[PATCH 3.16 130/132] KVM: coalesced_mmio: add bounds checking

[Ben Hutchings <ben@xxxxxxxxxxxxxxx>]

3.16.74-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Matt Delco <delco@xxxxxxxxxxxx>

commit b60fe990c6b07ef6d4df67bc0530c7c90a62623a upstream.

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.

Fixes: 5f94c1741bdc ("KVM: Add coalesced MMIO support (common part)")
Signed-off-by: Matt Delco <delco@xxxxxxxxxxxx>
Signed-off-by: Jim Mattson <jmattson@xxxxxxxxxx>
Reported-by: syzbot+983c866c3dd6efa3662a@xxxxxxxxxxxxxxxxxxxxxxxxx
[Use READ_ONCE. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
[bwh: Backported to 3.16:
 - Use ACCESS_ONCE() instead of READ_ONCE()
 - kvm_coalesced_mmio_zone::pio field is not supported]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 virt/kvm/coalesced_mmio.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

--- a/virt/kvm/coalesced_mmio.c
+++ b/virt/kvm/coalesced_mmio.c
@@ -39,7 +39,7 @@ static int coalesced_mmio_in_range(struc
 	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;
@@ -51,7 +51,7 @@ static int coalesced_mmio_has_room(struc
 	 * 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;
@@ -65,24 +65,27 @@ static int coalesced_mmio_write(struct k
 {
 	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 = ACCESS_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[insert].phys_addr = addr;
+	ring->coalesced_mmio[insert].len = len;
+	memcpy(ring->coalesced_mmio[insert].data, val, len);
 	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;
 }