net/packet: fix TOCTOU race on mmap'd vnet_hdr in tpacket_snd()

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>

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 4b043241fd56643e20a8a0903a3fe072f1010a7e..8e6f3a734ba0bde85321e0b3e113731ac6c0d17c 100644 (file)
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2718,7 +2718,8 @@ 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;
@@ -2819,16 +2820,20 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
                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,
@@ -2865,12 +2870,12 @@ 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;