KVM: x86: Return -E2BIG when KVM_GET_SUPPORTED_CPUID hits max entries

Fix a long-standing bug that causes KVM to return 0 instead of -E2BIG
when userspace's array is insufficiently sized.

This technically breaks backwards compatibility, e.g. a userspace with a
hardcoded cpuid->nent could theoretically be broken as it would see an
error instead of success if cpuid->nent is less than the number of
entries required to fully enumerate the host CPU.  But, the lowest known
cpuid->nent hardcoded by a VMM is 100 (lkvm and selftests), and the
limit for current processors on Intel and AMD is well under a 100.  E.g.
Intel's Icelake server with all the bells and whistles tops out at ~60
entries (variable due to SGX sub-leafs), and AMD's CPUID documentation
allows for less than 50.  CPUID 0xD sub-leaves on current kernels are
capped by the value of KVM_SUPPORTED_XCR0, and therefore so many subleaves
cannot have appeared on current kernels.

Note, while the Fixes: tag is accurate with respect to the immediate
bug, it's likely that similar bugs in KVM_GET_SUPPORTED_CPUID existed
prior to the refactoring, e.g. Qemu contains a workaround for the broken
KVM_GET_SUPPORTED_CPUID behavior that predates the buggy commit by over
two years.  The Qemu workaround is also likely the main reason the bug
has gone unreported for so long.

Qemu hack:
  commit 76ae317f7c16aec6b469604b1764094870a75470
  Author: Mark McLoughlin <markmc@redhat.com>
  Date:   Tue May 19 18:55:21 2009 +0100

    kvm: work around supported cpuid ioctl() brokenness

    KVM_GET_SUPPORTED_CPUID has been known to fail to return -E2BIG
    when it runs out of entries. Detect this by always trying again
    with a bigger table if the ioctl() fills the table.

Fixes: 831bf664 ("KVM: Refactor and simplify kvm_dev_ioctl_get_supported_cpuid")
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>

arch/x86/kvm/cpuid.c

@@ -908,9 +908,14 @@ int kvm_dev_ioctl_get_cpuid(struct kvm_cpuid2 *cpuid,
 			goto out_free;
 
 		limit = cpuid_entries[nent - 1].eax;
-		for (func = ent->func + 1; func <= limit && nent < cpuid->nent && r == 0; ++func)
+		for (func = ent->func + 1; func <= limit && r == 0; ++func) {
+			if (nent >= cpuid->nent) {
+				r = -E2BIG;
+				goto out_free;
+			}
 			r = do_cpuid_func(&cpuid_entries[nent], func,
 				          &nent, cpuid->nent, type);
+		}
 
 		if (r)
 			goto out_free;