Re: [PATCH net 1/2] openvswitch: only skb_tx_error() a packet we are about to drop

[Ilya Maximets <i.maximets@ovn.org>]

On 8/13/26 7:47 AM, Norbert Szetei wrote:
> queue_userspace_packet() borrows the packet skb -- it only copies it into
> a private netlink message (user_skb) and does not own it; on return
> do_execute_actions() keeps forwarding it through the flow's remaining
> actions. Its error path nevertheless calls skb_tx_error(skb), which via
> skb_zcopy_clear() does skb_shinfo(skb)->flags &= ~SKBFL_ALL_ZEROCOPY,
> stripping SKBFL_SHARED_FRAG from that live skb (skb_tx_error()'s kerneldoc
> says "skb must be freed afterwards").
> 
> For a MSG_ZEROCOPY skb carrying page-cache frags, SKBFL_SHARED_FRAG is
> what makes esp_input() skb_cow_data() before in-place AEAD; once it is
> stripped a later local ESP-in-UDP delivery decrypts in place over pages
> the sender does not own -- an unprivileged page-cache write (the
> "Fragnesia" primitive).
> do_execute_actions() ignores output_userspace()'s return value, so any
> action after a failed USERSPACE upcall inherits the stripped skb.
> 
> Move the skb_tx_error() to the flow-miss drop path - the "default"
> branch of ovs_dp_process_packet()'s switch(error), before kfree_skb().
> 
> The call has been here since commit 36d5fe6a0007 ("core, nfqueue,
> openvswitch: Orphan frags in skb_zerocopy and handle errors") but was
> harmless until esp_input() began relying on SKBFL_SHARED_FRAG to gate
> in-place decrypt; only then did stripping it on a still-forwarded skb
> become a page-cache write primitive.
> 
> Fixes: 36d5fe6a0007 ("core, nfqueue, openvswitch: Orphan frags in skb_zerocopy and handle errors")
> Fixes: f4c50a4034e6 ("xfrm: esp: avoid in-place decrypt on shared skb frags")
> Cc: stable@vger.kernel.org
> Assisted-by: Claude:claude-opus-5
> Signed-off-by: Norbert Szetei <norbert@doyensec.com>
> ---
Reviewed-by: Ilya Maximets <i.maximets@ovn.org>