e1000: verify we have buffers, upfront

The spec says: Any descriptor with a non-zero status byte has been
processed by the hardware, and is ready to be handled by the software.

Thus, once we change a descriptor status to non-zero we should
never move the head backwards and try to reuse this
descriptor from hardware.

This actually happened with a multibuffer packet
that arrives when we don't have enough buffers.

Fix by checking that we have enough buffers upfront
so we never need to discard the packet midway through.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Acked-by: NAlex Williamson <alex.williamson@redhat.com>
Acked-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

hw/e1000.c

@@ -631,6 +631,24 @@ e1000_can_receive(VLANClientState *nc)
     return (s->mac_reg[RCTL] & E1000_RCTL_EN);
 }
 
+static bool e1000_has_rxbufs(E1000State *s, size_t total_size)
+{
+    int bufs;
+    /* Fast-path short packets */
+    if (total_size <= s->rxbuf_size) {
+        return s->mac_reg[RDH] != s->mac_reg[RDT] || !s->check_rxov;
+    }
+    if (s->mac_reg[RDH] < s->mac_reg[RDT]) {
+        bufs = s->mac_reg[RDT] - s->mac_reg[RDH];
+    } else if (s->mac_reg[RDH] > s->mac_reg[RDT] || !s->check_rxov) {
+        bufs = s->mac_reg[RDLEN] /  sizeof(struct e1000_rx_desc) +
+            s->mac_reg[RDT] - s->mac_reg[RDH];
+    } else {
+        return false;
+    }
+    return total_size <= bufs * s->rxbuf_size;
+}
+
 static ssize_t
 e1000_receive(VLANClientState *nc, const uint8_t *buf, size_t size)
 {
@@ -671,17 +689,15 @@ e1000_receive(VLANClientState *nc, const uint8_t *buf, size_t size)
     rdh_start = s->mac_reg[RDH];
     desc_offset = 0;
     total_size = size + fcs_len(s);
+    if (!e1000_has_rxbufs(s, total_size)) {
+            set_ics(s, 0, E1000_ICS_RXO);
+            return -1;
+    }
     do {
         desc_size = total_size - desc_offset;
         if (desc_size > s->rxbuf_size) {
             desc_size = s->rxbuf_size;
         }
-        if (s->mac_reg[RDH] == s->mac_reg[RDT] && s->check_rxov) {
-            /* Discard all data written so far */
-            s->mac_reg[RDH] = rdh_start;
-            set_ics(s, 0, E1000_ICS_RXO);
-            return -1;
-        }
         base = ((uint64_t)s->mac_reg[RDBAH] << 32) + s->mac_reg[RDBAL] +
                sizeof(desc) * s->mac_reg[RDH];
         cpu_physical_memory_read(base, (void *)&desc, sizeof(desc));