[Qemu-devel] Re: [PATCH RFC] e1000: fix access 4 bytes beyond buffer end

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jul 12, 2010 at 04:07:21PM -0500, Anthony Liguori wrote:
> On 07/12/2010 12:48 PM, Michael S. Tsirkin wrote:
> >We do range check for size, and get size as buffer,
> >but copy size + 4 bytes (4 is for FCS).
> >Let's copy size bytes but put size + 4 in length.
> >
> >Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
> 
> I think I'd feel slightly better if we zero'd out the FCS before
> writing it to the guest.  It is potentially a data leak.

It's the buffer guest allocated, and we leave it untouched.
How does this leak data?

> Regards,
> 
> Anthony Liguori
> 
> >---
> >
> >Anthony, Alex, please review.
> >
> >  hw/e1000.c |    3 +--
> >  1 files changed, 1 insertions(+), 2 deletions(-)
> >
> >diff --git a/hw/e1000.c b/hw/e1000.c
> >index 0da65f9..70aba11 100644
> >--- a/hw/e1000.c
> >+++ b/hw/e1000.c
> >@@ -649,7 +649,6 @@ e1000_receive(VLANClientState *nc, const uint8_t *buf, size_t size)
> >      }
> >
> >      rdh_start = s->mac_reg[RDH];
> >-    size += 4; // for the header
> >      do {
> >          if (s->mac_reg[RDH] == s->mac_reg[RDT]&&  s->check_rxov) {
> >              set_ics(s, 0, E1000_ICS_RXO);
> >@@ -663,7 +662,7 @@ e1000_receive(VLANClientState *nc, const uint8_t *buf, size_t size)
> >          if (desc.buffer_addr) {
> >              cpu_physical_memory_write(le64_to_cpu(desc.buffer_addr),
> >                                        (void *)(buf + vlan_offset), size);
> >-            desc.length = cpu_to_le16(size);
> >+            desc.length = cpu_to_le16(size + 4 /* for FCS */);
> >              desc.status |= E1000_RXD_STAT_EOP|E1000_RXD_STAT_IXSM;
> >          } else // as per intel docs; skip descriptors with null buf addr
> >              DBGOUT(RX, "Null RX descriptor!!\n");