On 10/12/19 23:44, Sean Christopherson wrote: > KVM currently doesn't incorporate the platform's capabilities in its cr4 > reserved bit checks and instead checks only whether KVM is aware of the > feature in general and whether or not userspace has advertised the feature > to the guest. > > Lack of checking allows userspace/guest to set unsupported bits in cr4. > For the most part, setting unsupported bits will simply cause VM-Enter to > fail. The one existing exception is OSXSAVE, which is conditioned on host > support as checking only guest_cpuid_has() would result in KVM attempting > XSAVE, leading to faults and WARNs. > > 57-bit virtual addressing has introduced another case where setting an > unsupported bit (cr4.LA57) can induce a fault in the host. In the LA57 > case, userspace can set the guest's cr4.LA57 by advertising LA57 support > via CPUID and abuse the bogus cr4.LA57 to effectively bypass KVM's > non-canonical address check, ultimately causing a #GP when VMX writes > the guest's bogus address to MSR_KERNEL_GS_BASE during VM-Enter. > > Given that the best case scenario is a failed VM-Enter, there's no sane > reason to allow setting unsupported bits in cr4. Fix the LA57 bug by not > allowing userspace or the guest to set cr4 bits that are not supported > by the platform. > > Sean Christopherson (4): > KVM: x86: Don't let userspace set host-reserved cr4 bits > KVM: x86: Ensure all logical CPUs have consistent reserved cr4 bits > KVM: x86: Drop special XSAVE handling from guest_cpuid_has() > KVM: x86: Add macro to ensure reserved cr4 bits checks stay in sync > > arch/x86/kvm/cpuid.h | 4 --- > arch/x86/kvm/svm.c | 1 + > arch/x86/kvm/vmx/vmx.c | 1 + > arch/x86/kvm/x86.c | 65 +++++++++++++++++++++++++++++------------- > 4 files changed, 47 insertions(+), 24 deletions(-) > Queued, thanks. Paolo
