Re: [PATCH net] net: dsa: Fix skb ownership in taggers

[Kurt Kanzenbach <kurt@linutronix.de>]

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On Tue Jun 16 2026, Linus Walleij wrote:
> 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.
>
> 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>
> Assisted-by: Codex:gpt-5-5
> Signed-off-by: Linus Walleij <linusw@kernel.org>

Acked-by: Kurt Kanzenbach <kurt@linutronix.de> # hellcreek

> ---
>  net/dsa/tag_hellcreek.c     |  9 +++++++--

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