[Bionic][SRU][CVE-2019-19332][PATCH] KVM: x86: fix out-of-bounds write in KVM_GET_EMULATED_CPUID (CVE-2019-19332)

[Connor Kuehl]

From: Paolo Bonzini <pbonzini at redhat.com>

CVE-2019-19332

The bounds check was present in KVM_GET_SUPPORTED_CPUID but not
KVM_GET_EMULATED_CPUID.

Reported-by: syzbot+e3f4897236c4eeb8af4f at syzkaller.appspotmail.com
Fixes: 84cffe499b94 ("kvm: Emulate MOVBE", 2013-10-29)
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
(backported from commit 433f4ba1904100da65a311033f17a9bf586b287e)
[ Connor Kuehl: offset adjustments ]
Signed-off-by: Connor Kuehl <connor.kuehl at canonical.com>
---
 arch/x86/kvm/cpuid.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 803479c499e9..dd22f8f60798 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -405,7 +405,7 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
 
 	r = -E2BIG;
 
-	if (*nent >= maxnent)
+	if (WARN_ON(*nent >= maxnent))
 		goto out;
 
 	do_cpuid_1_ent(entry, function, index);
@@ -707,6 +707,9 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
 static int do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 func,
 			u32 idx, int *nent, int maxnent, unsigned int type)
 {
+	if (*nent >= maxnent)
+		return -E2BIG;
+
 	if (type == KVM_GET_EMULATED_CPUID)
 		return __do_cpuid_ent_emulated(entry, func, idx, nent, maxnent);
 
-- 
2.20.1