CVE-2026-63978: net/handshake: Drain pending requests at net namespace exit

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

net/handshake: Drain pending requests at net namespace exit

The arguments to listspliceinit() in handshakenetexit() are reversed. The call moves the local empty "requests" list onto hn->hnrequests, leaving the local list empty, so the subsequent drain loop runs zero iterations. Pending handshake requests that had not yet been accepted are not torn down when the net namespace is destroyed; each one keeps a reference on a socket file and on the handshakereq allocation.

Pass the source and destination in the documented order (listspliceinit(list, head) moves list onto head) so the pending list is transferred to the local scratch list and drained through handshakecomplete().

Fixing the splice direction exposes a list-corruption race. After the splice each req->hrlist still has non-empty link pointers, threading the stack-local scratch list rather than hnrequests. A concurrent handshakereqcancel() -- for example, from sunrpc's TLS timeout on a kernel socket whose netns reference was not taken -- finds the request through the rhashtable, calls removepending(), and sees !listempty(&req->hrlist). removependinglocked() then listdelinit()s an entry off the scratch list while the drain iterates, corrupting it. The same call arriving after the drain loop has run listdel() on an entry hits LISTPOISON instead.

Have removepending() check HANDSHAKEFNETDRAINING under hnlock and report not-found when drain is in progress. The drain has already taken ownership; handshakecomplete()'s existing testandset on HANDSHAKEFREQCOMPLETED still arbitrates between drain and cancel for who calls the consumer's hpdone. Use listdelinit() rather than listdel() in the drain so req->hrlist does not carry LISTPOISON after drain releases the entry.

The DRAINING guard in removepending() makes cancel return false, but cancel still falls through to testandsetbit on HANDSHAKEFREQCOMPLETED and drops the request's hrfile reference. Without another pin, if that is the last reference, skdestruct frees the request while it is still linked on the drain loop's local list. Pin each request's hrfile under hnlock before releasing the list, and drop that drain pin after the loop finishes with the request.