[PATCH] kvm: Ensure writes to the coalesced MMIO ring are within bounds

[Will Deacon]

When records are written to the coalesced MMIO ring in response to a
vCPU MMIO exit, the 'ring->last' field is used to index the ring buffer
page. Although we hold the 'kvm->ring_lock' at this point, the ring
structure is mapped directly into the host userspace and can therefore
be modified to point at arbitrary pages within the kernel.

Since this shouldn't happen in normal operation, simply bound the index
by KVM_COALESCED_MMIO_MAX to contain the accesses within the ring buffer
page.

Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxx> # 5.2.y
Fixes: 5f94c1741bdc ("KVM: Add coalesced MMIO support (common part)")
Reported-by: Bill Creasey <bcreasey@xxxxxxxxxx>
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
---

I think there are some other fixes kicking around for this, but they
still rely on 'ring->last' being stable, which isn't necessarily the
case. I'll send the -stable backport for kernels prior to 5.2 once this
hits mainline.

 virt/kvm/coalesced_mmio.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/virt/kvm/coalesced_mmio.c b/virt/kvm/coalesced_mmio.c
index 5294abb3f178..09b3e4421550 100644
--- a/virt/kvm/coalesced_mmio.c
+++ b/virt/kvm/coalesced_mmio.c
@@ -67,6 +67,7 @@ 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 last;
 
 	if (!coalesced_mmio_in_range(dev, addr, len))
 		return -EOPNOTSUPP;
@@ -79,13 +80,13 @@ static int coalesced_mmio_write(struct kvm_vcpu *vcpu,
 	}
 
 	/* 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;
+	last = ring->last % KVM_COALESCED_MMIO_MAX;
+	ring->coalesced_mmio[last].phys_addr = addr;
+	ring->coalesced_mmio[last].len = len;
+	memcpy(ring->coalesced_mmio[last].data, val, len);
+	ring->coalesced_mmio[last].pio = dev->zone.pio;
 	smp_wmb();
-	ring->last = (ring->last + 1) % KVM_COALESCED_MMIO_MAX;
+	ring->last = (last + 1) % KVM_COALESCED_MMIO_MAX;
 	spin_unlock(&dev->kvm->ring_lock);
 	return 0;
 }
-- 
2.23.0.351.gc4317032e6-goog