[PULL 5/5] net: pad packets to minimum length in qemu_receive_packet()

[Jason Wang <jasowang@redhat.com>]

From: Peter Maydell <peter.maydell@linaro.org>

In commits like 969e50b61a28 ("net: Pad short frames to minimum size
before sending from SLiRP/TAP") we switched away from requiring
network devices to handle short frames to instead having the net core
code do the padding of short frames out to the ETH_ZLEN minimum size.
We then dropped the code for handling short frames from the network
devices in a series of commits like 140eae9c8f7 ("hw/net: e1000:
Remove the logic of padding short frames in the receive path").

This missed one route where the device's receive code can still see a
short frame: if the device is in loopback mode and it transmits a
short frame via the qemu_receive_packet() function, this will be fed
back into its own receive code without being padded.

Add the padding logic to qemu_receive_packet().

This fixes a buffer overrun which can be triggered in the
e1000_receive_iov() logic via the loopback code path.

Other devices that use qemu_receive_packet() to implement loopback
are cadence_gem, dp8393x, lan9118, msf2-emac, pcnet, rtl8139
and sungem.

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3043
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>
---
 net/net.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/net/net.c b/net/net.c
index 27e0d27807..8aefdb3424 100644
--- a/net/net.c
+++ b/net/net.c
@@ -775,10 +775,20 @@ ssize_t qemu_send_packet(NetClientState *nc, const uint8_t *buf, int size)
 
 ssize_t qemu_receive_packet(NetClientState *nc, const uint8_t *buf, int size)
 {
+    uint8_t min_pkt[ETH_ZLEN];
+    size_t min_pktsz = sizeof(min_pkt);
+
     if (!qemu_can_receive_packet(nc)) {
         return 0;
     }
 
+    if (net_peer_needs_padding(nc)) {
+        if (eth_pad_short_frame(min_pkt, &min_pktsz, buf, size)) {
+            buf = min_pkt;
+            size = min_pktsz;
+        }
+    }
+
     return qemu_net_queue_receive(nc->incoming_queue, buf, size);
 }
 
-- 
2.42.0