KVM: x86: Check userspace CPUID array size after validating sub-leaf

Verify that the next sub-leaf of CPUID 0x4 (or 0x8000001d) is valid
before rejecting the entire KVM_GET_SUPPORTED_CPUID due to insufficent
space in the userspace array.

Note, although this is technically a bug, it's not visible to userspace
as KVM_GET_SUPPORTED_CPUID is guaranteed to fail on KVM_CPUID_SIGNATURE,
which is hardcoded to be added after the affected leafs.  The real
motivation for the change is to tightly couple the nent/maxnent and
do_host_cpuid() sequences in preparation for future cleanup.

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 11d5f31..e5cf1e0 100644 (file)
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -552,12 +552,12 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
 
                /* read more entries until cache_type is zero */
                for (i = 1; ; ++i) {
-                       if (*nent >= maxnent)
-                               goto out;
-
                        cache_type = entry[i - 1].eax & 0x1f;
                        if (!cache_type)
                                break;
+
+                       if (*nent >= maxnent)
+                               goto out;
                        do_host_cpuid(&entry[i], function, i);
                        ++*nent;
                }