[Stable-7.2.17,03/27] hw/net/smc91c111: Ignore attempt to pop from empty RX fifo

Commit Message

Michael Tokarev March 15, 2025, 9:14 a.m. UTC

From: Peter Maydell <[e-mail]>

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: [e-mail]
Fixes: 80337b66a8e7 ("NIC emulation for qemu arm-softmmu")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2780
Signed-off-by: Peter Maydell <[e-mail]>
Reviewed-by: Philippe Mathieu-Daudé <[e-mail]>
Message-ID: <[e-mail]>
Signed-off-by: Philippe Mathieu-Daudé <[e-mail]>
(cherry picked from commit 937df81af6757638a7f1908747560dd342947213)
Signed-off-by: Michael Tokarev <[e-mail]>

Patch

diff --git a/hw/net/smc91c111.c b/hw/net/smc91c111.c
index 4eda971ef3..6ce610cb23 100644
--- a/hw/net/smc91c111.c
+++ b/hw/net/smc91c111.c
@@ -183,6 +183,15 @@  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++)