[PATCH 7.0 37/42] net/packet: fix TOCTOU race on mmapd vnet_hdr in tpacket_snd()

Greg Kroah-Hartman
2026-04-24T13:33:10Z

From: Bingquan Chen <patzilla007@gmail.com>

commit 2c054e17d9d41f1020376806c7f750834ced4dc5 upstream.

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 between validation
and use, bypassing all safety checks.

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.

Fixes: 1d036d25e560 ("packet: tpacket_snd gso and checksum offload")
Signed-off-by: Bingquan Chen <patzilla007@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260418112006.78823-1-patzilla007@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/packet/af_packet.c | 21 +++++++++++++--------
1 file changed, 13 insertions(+), 8 deletions(-)

--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2717,7 +2717,8 @@ static int tpacket_snd(struct packet_soc
{
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;
@@ -2818,16 +2819,20 @@ static int tpacket_snd(struct packet_soc
hlen = LL_RESERVED_SPACE(dev);
tlen = dev->needed_tailroom;
if (vnet_hdr_sz) {
- vnet_hdr = data;
data += vnet_hdr_sz;
tp_len -= vnet_hdr_sz;
- if (tp_len < 0 ||
- __packet_snd_vnet_parse(vnet_hdr, tp_len)) {
+ if (tp_len < 0) {
+ tp_len = -EINVAL;
+ goto tpacket_error;
+ }
+ memcpy(&vnet_hdr, data - vnet_hdr_sz, sizeof(vnet_hdr));
+ if (__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,
@@ -2864,12 +2869,12 @@ tpacket_error:
}
}

- if (vnet_hdr_sz) {
- 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;