvm from making
> a similar DoS attack on the host?
> 
>>
>> 1. Split lock detection:https://lore.kernel.org/lkml/158031147976.396.8941798847364718785.tip-bot2@tip-bot2/
>> Some CPUs can raise an #AC trap when a split lock is attempted.
> 
> Would split_lock_detect=fatal be enough to prevent the above attacks?

NO.

There are two types bus lock:
1. split lock - lock on cacheable memory while the memory across two 
cache lines.
2. non-wb lock - lock on non-writableback memory (you can find it on 
Intel ISE chapter 8, 
https://software.intel.com/content/www/us/en/develop/download/intel-architecture-instruction-set-extensions-programming-reference.html)

split lock detection can only prevent 1)

> Is split_lock_detect=fatal the only available way to prevent them?

as above, 2) non-wb lock can be prevented by "non-wb lock disable" feature

> 
>>
>> 2. Bus lock Debug Exception:
>> https://lore.kernel.org/lkml/20210322135325.682257-1-fenghua.yu@intel.com/
>> The kernel can be notified by an #DB trap after a user instruction acquires
>> a bus lock and is executed.
> 
> I see a rate limit option mentioned at the above URL.  Would a
> host kernel bus lock rate limit option make this QEMU patch
> redundant?
> 

No. Bus lock Debug exception cannot be used to detect the bus lock 
happens in guest (vmx non-root mode).

We have patch to virtualize this feature for guest 
https://lore.kernel.org/kvm/20210202090433.13441-1-chenyi.qiang@intel.com/

that guest will have its own setting of bus lock debug exception on or off.

What's more important is that, even we force set the 
MSR_DEBUGCTL.BUS_LOCK_DETECT for guest, guest still can escape from it.
Because bus lock #DB is a trap which is delivered after the instruction 
completes. If the instruction acquires bus lock subsequently faults 
e.g., #PF, then no bus lock #DB generated. But the bus lock does happen.

But with bus lock VM exit, even the instruction faults, it will cause a 
BUS LOCK VM exit.