Re: [Qemu-block] [PATCH] nvme: fix oob access issue(CVE-2018-16847)

[Keith Busch]

On Fri, Nov 02, 2018 at 11:54:21AM +0100, Kevin Wolf wrote:
> Am 02.11.2018 um 02:22 hat Li Qiang geschrieben:
> > Currently, the nvme_cmb_ops mr doesn't check the addr and size.
> > This can lead an oob access issue. This is triggerable in the guest.
> > Add check to avoid this issue.
> > 
> > Fixes CVE-2018-16847.
> > 
> > Reported-by: Li Qiang <address@hidden>
> > Reviewed-by: Paolo Bonzini <address@hidden>
> > Signed-off-by: Li Qiang <address@hidden>
> > ---
> >  hw/block/nvme.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/hw/block/nvme.c b/hw/block/nvme.c
> > index fc7dacb..d097add 100644
> > --- a/hw/block/nvme.c
> > +++ b/hw/block/nvme.c
> > @@ -1175,6 +1175,10 @@ static void nvme_cmb_write(void *opaque, hwaddr 
> > addr, uint64_t data,
> >      unsigned size)
> >  {
> >      NvmeCtrl *n = (NvmeCtrl *)opaque;
> > +
> > +    if (addr + size > NVME_CMBSZ_GETSIZE(n->bar.cmbsz)) {
> 
> What prevents a guest from moving the device to the end of the address
> space and causing an integer overflow in addr + size?
> 
> If this happens, we still have .max_access_size = 8. The next question is
> then, is NVME_CMBSZ_GETSIZE guaranteed to be at least 8? I suppose yes,
> but do we want to rely on this for security?
> 
> Kevin

The nvme spec at least doesn't allow a way to express a CMB that isn't
at a minimum 4k aligned and sized, so 8 byte access should always be
within the boundary.