ACK: [SRU][Disco][CVE-2019-14821][PATCH] KVM: coalesced_mmio: add bounds checking

[Connor Kuehl]

On 9/24/19 12:47 AM, Juerg Haefliger wrote:
> From: Matt Delco <delco at chromium.org>
> 
> 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 at vger.kernel.org
> Fixes: 5f94c1741bdc ("KVM: Add coalesced MMIO support (common part)")
> Signed-off-by: Matt Delco <delco at chromium.org>
> Signed-off-by: Jim Mattson <jmattson at google.com>
> Reported-by: syzbot+983c866c3dd6efa3662a at syzkaller.appspotmail.com
> [Use READ_ONCE. - Paolo]
> Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
> 
> CVE-2019-14821
> 
> (cherry picked from commit b60fe990c6b07ef6d4df67bc0530c7c90a62623a)
> Signed-off-by: Juerg Haefliger <juergh at canonical.com>

Acked-by: Connor Kuehl <connor.kuehl at canonical.com>

> ---
>   virt/kvm/coalesced_mmio.c | 19 +++++++++++--------
>   1 file changed, 11 insertions(+), 8 deletions(-)
> 
> diff --git a/virt/kvm/coalesced_mmio.c b/virt/kvm/coalesced_mmio.c
> index 6855cce3e528..f52ae1e52591 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;
>   }
>