hw/net/smc91c111: Ignore attempt to pop from empty RX fifo

The SMC91C111 includes an MMU Command register which permits
the guest to remove entries from the RX FIFO. The datasheet
does not specify what happens if the guest tries to do this
when the FIFO is already empty; there are no status registers
containing error bits which might be applicable.

Currently we don't guard at all against pop of an empty
RX FIFO, with the result that we allow the guest to drive
the rx_fifo_len index to negative values, which will cause
smc91c111_receive() to write to the rx_fifo[] array out of
bounds when we receive the next packet.

Instead ignore attempts to pop an empty RX FIFO.

Cc: qemu-stable@nongnu.org
Fixes: 80337b66a8 ("NIC emulation for qemu arm-softmmu")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2780
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250207151157.3151776-1-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

hw/net/smc91c111.c

@@ -182,6 +182,15 @@ static void smc91c111_pop_rx_fifo(smc91c111_state *s)
 {
     int i;
 
+    if (s->rx_fifo_len == 0) {
+        /*
+         * The datasheet doesn't document what the behaviour is if the
+         * guest tries to pop an empty RX FIFO, and there's no obvious
+         * error status register to report it. Just ignore the attempt.
+         */
+        return;
+    }
+
     s->rx_fifo_len--;
     if (s->rx_fifo_len) {
         for (i = 0; i < s->rx_fifo_len; i++)