From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	Dongli Zhang <dongli.zhang@oracle.com>,
	Willem de Bruijn <willemb@google.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Jakub Kicinski <kuba@kernel.org>,
	Sasha Levin <sashal@kernel.org>
Subject: [PATCH 6.12 175/220] net: tap: set skb->dev before parsing virtio net header in tap_get_user_xdp()
Date: Thu, 20 Aug 2026 16:56:05 +0200
Message-ID: <20260820145228.735943412@linuxfoundation.org>
In-Reply-To: <20260820145223.480031205@linuxfoundation.org>
References: <20260820145223.480031205@linuxfoundation.org>
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6.12-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Dongli Zhang <dongli.zhang@oracle.com>

[ Upstream commit 3874892dd27d5387aa9a06f58d9060f18f351d24 ]

The commit 4f61f133f354 ("net: tap: NULL pointer derefence in
dev_parse_header_protocol when skb->dev is null") fixed a crash in
tap_get_user() by assigning skb->dev before calling tun_vnet_hdr_to_skb().
This is required because virtio_net_hdr_to_skb() may invoke
dev_parse_header_protocol(), which dereferences skb->dev. Without the
assignment, a NULL pointer dereference can occur.

However, tap_get_user_xdp() still parses the virtio-net header before
assigning skb->dev. When the vhost TX path passes an XDP buffer containing
a GSO virtio-net header but the protocol is set to zero on purpose,
tun_vnet_hdr_to_skb() can reach dev_parse_header_protocol() while skb->dev
is still NULL, resulting in a crash.

Fix this by looking up the tap device and assigning skb->dev before calling
tun_vnet_hdr_to_skb(), matching the ordering already used in
tap_get_user(). Preserve the existing RCU read-side critical section across
dev_queue_xmit().

Fixes: 924a9bc362a5 ("net: check if protocol extracted by virtio_net_hdr_set_proto is correct")
Cc: stable@vger.kernel.org
Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://patch.msgid.link/20260802224612.264563-1-dongli.zhang@oracle.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/net/tap.c |   24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)

--- a/drivers/net/tap.c
+++ b/drivers/net/tap.c
@@ -1199,10 +1199,21 @@ static int tap_get_user_xdp(struct tap_q
 	skb_reset_mac_header(skb);
 	skb->protocol = eth_hdr(skb)->h_proto;
 
+	rcu_read_lock();
+	tap = rcu_dereference(q->tap);
+	if (!tap) {
+		kfree_skb(skb);
+		rcu_read_unlock();
+		return 0;
+	}
+	skb->dev = tap->dev;
+
 	if (vnet_hdr_len) {
 		err = virtio_net_hdr_to_skb(skb, gso, tap_is_little_endian(q));
-		if (err)
+		if (err) {
+			rcu_read_unlock();
 			goto err_kfree;
+		}
 	}
 
 	/* Move network header to the right position for VLAN tagged packets */
@@ -1210,15 +1221,8 @@ static int tap_get_user_xdp(struct tap_q
 	    vlan_get_protocol_and_depth(skb, skb->protocol, &depth) != 0)
 		skb_set_network_header(skb, depth);
 
-	rcu_read_lock();
-	tap = rcu_dereference(q->tap);
-	if (tap) {
-		skb->dev = tap->dev;
-		skb_probe_transport_header(skb);
-		dev_queue_xmit(skb);
-	} else {
-		kfree_skb(skb);
-	}
+	skb_probe_transport_header(skb);
+	dev_queue_xmit(skb);
 	rcu_read_unlock();
 
 	return 0;