KVM: coalesced_mmio: add bounds checking

The first/last indexes are typically shared with a user app.
The app can change the 'last' index that the kernel uses
to store the next result.  This change sanity checks the index
before using it for writing to a potentially arbitrary address.

This fixes CVE-2019-14821.

Cc: stable@vger.kernel.org
Fixes: 5f94c1741bdc ("KVM: Add coalesced MMIO support (common part)")
Signed-off-by: Matt Delco <delco@chromium.org>
Signed-off-by: Jim Mattson <jmattson@google.com>
Reported-by: syzbot+983c866c3dd6efa3662a@syzkaller.appspotmail.com
[Use READ_ONCE. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 11 insertions, 8 deletions

diff --git a/virt/kvm/coalesced_mmio.c b/virt/kvm/coalesced_mmio.c
index 5294abb3f178..8ffd07e2a160 100644
--- a/virt/kvm/coalesced_mmio.c
+++ b/virt/kvm/coalesced_mmio.c
@@ -40,7 +40,7 @@ static int coalesced_mmio_in_range(struct kvm_coalesced_mmio_dev *dev,
         return 1;
 }
 
-static int coalesced_mmio_has_room(struct kvm_coalesced_mmio_dev *dev)
+static int coalesced_mmio_has_room(struct kvm_coalesced_mmio_dev *dev, u32 last)
 {
         struct kvm_coalesced_mmio_ring *ring;
         unsigned avail;
@@ -52,7 +52,7 @@ static int coalesced_mmio_has_room(struct kvm_coalesced_mmio_dev *dev)
          * there is always one unused entry in the buffer
          */
         ring = dev->kvm->coalesced_mmio_ring;
-        avail = (ring->first - ring->last - 1) % KVM_COALESCED_MMIO_MAX;
+        avail = (ring->first - last - 1) % KVM_COALESCED_MMIO_MAX;
         if (avail == 0) {
                 /* full */
                 return 0;
@@ -67,25 +67,28 @@ static int coalesced_mmio_write(struct kvm_vcpu *vcpu,
 {
         struct kvm_coalesced_mmio_dev *dev = to_mmio(this);
         struct kvm_coalesced_mmio_ring *ring = dev->kvm->coalesced_mmio_ring;
+        __u32 insert;
 
         if (!coalesced_mmio_in_range(dev, addr, len))
                 return -EOPNOTSUPP;
 
         spin_lock(&dev->kvm->ring_lock);
 
-        if (!coalesced_mmio_has_room(dev)) {
+        insert = READ_ONCE(ring->last);
+        if (!coalesced_mmio_has_room(dev, insert) ||
+            insert >= KVM_COALESCED_MMIO_MAX) {
                 spin_unlock(&dev->kvm->ring_lock);
                 return -EOPNOTSUPP;
         }
 
         /* copy data in first free entry of the ring */
 
-        ring->coalesced_mmio[ring->last].phys_addr = addr;
-        ring->coalesced_mmio[ring->last].len = len;
-        memcpy(ring->coalesced_mmio[ring->last].data, val, len);
-        ring->coalesced_mmio[ring->last].pio = dev->zone.pio;
+        ring->coalesced_mmio[insert].phys_addr = addr;
+        ring->coalesced_mmio[insert].len = len;
+        memcpy(ring->coalesced_mmio[insert].data, val, len);
+        ring->coalesced_mmio[insert].pio = dev->zone.pio;
         smp_wmb();
-        ring->last = (ring->last + 1) % KVM_COALESCED_MMIO_MAX;
+        ring->last = (insert + 1) % KVM_COALESCED_MMIO_MAX;
         spin_unlock(&dev->kvm->ring_lock);
         return 0;
 }