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

[Chenyi Qiang]

On 4/21/2021 11:18 PM, Eduardo Habkost wrote:
> 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?

Hi Eduardo,

Just make it more clear.

Bus lock detection contains two sub-features. One is bus lock debug 
exception, and the other is bus lock VM exit.

Bus lock #DB exception can help detect the bus locks acquired in user 
space and bus lock VM exit detects the bus locks insides VMs. To address 
the attacks from non-VM userspace attackers against VM, Bus lock #DB 
exception can help.

The Bus lock #DB exception support 
(https://lore.kernel.org/lkml/20210322135325.682257-3-fenghua.yu@intel.com/) 
extends the  existing kernel command line parameter "split_lock_detect=" 
also applying to non-wb bus lock.
For example, split_lock_detect=fatal will send SIGBUS to the attackers 
once this kind of #DB is detected.

> > > > 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.