Re: [PATCH] macsec: skip PACKET_LOOPBACK frames in macsec_handle_frame()

[Eric Dumazet]

On Mon, Feb 9, 2026 at 5:29 PM Daniel Hodges <git@xxxxxxxxxxxxxxxx> wrote:
>
> macsec_handle_frame() assumes all incoming frames have a valid Ethernet
> header at skb_mac_header(skb) and reads hdr->eth.h_proto to determine
> whether to process the frame as MACsec.  However, loopback packets are
> delivered with pkt_type PACKET_LOOPBACK and carry only a
> protocol-specific header (e.g. 7-byte phonethdr), not a full Ethernet
> header. Reading 14 bytes of ethhdr from such a short header results in a
> slab-out-of-bounds / uninit-value access.
>
> Fix this by returning RX_HANDLER_PASS early for PACKET_LOOPBACK frames,
> consistent with how macvlan_handle_frame() handles this case.

In the syzbot report, I do not see dev_loopback_xmit() being involved ?

>
> Reported-by: syzbot+0e665e4b99cb925286a0@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: c09440f7dcb3 ("macsec: introduce IEEE 802.1AE driver")
> Signed-off-by: Daniel Hodges <git@xxxxxxxxxxxxxxxx>
> ---
>  drivers/net/macsec.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/drivers/net/macsec.c b/drivers/net/macsec.c
> index 0206b84284ab..edcc51f82327 100644
> --- a/drivers/net/macsec.c
> +++ b/drivers/net/macsec.c
> @@ -1103,6 +1103,13 @@ static rx_handler_result_t macsec_handle_frame(struct sk_buff **pskb)
>         bool pulled_sci;
>         int ret;
>
> +       /* Loopback packets (e.g. from phonet) don't have L2 headers, so
> +        * attempting to interpret the mac header as Ethernet would read
> +        * uninitialized memory.  Let them pass through unmodified.
> +        */
> +       if (unlikely(skb->pkt_type == PACKET_LOOPBACK))
> +               return RX_HANDLER_PASS;
> +
>         if (skb_headroom(skb) < ETH_HLEN)
>                 goto drop_direct;
>
> --
> 2.52.0

I think the issue is in this skb_headroom() check really, this is not
really doing what we need.

Note that the existing check (if (hdr->eth.h_proto !=
htons(ETH_P_MACSEC))) should kick properly
if phonet was able to send a bigger packet.

diff --git a/drivers/net/macsec.c b/drivers/net/macsec.c
index c2cb2d20976b..ef4cdf25d425 100644
--- a/drivers/net/macsec.c
+++ b/drivers/net/macsec.c
@@ -1141,7 +1141,7 @@ static rx_handler_result_t
macsec_handle_frame(struct sk_buff **pskb)
        bool pulled_sci;
        int ret;

-       if (skb_headroom(skb) < ETH_HLEN)
+       if (!pskb_may_pull(skb, skb_mac_offset(skb) + ETH_HLEN))
                goto drop_direct;

        hdr = macsec_ethhdr(skb);