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

[Bingquan Chen <patzilla007@gmail.com>]

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>
---
 net/packet/af_packet.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 4b043241fd56..8e6f3a734ba0 100644
--- 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 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
 			}
 		}
 
-		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;
-- 
2.53.0