CVE-2025-21921

In the Linux kernel, the following vulnerability has been resolved:

net: ethtool: netlink: Allow NULL nlattrs when getting a phy_device

ethnl_req_get_phydev() is used to lookup a phy_device, in the case an ethtool netlink command targets a specific phydev within a netdevs topology.

It takes as a parameter a const struct nlattr *header thats used for error handling :

   if (!phydev) {
           NL_SET_ERR_MSG_ATTR(extack, header,
                               no phy matching phyindex);
           return ERR_PTR(-ENODEV);
   }

In the notify path after a ->set operation however, theres no request attributes available.

The typical callsite for the above function looks like:

phydev = ethnl_req_get_phydev(req_base, tb[ETHTOOL_A_XXX_HEADER],
			      info->extack);

So, when tb is NULL (such as in the ethnl notify path), we have a nice crash.

It turns out that theres only the PLCA command that is in that case, as the other phydev-specific commands dont have a notification.

This commit fixes the crash by passing the cmd index and the nlattr array separately, allowing NULL-checking it directly inside the helper.

Weakness

The product dereferences a pointer that it expects to be valid but is NULL.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/1f458fa42c29144cef280e05bc49fc21b873d897
• https://git.kernel.org/stable/c/637399bf7e77797811adf340090b561a8f9d1213
• https://git.kernel.org/stable/c/639c70352958735addbba5ae7dd65985da96e061