Re: [Qemu-devel] [PATCH 15/23] pxa2xx: avoid buffer overrun on incoming migration

[Michael Roth <mdroth@linux.vnet.ibm.com>]

Quoting Don Koch (2013-12-03 13:46:24)
> On 12/03/2013 11:29 AM, Michael S. Tsirkin wrote:
> > From: Michael Roth <mdroth@linux.vnet.ibm.com>
> > 
> > CVE-2013-4533
> > 
> > s->rx_level is read from the wire and used to determine how many bytes
> > to subsequently read into s->rx_fifo[]. If s->rx_level exceeds the
> > length of s->rx_fifo[] the buffer can be overrun with arbitrary data
> > from the wire.
> > 
> > Fix this by introducing a constant, RX_FIFO_SZ, that defines the length
> > of s->rx_fifo[], and taking the wire value modulo RX_FIFO_SZ (as is done
> > elsewhere in the emulation code when s->rx_level exceeds RX_FIFO_SZ).
> > 
> > Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >  hw/arm/pxa2xx.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/hw/arm/pxa2xx.c b/hw/arm/pxa2xx.c
> > index 02b7016..41d3c39 100644
> > --- a/hw/arm/pxa2xx.c
> > +++ b/hw/arm/pxa2xx.c
> > @@ -457,6 +457,8 @@ static const VMStateDescription vmstate_pxa2xx_mm = {
> >      }
> >  };
> >  
> > +#define RX_FIFO_SZ 16
> > +
> >  #define TYPE_PXA2XX_SSP "pxa2xx-ssp"
> >  #define PXA2XX_SSP(obj) \
> >      OBJECT_CHECK(PXA2xxSSPState, (obj), TYPE_PXA2XX_SSP)
> > @@ -481,7 +483,7 @@ typedef struct {
> >      uint8_t ssrsa;
> >      uint8_t ssacd;
> >  
> > -    uint32_t rx_fifo[16];
> > +    uint32_t rx_fifo[RX_FIFO_SZ];
> >      int rx_level;
> >      int rx_start;
> >  } PXA2xxSSPState;
> > @@ -756,7 +758,7 @@ static int pxa2xx_ssp_load(QEMUFile *f, void *opaque, int version_id)
> >      qemu_get_8s(f, &s->ssrsa);
> >      qemu_get_8s(f, &s->ssacd);
> >  
> > -    s->rx_level = qemu_get_byte(f);
> > +    s->rx_level = qemu_get_byte(f) % RX_FIFO_SZ;
> 
> This looks like it could leave garbage to be read in later. Why not
> check for s->rx_level > RX_FIFO_SZ and return an error like the others?

When I looked at the code before it seemed like s->rx_level was a running index
into a circular buffer, but I see now it never gets incremented beyond 16:

        if (s->enable) {
            uint32_t readval;
            readval = ssi_transfer(s->bus, value);
            if (s->rx_level < 0x10) {
                s->rx_fifo[(s->rx_start + s->rx_level ++) & 0xf] = readval;
            } else {
                s->sssr |= SSSR_ROR;
            }
        }

So it probably makes more sense to just fail migration if it exceeds 16. I
think that would also address the issue Peter pointed out.

> 
> >      s->rx_start = 0;
> >      for (i = 0; i < s->rx_level; i ++)
> >          s->rx_fifo[i] = qemu_get_byte(f);
> > 
> 
> -d