KVM: x86: Fix kernel info-leak in KVM_HC_CLOCK_PAIRING hypercall

kvm_pv_clock_pairing() allocates local var
"struct kvm_clock_pairing clock_pairing" on stack and initializes
all it's fields besides padding (clock_pairing.pad[]).

Because clock_pairing var is written completely (including padding)
to guest memory, failure to init struct padding results in kernel
info-leak.

Fix the issue by making sure to also init the padding with zeroes.

Fixes: 55dd00a73a51 ("KVM: x86: add KVM_HC_CLOCK_PAIRING hypercall")
Reported-by: syzbot+a8ef68d71211ba264f56@syzkaller.appspotmail.com
Reviewed-by: Mark Kanda <mark.kanda@oracle.com>
Signed-off-by: Liran Alon <liran.alon@oracle.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -6918,6 +6918,7 @@ static int kvm_pv_clock_pairing(struct kvm_vcpu *vcpu, gpa_t paddr,
 	clock_pairing.nsec = ts.tv_nsec;
 	clock_pairing.tsc = kvm_read_l1_tsc(vcpu, cycle);
 	clock_pairing.flags = 0;
+	memset(&clock_pairing.pad, 0, sizeof(clock_pairing.pad));
 
 	ret = 0;
 	if (kvm_write_guest(vcpu->kvm, paddr, &clock_pairing,