KVM: SVM: Zero DEBUGCTL before VMRUN if necessary

[PATCH 0/3] KVM: SVM: Zero DEBUGCTL before VMRUN if necessary

Posted by Sean Christopherson 9 months, 3 weeks ago

PeterZ,

Can you confirm that the last patch (snapshot and restore DEBUGCTL with
IRQs disabled) is actually necessary?  I'm 99% certain it is, but I'm
holding out hope that it somehow isn't, because I don't love the idea of
adding a RDMSR to every VM-Entry.

Assuming DEBUGCTL can indeed get modified in IRQ context, it probably
makes sense to add a per-CPU cache to eliminate the RDMSR.  Unfortunately,
there are quite a few open-coded WRMSRs, so it's not a trivial change.

On to the main event...

Fix a long-lurking bug in SVM where KVM runs the guest with the host's
DEBUGCTL if LBR virtualization is disabled.  AMD CPUs rather stupidly
context switch DEBUGCTL if and only if LBR virtualization is enabled (not
just supported, but fully enabled).

The bug has gone unnoticed because until recently, the only bits that
KVM would leave set were things like BTF, which are guest visible but
won't cause functional problems unless guest software is being especially
particular about #DBs.

The bug was exposed by the addition of BusLockTrap ("Detect" in the kernel),
as the resulting #DBs due to split-lock accesses in guest userspace (lol
Steam) get reflected into the guest by KVM.

Sean Christopherson (3):
  KVM: x86: Snapshot the host's DEBUGCTL in common x86
  KVM: SVM: Manually zero/restore DEBUGCTL if LBR virtualization is
    disabled
  KVM: x86: Snapshot the host's DEBUGCTL after disabling IRQs

 arch/x86/include/asm/kvm_host.h |  1 +
 arch/x86/kvm/svm/svm.c          | 14 ++++++++++++++
 arch/x86/kvm/vmx/vmx.c          |  8 ++------
 arch/x86/kvm/vmx/vmx.h          |  2 --
 arch/x86/kvm/x86.c              |  2 ++
 5 files changed, 19 insertions(+), 8 deletions(-)


base-commit: fed48e2967f402f561d80075a20c5c9e16866e53
-- 
2.48.1.658.g4767266eb4-goog

Re: [PATCH 0/3] KVM: SVM: Zero DEBUGCTL before VMRUN if necessary

Posted by Peter Zijlstra 9 months, 3 weeks ago

On Mon, Feb 24, 2025 at 10:13:12AM -0800, Sean Christopherson wrote:
> PeterZ,
> 
> Can you confirm that the last patch (snapshot and restore DEBUGCTL with
> IRQs disabled) is actually necessary?  I'm 99% certain it is, but I'm
> holding out hope that it somehow isn't, because I don't love the idea of
> adding a RDMSR to every VM-Entry.

I think you're right. I mean, I'd have to go double check and trace the
various call paths again, but I'd be very surprised if we can't change
DEBUGCTL from NMI context.

> Assuming DEBUGCTL can indeed get modified in IRQ context, it probably
> makes sense to add a per-CPU cache to eliminate the RDMSR.  Unfortunately,
> there are quite a few open-coded WRMSRs, so it's not a trivial change.

This, I'm surprised we've not yet done that.

> On to the main event...
> 
> Fix a long-lurking bug in SVM where KVM runs the guest with the host's
> DEBUGCTL if LBR virtualization is disabled.  AMD CPUs rather stupidly
> context switch DEBUGCTL if and only if LBR virtualization is enabled (not
> just supported, but fully enabled).
> 
> The bug has gone unnoticed because until recently, the only bits that
> KVM would leave set were things like BTF, which are guest visible but
> won't cause functional problems unless guest software is being especially
> particular about #DBs.
> 
> The bug was exposed by the addition of BusLockTrap ("Detect" in the kernel),
> as the resulting #DBs due to split-lock accesses in guest userspace (lol
> Steam) get reflected into the guest by KVM.

Hehe, yeah, games. Yeah we ran into that with bus-lock on intel too :-)