CVE-2026-74684 in Linux
Summary
by MITRE • 08/22/2026
In the Linux kernel, the following vulnerability has been resolved:
net: tap: set skb->dev before parsing virtio net header in tap_get_user_xdp()
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().
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Analysis
by VulDB Data Team • 08/22/2026
The Linux kernel networking subsystem contains a vulnerability within the TAP (Tunnel Access Point) driver that results in a NULL pointer dereference during packet processing. This flaw specifically affects the tap_get_user_xdp() function, which handles data transmission from user space to the network interface using eXpress Data Path buffers. The root cause lies in the incorrect ordering of operations when parsing virtio-net headers. Specifically, the code attempts to parse the virtual network header and invoke dev_parse_header_protocol before assigning a valid device pointer to the socket buffer structure. This oversight creates a critical execution path where the kernel dereferences a NULL pointer, leading to an immediate system crash or denial of service for any process relying on this interface.
The technical flaw originates from a previous fix that addressed a similar issue in tap_get_user() but failed to apply the same correction to its XDP counterpart. In the standard tap_get_user path, developers correctly assigned skb->dev before calling tun_vnet_hdr_to_skb(). However, in tap_get_user_xdp(), the assignment of the device pointer was omitted prior to header parsing. When a vhost TX path passes an XDP buffer containing a Generic Segmentation Offload virtio-net header with a protocol field set to zero, the kernel invokes dev_parse_header_protocol() while skb->dev remains NULL. Since this function expects a valid network device structure to determine link-layer headers, the absence of this pointer triggers a fatal exception in the kernel space.
From an operational perspective, this vulnerability allows for a local denial of service attack. An unprivileged user with access to the TAP device can craft specific packet sequences that exploit this race condition or logical error by ensuring the protocol field is zero while triggering GSO processing. The resulting NULL pointer dereference causes the kernel to panic, effectively crashing the host system or isolating network connectivity for all users on that machine. This impacts system stability and availability, particularly in virtualized environments where TAP interfaces are commonly used to connect guest operating systems to external networks.
This vulnerability is categorized under CWE-476, which denotes a NULL pointer dereference, as it involves accessing memory through an uninitialized or null reference. In the context of the MITRE ATT&CK framework, this aligns with techniques related to resource exhaustion and denial of service via kernel exploitation. The fix requires reordering the code in tap_get_user_xdp() to ensure that the TAP device is looked up and assigned to skb->dev before any parsing functions are called. This change mirrors the logic already present in tap_get_user(), ensuring consistency across both processing paths. Additionally, developers must preserve the existing RCU read-side critical section around dev_queue_xmit() to maintain proper synchronization and prevent race conditions during packet transmission.