Re: [PATCH v2 1/4] virtio-mem: validate device-reported block size

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Jul 16, 2026 at 10:55:42AM +0200, David Hildenbrand (Arm) wrote:
> On 7/15/26 18:41, Hari Mishal wrote:
> > The device_block_size read from the virtio-mem config space is used as a
> > divisor and also in ALIGN_DOWN() further down the code path in the
> > driver without further validation. A zero value leads to a division by
> > zero, and a non-power-of-two value corrupts the ALIGN_DOWN() bitmask
> > arithmetic leading to a misreporting of guest-usable ram, post crash.
> > 
> > Reject both at init time instead of trusting the device.
> > 
> > Signed-off-by: Hari Mishal <harimishal1@gmail.com>
> > ---
> > v2: dropped the redundant explicit zero check, since
> >     is_power_of_2(0) already returns false.
> > 
> >  drivers/virtio/virtio_mem.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/drivers/virtio/virtio_mem.c b/drivers/virtio/virtio_mem.c
> > index 11c441501582..0e04fec458af 100644
> > --- a/drivers/virtio/virtio_mem.c
> > +++ b/drivers/virtio/virtio_mem.c
> > @@ -2847,6 +2847,12 @@ static int virtio_mem_init(struct virtio_mem *vm)
> >  			&vm->plugged_size);
> >  	virtio_cread_le(vm->vdev, struct virtio_mem_config, block_size,
> >  			&vm->device_block_size);
> > +	if (!is_power_of_2(vm->device_block_size)) {
> > +		dev_err(&vm->vdev->dev,
> > +			"invalid device block size: 0x%llx\n",
> > +			(unsigned long long)vm->device_block_size);
> > +		return -EINVAL;
> > +	}
> 
> The spec states "The device MUST set block_size to a power of two."
> 
> I'm missing the point here.

So what happens if we have a non-spec-compliant device?  Shouldn't we be
attempting to verify this before doing something with the data?

Or do we just always trust virtio mem devices explicitly?

thanks,
greg k-h