pcnet: fix Negative array index read
    
From: Gonglei <arei.gonglei@huawei.com>

s->xmit_pos maybe assigned to a negative value (-1),
but in this branch variable s->xmit_pos as an index to
array s->buffer. Let's add a check for s->xmit_pos.
    
upstream-commit-id: 7b50d00911ddd6d56a766ac5671e47304c20a21b

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

diff --git a/hw/pcnet.c b/hw/pcnet.c
index 7cc0637..9f3e1cc 100644
--- a/tools/qemu-xen-traditional/hw/pcnet.c
+++ b/tools/qemu-xen-traditional/hw/pcnet.c
@@ -1250,7 +1250,7 @@ static void pcnet_transmit(PCNetState *s)
     target_phys_addr_t xmit_cxda = 0;
     int count = CSR_XMTRL(s)-1;
     int add_crc = 0;
-
+    int bcnt;
     s->xmit_pos = -1;
 
     if (!CSR_TXON(s)) {
@@ -1276,34 +1276,39 @@ static void pcnet_transmit(PCNetState *s)
             if (BCR_SWSTYLE(s) != 1)
                 add_crc = GET_FIELD(tmd.status, TMDS, ADDFCS);
         }
+
+        if (s->xmit_pos < 0) {
+            goto txdone;
+        }
+
+        bcnt = 4096 - GET_FIELD(tmd.length, TMDL, BCNT);
+        s->phys_mem_read(s->dma_opaque, PHYSADDR(s, tmd.tbadr),
+                         s->buffer + s->xmit_pos, bcnt, CSR_BSWP(s));
+        s->xmit_pos += bcnt;
+
         if (!GET_FIELD(tmd.status, TMDS, ENP)) {
-            int bcnt = 4096 - GET_FIELD(tmd.length, TMDL, BCNT);
-            s->phys_mem_read(s->dma_opaque, PHYSADDR(s, tmd.tbadr),
-                             s->buffer + s->xmit_pos, bcnt, CSR_BSWP(s));
-            s->xmit_pos += bcnt;
-        } else if (s->xmit_pos >= 0) {
-            int bcnt = 4096 - GET_FIELD(tmd.length, TMDL, BCNT);
-            s->phys_mem_read(s->dma_opaque, PHYSADDR(s, tmd.tbadr),
-                             s->buffer + s->xmit_pos, bcnt, CSR_BSWP(s));
-            s->xmit_pos += bcnt;
+            goto txdone;
+        }
 #ifdef PCNET_DEBUG
-            printf("pcnet_transmit size=%d\n", s->xmit_pos);
+        printf("pcnet_transmit size=%d\n", s->xmit_pos);
 #endif
-            if (CSR_LOOP(s)) {
-                if (BCR_SWSTYLE(s) == 1)
-                    add_crc = !GET_FIELD(tmd.status, TMDS, NOFCS);
-                s->looptest = add_crc ? PCNET_LOOPTEST_CRC : PCNET_LOOPTEST_NOCRC;
-                pcnet_receive(s, s->buffer, s->xmit_pos);
-                s->looptest = 0;
-            } else
-                if (s->vc)
-                    qemu_send_packet(s->vc, s->buffer, s->xmit_pos);
-
-            s->csr[0] &= ~0x0008;   /* clear TDMD */
-            s->csr[4] |= 0x0004;    /* set TXSTRT */
-            s->xmit_pos = -1;
+        if (CSR_LOOP(s)) {
+            if (BCR_SWSTYLE(s) == 1)
+                add_crc = !GET_FIELD(tmd.status, TMDS, NOFCS);
+            s->looptest = add_crc ? PCNET_LOOPTEST_CRC : PCNET_LOOPTEST_NOCRC;
+            pcnet_receive(s, s->buffer, s->xmit_pos);
+            s->looptest = 0;
+        } else {
+            if (s->vc) {
+                qemu_send_packet(s->vc, s->buffer, s->xmit_pos);
+            }
         }
 
+        s->csr[0] &= ~0x0008;   /* clear TDMD */
+        s->csr[4] |= 0x0004;    /* set TXSTRT */
+        s->xmit_pos = -1;
+
+    txdone:
         SET_FIELD(&tmd.status, TMDS, OWN, 0);
         TMDSTORE(&tmd, PHYSADDR(s,CSR_CXDA(s)));
         if (!CSR_TOKINTD(s) || (CSR_LTINTEN(s) && GET_FIELD(tmd.status, TMDS, LTINT)))