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>

diff --git a/virt/kvm/coalesced_mmio.c b/virt/kvm/coalesced_mmio.c
index 5294abb3f1788cbe07fa4835e18cdcf95e2ecd49..8ffd07e2a1602f2ceee0c2c08cec1a666f32b7ca 100644 (file)
--- 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;
 }