Re: [PATCH] net: pad packets to minimum length in qemu_receive_packet()

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 28/10/25 17:00, 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: qemu-stable@nongnu.org
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3043
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> 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);
>   }

Nitpicking, variables scope can be reduced:

-- >8 --
@@ -777,5 +777,2 @@ 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)) {
@@ -785,2 +782,5 @@ ssize_t qemu_receive_packet(NetClientState *nc, 
const uint8_t *buf, int size)
      if (net_peer_needs_padding(nc)) {
+        uint8_t min_pkt[ETH_ZLEN];
+        size_t min_pktsz = sizeof(min_pkt);
+
          if (eth_pad_short_frame(min_pkt, &min_pktsz, buf, size)) {
---

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>