CVE-2026-52947: net: qrtr: fix refcount saturation and potential UAF in qrtr_port_remove

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

net: qrtr: fix refcount saturation and potential UAF in qrtrportremove

In qrtrportremove(), the socket reference count is decremented via sockput() before the port is removed from the qrtrports XArray and before the RCU grace period elapses.

This breaks the fundamental RCU update paradigm. It exposes a race window where a concurrent RCU reader (such as qrtrresetports() or qrtrportlookup()) can obtain a pointer to the socket from the XArray, and attempt to call sockhold() on a socket whose reference count has already dropped to zero.

This exact race condition was hit during syzkaller fuzzing, leading to the following refcount saturation warning and a potential Use-After-Free:

refcountt: saturated; leaking memory. WARNING: CPU: 3 PID: 1273 at lib/refcount.c:22 refcountwarnsaturate+0xae/0x1d0 Modules linked in: qrtr(+) bochs drmshmemhelper ... Call Trace: <TASK> qrtrresetports net/qrtr/afqrtr.c:768 [inline] [qrtr] qrtrbind.isra.0+0x48b/0x570 net/qrtr/afqrtr.c:805 [qrtr] qrtrbind+0x17d/0x210 net/qrtr/afqrtr.c:901 [qrtr] kernelbind+0xe4/0x120 net/socket.c:3592 qrtrnsinit+0x1a6/0x380 net/qrtr/ns.c:715 [qrtr] qrtrprotoinit+0x3b/0xff0 net/qrtr/afqrtr.c:169 [qrtr] dooneinitcall+0xf5/0x5e0 init/main.c:1283 ... </TASK>

Fix this by deferring the reference count decrement until after the xaerase() and the synchronizercu() complete.

(Note: The v1 of this patch incorrectly replaced sockput() with sockput(). As Simon Horman pointed out, the callers of qrtrportremove() still hold a reference to the socket, so freeing the socket memory here would lead to a subsequent UAF in the caller. Thus, the sockput() is kept, but only repositioned to close the RCU race.)