net: dsa: Fix skb ownership in taggers

net: dsa: Fix skb ownership in taggers

The tag_8021q.c tagger calls vlan_insert_tag() in dsa_8021q_xmit().
vlan_insert_tag() will consume the skb with kfree_skb() on failure
and return NULL.

When NULL is returned as error code to ->xmit() in dsa_user_xmit()
it will free the same skb again leading to a double-free.

The idea of dsa_user_xmit() and dsa_switch_rcv() dropping the skb
they held before the call to ->xmit() and ->rcv() is conceptually
wrong: the pattern elsewhere in the networking code is that consumers
drop their skb:s on failure.

Modify the ->xmit() and ->rcv() call sites to not drop the SKB if
the taggers return NULL from any of these calls. Move those drops into
the taggers so every callback error path that retains ownership consumes
the skb before returning NULL.

Keep the existing helper ownership rules: VLAN insertion helpers already
free on failure (this is the case in tag_8021q.c), while deferred
transmit paths either transfer the skb reference to worker context or
hold a worker reference with skb_get() and drop the caller's reference.

For SJA1105 meta RX, transfer the buffered stampable skb under the meta
lock and return NULL while the skb is waiting for its meta frame: the
skb is not dropped in this case.

NOTICE: Backporting patches to taggers (e.g. for stable kernels) after
this point cannot be mechanical or they will introduce double
kfree_skb().

Reported-by: Sashiko AI Review <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/r/20260610153952.1685895-1-kuba@kernel.org/
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: David Yang <mmyangfl@gmail.com> # yt921x
Acked-by: Kurt Kanzenbach <kurt@linutronix.de> # hellcreek
Reviewed-by: Wei Fang <wei.fang@nxp.com> # netc
Signed-off-by: Linus Walleij <linusw@kernel.org>
Link: https://patch.msgid.link/20260625-dsa-fix-free-skb-v5-1-b5931e4cbdb0@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>