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

[Eduardo Habkost]

On Wed, Apr 21, 2021 at 10:50:10PM +0800, Xiaoyao Li wrote:
> 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?

Thanks for all the clarifications below.  Considering them,
what's the answer to the question above?

> > 
> > > 
> > > 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

Bus lock VM exit applies to both 1 and 2, correct?

> 
> > 
> > > 
> > > 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.
> 
> 

-- 
Eduardo