6.12-stable review patch. If anyone has any objections, please let me know.
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From: Sean Christopherson seanjc@google.com
commit 433265870ab3455b418885bff48fa5fd02f7e448 upstream.
Manually load the guest's DEBUGCTL prior to VMRUN (and restore the host's value on #VMEXIT) if it diverges from the host's value and LBR virtualization is disabled, as hardware only context switches DEBUGCTL if LBR virtualization is fully enabled. Running the guest with the host's value has likely been mildly problematic for quite some time, e.g. it will result in undesirable behavior if BTF diverges (with the caveat that KVM now suppresses guest BTF due to lack of support).
But the bug became fatal with the introduction of Bus Lock Trap ("Detect" in kernel paralance) support for AMD (commit 408eb7417a92 ("x86/bus_lock: Add support for AMD")), as a bus lock in the guest will trigger an unexpected #DB.
Note, suppressing the bus lock #DB, i.e. simply resuming the guest without injecting a #DB, is not an option. It wouldn't address the general issue with DEBUGCTL, e.g. for things like BTF, and there are other guest-visible side effects if BusLockTrap is left enabled.
If BusLockTrap is disabled, then DR6.BLD is reserved-to-1; any attempts to clear it by software are ignored. But if BusLockTrap is enabled, software can clear DR6.BLD:
Software enables bus lock trap by setting DebugCtl MSR[BLCKDB] (bit 2) to 1. When bus lock trap is enabled, ... The processor indicates that this #DB was caused by a bus lock by clearing DR6[BLD] (bit 11). DR6[11] previously had been defined to be always 1.
and clearing DR6.BLD is "sticky" in that it's not set (i.e. lowered) by other #DBs:
All other #DB exceptions leave DR6[BLD] unmodified
E.g. leaving BusLockTrap enable can confuse a legacy guest that writes '0' to reset DR6.
Reported-by: rangemachine@gmail.com Reported-by: whanos@sergal.fun Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219787 Closes: https://lore.kernel.org/all/bug-219787-28872@https.bugzilla.kernel.org%2F Cc: Ravi Bangoria ravi.bangoria@amd.com Cc: stable@vger.kernel.org Reviewed-and-tested-by: Ravi Bangoria ravi.bangoria@amd.com Link: https://lore.kernel.org/r/20250227222411.3490595-5-seanjc@google.com Signed-off-by: Sean Christopherson seanjc@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/kvm/svm/svm.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+)
--- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -4275,6 +4275,16 @@ static __no_kcsan fastpath_t svm_vcpu_ru clgi(); kvm_load_guest_xsave_state(vcpu);
+ /* + * Hardware only context switches DEBUGCTL if LBR virtualization is + * enabled. Manually load DEBUGCTL if necessary (and restore it after + * VM-Exit), as running with the host's DEBUGCTL can negatively affect + * guest state and can even be fatal, e.g. due to Bus Lock Detect. + */ + if (!(svm->vmcb->control.virt_ext & LBR_CTL_ENABLE_MASK) && + vcpu->arch.host_debugctl != svm->vmcb->save.dbgctl) + update_debugctlmsr(svm->vmcb->save.dbgctl); + kvm_wait_lapic_expire(vcpu);
/* @@ -4302,6 +4312,10 @@ static __no_kcsan fastpath_t svm_vcpu_ru if (unlikely(svm->vmcb->control.exit_code == SVM_EXIT_NMI)) kvm_before_interrupt(vcpu, KVM_HANDLING_NMI);
+ if (!(svm->vmcb->control.virt_ext & LBR_CTL_ENABLE_MASK) && + vcpu->arch.host_debugctl != svm->vmcb->save.dbgctl) + update_debugctlmsr(vcpu->arch.host_debugctl); + kvm_load_host_xsave_state(vcpu); stgi();