CVE-2025-38443: nbd: fix uaf in nbd_genl_connect() error path

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

nbd: fix uaf in nbdgenlconnect() error path

There is a use-after-free issue in nbd:

block nbd6: Receive control failed (result -104) block nbd6: shutting down sockets ================================================================== BUG: KASAN: slab-use-after-free in recvwork+0x694/0xa80 drivers/block/nbd.c:1022 Write of size 4 at addr ffff8880295de478 by task kworker/u33:0/67

CPU: 2 UID: 0 PID: 67 Comm: kworker/u33:0 Not tainted 6.15.0-rc5-syzkaller-00123-g2c89c1b655c0 #0 PREEMPT(full) Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014 Workqueue: nbd6-recv recvwork Call Trace: <TASK> dumpstack lib/dumpstack.c:94 [inline] dumpstacklvl+0x116/0x1f0 lib/dumpstack.c:120 printaddressdescription mm/kasan/report.c:408 [inline] printreport+0xc3/0x670 mm/kasan/report.c:521 kasanreport+0xe0/0x110 mm/kasan/report.c:634 checkregioninline mm/kasan/generic.c:183 [inline] kasancheckrange+0xef/0x1a0 mm/kasan/generic.c:189 instrumentatomicreadwrite include/linux/instrumented.h:96 [inline] atomicdec include/linux/atomic/atomic-instrumented.h:592 [inline] recvwork+0x694/0xa80 drivers/block/nbd.c:1022 processonework+0x9cc/0x1b70 kernel/workqueue.c:3238 processscheduledworks kernel/workqueue.c:3319 [inline] workerthread+0x6c8/0xf10 kernel/workqueue.c:3400 kthread+0x3c2/0x780 kernel/kthread.c:464 retfromfork+0x45/0x80 arch/x86/kernel/process.c:153 retfromforkasm+0x1a/0x30 arch/x86/entry/entry64.S:245 </TASK>

nbdgenlconnect() does not properly stop the device on certain error paths after nbdstartdevice() has been called. This causes the error path to put nbd->config while recvwork continue to use the config after putting it, leading to use-after-free in recvwork.

This patch moves nbdstartdevice() after the backend file creation.