CVE-2026-99211: netlink: reset skb headers before unicast and multicast filtering

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

netlink: reset skb headers before unicast and multicast filtering

Socket filters can use SKFNETOFF and SKFLLOFF to read relative to an skb's network and MAC headers. Netlink producers may reserve headroom without resetting those headers, allowing a receiving socket's filter to read data before the initialized message.

For example, IWPM HELLO replies are allocated with devallocskb(), which reserves NETSKBPAD bytes. An unprivileged NETLINKRDMA user can attach a classic socket filter and disclose this headroom through the length of the delivered reply. The resets in netlinkdump() do not help because IWPM sends a separately allocated skb through netlinkunicast().

Reset both headers before running the receiver's filter in unicast and broadcast delivery, as is already done for dump skbs. This also covers other netlink producers that reserve headroom without changing their allocation or message-building code.