Subject: Re: [PATCH ipsec v2] xfrm: iptfs: fix skb_put() panic on non-linear skb during reassembly

From: Antony Antony <[email-masked]>
Date: Mon, 9 Mar 2026 18:37:04 +0100

On Wed, Mar 04, 2026 at 03:09:35PM +0100, Fernando Fernandez Mancera wrote:
> In iptfs_reassem_cont(), IP-TFS attempts to append data to the new inner
> packet 'newskb' that is being reassembled. First a zero-copy approach is
> tried if it succeeds then newskb becomes non-linear.
> 
> When a subsequent fragment in the same datagram does not meet the
> fast-path conditions, a memory copy is performed. It calls skb_put() to
> append the data and as newskb is non-linear it triggers
> SKB_LINEAR_ASSERT check.

I wonder if linearizing has a performance penalty? Did you test that?

I tried to reproduce it but had no luck so far. It seems the NICs I tested,
CX4 and virtio, veth, deliver the inner packet as a linear skb.

Any tricks to force Mellanox CX4 and virtio to deliver
non-linear skbs?

-antony

> 
>  Oops: invalid opcode: 0000 [#1] SMP NOPTI
>  [...]
>  RIP: 0010:skb_put+0x3c/0x40
>  [...]
>  Call Trace:
>   <IRQ>
>   iptfs_reassem_cont+0x1ab/0x5e0 [xfrm_iptfs]
>   iptfs_input_ordered+0x2af/0x380 [xfrm_iptfs]
>   iptfs_input+0x122/0x3e0 [xfrm_iptfs]
>   xfrm_input+0x91e/0x1a50
>   xfrm4_esp_rcv+0x3a/0x110
>   ip_protocol_deliver_rcu+0x1d7/0x1f0
>   ip_local_deliver_finish+0xbe/0x1e0
>   __netif_receive_skb_core.constprop.0+0xb56/0x1120
>   __netif_receive_skb_list_core+0x133/0x2b0
>   netif_receive_skb_list_internal+0x1ff/0x3f0
>   napi_complete_done+0x81/0x220
>   virtnet_poll+0x9d6/0x116e [virtio_net]
>   __napi_poll.constprop.0+0x2b/0x270
>   net_rx_action+0x162/0x360
>   handle_softirqs+0xdc/0x510
>   __irq_exit_rcu+0xe7/0x110
>   irq_exit_rcu+0xe/0x20
>   common_interrupt+0x85/0xa0
>   </IRQ>
>   <TASK>
> 
> Fix this by checking if the skb is non-linear. If it is, linearize it by
> calling skb_linearize(). As the initial allocation of newskb originally
> reserved enough tailroom for the entire reassembled packet we do not
> need to check if we have enough tailroom or extend it.
> 
> Fixes: 5f2b6a909574 ("xfrm: iptfs: add skb-fragment sharing code")
> Reported-by: Hao Long <[email-masked]>
> Closes: https://lore.kernel.org/netdev/[email-masked]/
> Signed-off-by: Fernando Fernandez Mancera <[email-masked]>
> ---
> v2: adjusted fixes tag and dropped skb_is_nonlinear() as skb_linearize()
> is enough.
> ---
>  net/xfrm/xfrm_iptfs.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/net/xfrm/xfrm_iptfs.c b/net/xfrm/xfrm_iptfs.c
> index 050a82101ca5..c96bec84bfa6 100644
> --- a/net/xfrm/xfrm_iptfs.c
> +++ b/net/xfrm/xfrm_iptfs.c
> @@ -901,6 +901,12 @@ static u32 iptfs_reassem_cont(struct xfrm_iptfs_data *xtfs, u64 seq,
>  	    iptfs_skb_can_add_frags(newskb, fragwalk, data, copylen)) {
>  		iptfs_skb_add_frags(newskb, fragwalk, data, copylen);
>  	} else {
> +		if (skb_linearize(newskb)) {
> +			XFRM_INC_STATS(xs_net(xtfs->x),
> +				       LINUX_MIB_XFRMINBUFFERERROR);
> +			goto abandon;
> +		}
> +
>  		/* copy fragment data into newskb */
>  		if (skb_copy_seq_read(st, data, skb_put(newskb, copylen),
>  				      copylen)) {
> -- 
> 2.53.0
> 
>