hw/net/smc91c111: Sanitize packet length on tx

When the smc91c111 transmits a packet, it must read a control byte
which is at the end of the data area and CRC.  However, we don't
sanitize the length field in the packet buffer, so if the guest sets
the length field to something large we will try to read past the end
of the packet data buffer when we access the control byte.

As usual, the datasheet says nothing about the behaviour of the
hardware if the guest misprograms it in this way.  It says only that
the maximum valid length is 2048 bytes.  We choose to log the guest
error and silently drop the packet.

This requires us to factor out the "mark the tx packet as complete"
logic, so we can call it for this "drop packet" case as well as at
the end of the loop when we send a valid packet.

Cc: [email protected]
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2742
Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-ID: <[email protected]>
[PMD: Update smc91c111_do_tx() as len > MAX_PACKET_SIZE]
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>

Author: pm215

hw/net/smc91c111.c

@@ -22,6 +22,13 @@
 
 /* Number of 2k memory pages available.  */
 #define NUM_PACKETS 4
+/*
+ * Maximum size of a data frame, including the leading status word
+ * and byte count fields and the trailing CRC, last data byte
+ * and control byte (per figure 8-1 in the Microchip Technology
+ * LAN91C111 datasheet).
+ */
+#define MAX_PACKET_SIZE 2048
 
 #define TYPE_SMC91C111 "smc91c111"
 OBJECT_DECLARE_SIMPLE_TYPE(smc91c111_state, SMC91C111)
@@ -240,6 +247,16 @@ static void smc91c111_release_packet(smc91c111_state *s, int packet)
     smc91c111_flush_queued_packets(s);
 }
 
+static void smc91c111_complete_tx_packet(smc91c111_state *s, int packetnum)
+{
+    if (s->ctr & CTR_AUTO_RELEASE) {
+        /* Race?  */
+        smc91c111_release_packet(s, packetnum);
+    } else if (s->tx_fifo_done_len < NUM_PACKETS) {
+        s->tx_fifo_done[s->tx_fifo_done_len++] = packetnum;
+    }
+}
+
 /* Flush the TX FIFO.  */
 static void smc91c111_do_tx(smc91c111_state *s)
 {
@@ -263,6 +280,17 @@ static void smc91c111_do_tx(smc91c111_state *s)
         *(p++) = 0x40;
         len = *(p++);
         len |= ((int)*(p++)) << 8;
+        if (len > MAX_PACKET_SIZE) {
+            /*
+             * Datasheet doesn't say what to do here, and there is no
+             * relevant tx error condition listed. Log, and drop the packet.
+             */
+            qemu_log_mask(LOG_GUEST_ERROR,
+                          "smc91c111: tx packet with bad length %d, dropping\n",
+                          len);
+            smc91c111_complete_tx_packet(s, packetnum);
+            continue;
+        }
         len -= 6;
         control = p[len + 1];
         if (control & 0x20)
@@ -291,11 +319,7 @@ static void smc91c111_do_tx(smc91c111_state *s)
             }
         }
 #endif
-        if (s->ctr & CTR_AUTO_RELEASE)
-            /* Race?  */
-            smc91c111_release_packet(s, packetnum);
-        else if (s->tx_fifo_done_len < NUM_PACKETS)
-            s->tx_fifo_done[s->tx_fifo_done_len++] = packetnum;
+        smc91c111_complete_tx_packet(s, packetnum);
         qemu_send_packet(qemu_get_queue(s->nic), p, len);
     }
     s->tx_fifo_len = 0;