During procss_tx_desc(), driver can try to chain data descriptor with
legacy descriptor, when will lead underflow for the following
calculation in process_tx_desc() for bytes:

            if (tp->size + bytes > msh)
                bytes = msh - tp->size;

This will lead a infinite loop. So check and fail early if tp->size if
greater or equal to msh.

Reported-by: Alexander Bulekov <alxndr@bu.edu>
Reported-by: Cheolwoo Myung <cwmyung@snu.ac.kr>
Reported-by: Ruhr-University Bochum <bugs-syssec@rub.de>
Cc: Prasad J Pandit <ppandit@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Jason Wang <jasowang@redhat.com>
---
 hw/net/e1000.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/hw/net/e1000.c b/hw/net/e1000.c
index d8da2f6..4345d86 100644
--- a/hw/net/e1000.c
+++ b/hw/net/e1000.c
@@ -670,6 +670,9 @@ process_tx_desc(E1000State *s, struct e1000_tx_desc *dp)
         msh = tp->tso_props.hdr_len + tp->tso_props.mss;
         do {
             bytes = split_size;
+            if (tp->size >= msh) {
+                goto eop;
+            }
             if (tp->size + bytes > msh)
                 bytes = msh - tp->size;
 
@@ -695,6 +698,7 @@ process_tx_desc(E1000State *s, struct e1000_tx_desc *dp)
         tp->size += split_size;
     }
 
+eop:
     if (!(txd_lower & E1000_TXD_CMD_EOP))
         return;
     if (!(tp->cptse && tp->size < tp->tso_props.hdr_len)) {
-- 
2.7.4