Re: [PATCH v4 5/9] pcie_sriov: Validate NumVFs

[Akihiko Odaki]

On 2024/02/14 15:52, Michael S. Tsirkin wrote:
> On Wed, Feb 14, 2024 at 02:13:43PM +0900, Akihiko Odaki wrote:
> > The guest may write NumVFs greater than TotalVFs and that can lead
> > to buffer overflow in VF implementations.
> >
> > Fixes: 7c0fa8dff811 ("pcie: Add support for Single Root I/O Virtualization 
> > (SR/IOV)")
> > Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> > ---
> >   hw/pci/pcie_sriov.c | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/hw/pci/pcie_sriov.c b/hw/pci/pcie_sriov.c
> > index a1fe65f5d801..da209b7f47fd 100644
> > --- a/hw/pci/pcie_sriov.c
> > +++ b/hw/pci/pcie_sriov.c
> > @@ -176,6 +176,9 @@ static void register_vfs(PCIDevice *dev)
> >   
> >       assert(sriov_cap > 0);
> >       num_vfs = pci_get_word(dev->config + sriov_cap + PCI_SRIOV_NUM_VF);
> > +    if (num_vfs > pci_get_word(dev->config + sriov_cap + PCI_SRIOV_TOTAL_VF)) {
> > +        return;
> > +    }
>
>
> yes but with your change PCI_SRIOV_NUM_VF no longer reflects the
> number of registered VFs and that might lead to uninitialized
> data read which is not better :(.
>
> How about just forcing the PCI_SRIOV_NUM_VF register to be
> below PCI_SRIOV_TOTAL_VF at all times?

PCI_SRIOV_NUM_VF is already divergent from the number of registered VFs. 
It may have a number greater than the current registered VFs before 
setting VF Enable.

The guest may also change PCI_SRIOV_NUM_VF while VF Enable is set; the 
behavior is undefined in such a case but we still accept such a write. A 
value written in such a case won't be reflected to the actual number of 
enabled VFs.