On 10/28/25 19:00, Peter Maydell wrote:
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: [email protected]
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3043
Signed-off-by: Peter Maydell <[email protected]>
Hi!
Somehow I ended up picking up this change to all active-at-the-time
qemu stable series, including 7.2.x. Without realizing the problem
this change fixes does not exist in 7.2 to begin with - since commit
140eae9c8f7 is qemu 8.1, not 7.2.
Am I right this change is harmful but not very useful for 7.2?
Thanks,
/mjt
---
I think this is the right fix, but I'm not very familiar
with the net internals...
---
net/net.c | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/net/net.c b/net/net.c
index 27e0d278071..8aefdb3424f 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);
}
