Re: [Xen-devel] [PATCH v1 03/10] xen/pt: Check if reg->init function sets the 'data' past the reg->size

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Jul 17, 2015 at 05:03:44PM +0100, Stefano Stabellini wrote:
> On Thu, 2 Jul 2015, Konrad Rzeszutek Wilk wrote:
> > It should never happen, but in case it does (an developer adds
> > a new register and the 'init_val' expands past the register
> > size) we want to report. The code will only write up to
> > reg->size so there is no runtime danger of the register spilling
> > across other ones - however to catch this sort of thing
> > we still return an error.
> > 
> > Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > ---
> >  hw/xen/xen_pt_config_init.c | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/hw/xen/xen_pt_config_init.c b/hw/xen/xen_pt_config_init.c
> > index 3938afd..09309ba 100644
> > --- a/hw/xen/xen_pt_config_init.c
> > +++ b/hw/xen/xen_pt_config_init.c
> > @@ -1904,9 +1904,15 @@ static int 
> > xen_pt_config_reg_init(XenPCIPassthroughState *s,
> >          } else
> >              val = data;
> >  
> > +        if (val & ~size_mask) {
> > +            XEN_PT_ERR(&s->dev,"Offset 0x%04x:0x%04x expands past register 
> > size(%d)!\n",
> > +                       offset, val, reg->size);
> > +            g_free(reg_entry);
> > +            return -ENXIO;
> > +        }
> 
> If we worry about changes to init_val, wouldn't it be better to add
> QEMU_BUILD_BUG_ON(data & ~size_mask)?

I couldnt' figure out how to make that work nicely.

The QEMU_BUILD_BUG_ON look to be build time - not run-time.

Which means that doing:
        for (i = 0; i < grp_entries; i++)
        {
                entries = grp_entries[i]...
                for (j = 0; j < entries; j++)
                        QEMU_BUILD_BUG_ON(entries[j].init_val & ~size_mask)
        }

is not something I can image the compiler working with?

> 
> 
> >          /* This could be just pci_set_long as we don't modify the bits
> > -         * past reg->size, but in case this routine is run in parallel
> > -         * we do not want to over-write other registers. */
> > +         * past reg->size, but in case this routine is run in parallel or 
> > the
> > +         * init value is larger, we do not want to over-write registers. */
> >          switch (reg->size) {
> >          case 1: pci_set_byte(s->dev.config + offset, (uint8_t)val); break;
> >          case 2: pci_set_word(s->dev.config + offset, (uint16_t)val); break;
> > -- 
> > 2.1.0
> > 

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