CVE-2016-9777: Out-of-bounds Read (CWE-125)
value: KVM in the Linux kernel before 4.8.12, when I/O APIC is enabled, does not properly restrict the VCPU index, which allows guest OS users to gain host OS privileges or cause a denial of service (out-of-bounds array access and host OS crash) via a crafted interrupt request, related to arch/x86/kvm/ioapic.c and arch/x86/kvm/ioapic.h.
CWE description: The product reads data past the end, or before the beginning, of the intended buffer.
CVSS v3 base score: 7.8
Published: 2016-12-28T07:59Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: arch/x86/kvm/ioapic.c, arch/x86/kvm/ioapic.h
Commit date: 2016-11-24 18:37:19 +0100
Commit message: KVM: x86: fix out-of-bounds accesses of rtc_eoi map

KVM was using arrays of size KVM_MAX_VCPUS with vcpu_id, but ID can be
bigger that the maximal number of VCPUs, resulting in out-of-bounds
access.

Found by syzkaller:

  BUG: KASAN: slab-out-of-bounds in __apic_accept_irq+0xb33/0xb50 at addr [...]
  Write of size 1 by task a.out/27101
  CPU: 1 PID: 27101 Comm: a.out Not tainted 4.9.0-rc5+ #49
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
   [...]
  Call Trace:
   [...] __apic_accept_irq+0xb33/0xb50 arch/x86/kvm/lapic.c:905
   [...] kvm_apic_set_irq+0x10e/0x180 arch/x86/kvm/lapic.c:495
   [...] kvm_irq_delivery_to_apic+0x732/0xc10 arch/x86/kvm/irq_comm.c:86
   [...] ioapic_service+0x41d/0x760 arch/x86/kvm/ioapic.c:360
   [...] ioapic_set_irq+0x275/0x6c0 arch/x86/kvm/ioapic.c:222
   [...] kvm_ioapic_inject_all arch/x86/kvm/ioapic.c:235
   [...] kvm_set_ioapic+0x223/0x310 arch/x86/kvm/ioapic.c:670
   [...] kvm_vm_ioctl_set_irqchip arch/x86/kvm/x86.c:3668
   [...] kvm_arch_vm_ioctl+0x1a08/0x23c0 arch/x86/kvm/x86.c:3999
   [...] kvm_vm_ioctl+0x1fa/0x1a70 arch/x86/kvm/../../../virt/kvm/kvm_main.c:3099

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: stable@vger.kernel.org
Fixes: af1bae5497b9 ("KVM: x86: bump KVM_MAX_VCPU_ID to 1023")
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

Diff: diff --git a/arch/x86/kvm/ioapic.c b/arch/x86/kvm/ioapic.c
index 1a22de70f7f7d4..6e219e5c07d27c 100644
--- a/arch/x86/kvm/ioapic.c
+++ b/arch/x86/kvm/ioapic.c
@@ -92,11 +92,11 @@ static unsigned long ioapic_read_indirect(struct kvm_ioapic *ioapic,
 }
 
 static void rtc_irq_eoi_tracking_reset(struct kvm_ioapic *ioapic)
 {
 	ioapic->rtc_status.pending_eoi = 0;
-	bitmap_zero(ioapic->rtc_status.dest_map.map, KVM_MAX_VCPUS);
+	bitmap_zero(ioapic->rtc_status.dest_map.map, KVM_MAX_VCPU_ID);
 }
 
 static void kvm_rtc_eoi_tracking_restore_all(struct kvm_ioapic *ioapic);
 
 static void rtc_status_pending_eoi_check_valid(struct kvm_ioapic *ioapic)
diff --git a/arch/x86/kvm/ioapic.h b/arch/x86/kvm/ioapic.h
index 7d2692a4965756..1cc6e54436dbaa 100644
--- a/arch/x86/kvm/ioapic.h
+++ b/arch/x86/kvm/ioapic.h
@@ -40,17 +40,17 @@ struct kvm_vcpu;
 #define RTC_GSI -1U
 #endif
 
 struct dest_map {
 	/* vcpu bitmap where IRQ has been sent */
-	DECLARE_BITMAP(map, KVM_MAX_VCPUS);
+	DECLARE_BITMAP(map, KVM_MAX_VCPU_ID);
 
 	/*
 	 * Vector sent to a given vcpu, only valid when
 	 * the vcpu's bit in map is set
 	 */
-	u8 vectors[KVM_MAX_VCPUS];
+	u8 vectors[KVM_MAX_VCPU_ID];
 };
 
 
 struct rtc_status {
 	int pending_eoi;

Vulnerable code: 	bitmap_zero(ioapic->rtc_status.dest_map.map, KVM_MAX_VCPUS);
	DECLARE_BITMAP(map, KVM_MAX_VCPUS);
	u8 vectors[KVM_MAX_VCPUS];
Fixed code: 	bitmap_zero(ioapic->rtc_status.dest_map.map, KVM_MAX_VCPU_ID);
	DECLARE_BITMAP(map, KVM_MAX_VCPU_ID);
	u8 vectors[KVM_MAX_VCPU_ID];