Subject: Re: [PATCH] net/packet: fix TOCTOU race on mmap'd vnet_hdr in tpacket_snd()

From: Willem de Bruijn <[email-masked]>
Date: Fri, 17 Apr 2026 04:15:52 -0400

Zero Mark wrote:
> In tpacket_snd(), when PACKET_VNET_HDR is enabled, vnet_hdr points
> directly into the mmap'd TX ring buffer shared with userspace. The
> kernel validates the header via __packet_snd_vnet_parse() but then
> re-reads all fields later in virtio_net_hdr_to_skb(). A concurrent
> userspace thread can modify the vnet_hdr fields (gso_type, gso_size,
> flags, csum_start, csum_offset) between validation and use, bypassing
> all safety checks.
> 
> This can lead to:
>  - Out-of-bounds checksum writes via crafted csum_start/csum_offset
>  - Malicious GSO segmentation parameters
>  - Kernel memory corruption and potential local privilege escalation
> 
> The non-TPACKET path (packet_snd()) already correctly copies vnet_hdr
> to a stack-local variable. All other vnet_hdr consumers in the kernel
> (tun.c, tap.c, virtio_net.c) also use stack copies. The TPACKET TX
> path is the only caller of virtio_net_hdr_to_skb() that reads directly
> from user-controlled shared memory.
> 
> Fix this by copying vnet_hdr from the mmap'd ring buffer to a
> stack-local variable before validation and use, consistent with the
> approach used in packet_snd() and all other callers.
> 
> Exploitation requires CAP_NET_RAW, which can be obtained without
> special privileges via user namespaces.
> 
> Confirmed with a PoC on Linux 6.8.0 (Ubuntu): kprobe tracing on
> skb_partial_csum_set captured 77 race wins in 500,000 iterations.

No need to add such details on exploitability of bugs.

> Affects all kernels since PACKET_VNET_HDR support was added to the
> TPACKET TX path (~v3.14).
> 
> Fixes: 9ed988e5 ("packet: add vnet_hdr support for tpacket_snd")

This patch does not exist. Also 12-char SHA1.

I think this should be

Fixes: 1d036d25e560 ("packet: tpacket_snd gso and checksum offload")

> Signed-off-by: Zero Mark <[email-masked]>

Thanks for the fix!

Only it does not apply cleanly. Please mark fixes [PATCH net] and
ensure that they apply to current netdev-net/main

https://www.kernel.org/doc/html/latest/process/maintainer-netdev.html

> ---
>  net/packet/af_packet.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index abcdef012345..fedcba654321 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -2725,7 +2725,8 @@ static int tpacket_parse_header(struct packet_sock *po, void *frame,
>  static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
>  {
>  	struct sk_buff *skb = NULL;
>  	struct net_device *dev;
> -	struct virtio_net_hdr *vnet_hdr = NULL;
> +	struct virtio_net_hdr vnet_hdr;
> +	bool has_vnet_hdr = false;
>  	struct sockcm_cookie sockc;
>  	__be16 proto;
>  	int err, reserve = 0;
> @@ -2828,16 +2829,17 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
>  		if (po->has_vnet_hdr) {
> -			vnet_hdr = data;
> -			data += sizeof(*vnet_hdr);
> -			tp_len -= sizeof(*vnet_hdr);
> +			memcpy(&vnet_hdr, data, sizeof(vnet_hdr));

Move the tp_len < 0 check before memcpy

> +			data += sizeof(vnet_hdr);
> +			tp_len -= sizeof(vnet_hdr);
>  			if (tp_len < 0 ||
> -			    __packet_snd_vnet_parse(vnet_hdr, tp_len)) {
> +			    __packet_snd_vnet_parse(&vnet_hdr, tp_len)) {
>  				tp_len = -EINVAL;
>  				goto tpacket_error;
>  			}
>  			copylen = __virtio16_to_cpu(vio_le(),
> -						    vnet_hdr->hdr_len);
> +						    vnet_hdr.hdr_len);
> +			has_vnet_hdr = true;
>  		}
>  		copylen = max_t(int, copylen, dev->hard_header_len);
>  		skb = sock_alloc_send_skb(&po->sk,
> @@ -2875,11 +2877,11 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
>  		}
> 
> -		if (po->has_vnet_hdr) {
> -			if (virtio_net_hdr_to_skb(skb, vnet_hdr, vio_le())) {
> +		if (has_vnet_hdr) {
> +			if (virtio_net_hdr_to_skb(skb, &vnet_hdr, vio_le())) {
>  				tp_len = -EINVAL;
>  				goto tpacket_error;
>  			}
> -			virtio_net_hdr_set_proto(skb, vnet_hdr);
> +			virtio_net_hdr_set_proto(skb, &vnet_hdr);
>  		}
> 
>  		skb->destructor = tpacket_destruct_skb;
> --
> 2.43.0
>