On Thu, Aug 14, 2025, Sasha Levin wrote:
From: Maxim Levitsky mlevitsk@redhat.com
[ Upstream commit 095686e6fcb4150f0a55b1a25987fad3d8af58d6 ]
Add a consistency check for L2's guest_ia32_debugctl, as KVM only supports a subset of hardware functionality, i.e. KVM can't rely on hardware to detect illegal/unsupported values. Failure to check the vmcs12 value would allow the guest to load any harware-supported value while running L2.
Take care to exempt BTF and LBR from the validity check in order to match KVM's behavior for writes via WRMSR, but without clobbering vmcs12. Even if VM_EXIT_SAVE_DEBUG_CONTROLS is set in vmcs12, L1 can reasonably expect that vmcs12->guest_ia32_debugctl will not be modified if writes to the MSR are being intercepted.
Arguably, KVM _should_ update vmcs12 if VM_EXIT_SAVE_DEBUG_CONTROLS is set *and* writes to MSR_IA32_DEBUGCTLMSR are not being intercepted by L1, but that would incur non-trivial complexity and wouldn't change the fact that KVM's handling of DEBUGCTL is blatantly broken. I.e. the extra complexity is not worth carrying.
Cc: stable@vger.kernel.org Signed-off-by: Maxim Levitsky mlevitsk@redhat.com Co-developed-by: Sean Christopherson seanjc@google.com Link: https://lore.kernel.org/r/20250610232010.162191-7-seanjc@google.com Signed-off-by: Sean Christopherson seanjc@google.com Stable-dep-of: 7d0cce6cbe71 ("KVM: VMX: Wrap all accesses to IA32_DEBUGCTL with getter/setter APIs") Signed-off-by: Sasha Levin sashal@kernel.org
Acked-by: Sean Christopherson seanjc@google.com
Once again with the caveat that this lands after the RTM_DEBUG patch (which I also acked).
https://lore.kernel.org/all/20250813183728.2070321-1-sashal@kernel.org