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

[Paolo Bonzini]

On 18/09/19 15:15, Will Deacon wrote:
> 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.

Google's patch, which checks if ring->last is not in range and fails
with -EOPNOTSUPP if not, is slightly better.  I'll send it in a second
and Cc you (and also send it as a pull request to Linus).

Paolo

>  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;
>  }
>