char: xillybus: fix msg_ep UAF in xillyusb_probe()

When endpoint_alloc() return failed in xillyusb_setup_base_eps(),
'xdev->msg_ep' will be freed but not set to NULL. That lets program
enter fail handling to cleanup_dev() in xillyusb_probe(). Check for
'xdev->msg_ep' is invalid in cleanup_dev() because 'xdev->msg_ep' did
not set to NULL when was freed. So the UAF problem for 'xdev->msg_ep'
is triggered.

==================================================================
BUG: KASAN: use-after-free in fifo_mem_release+0x1f4/0x210
CPU: 0 PID: 166 Comm: kworker/0:2 Not tainted 5.15.0-rc5+ #19
Call Trace:
 dump_stack_lvl+0xe2/0x152
 print_address_description.constprop.0+0x21/0x140
 ? fifo_mem_release+0x1f4/0x210
 kasan_report.cold+0x7f/0x11b
 ? xillyusb_probe+0x530/0x700
 ? fifo_mem_release+0x1f4/0x210
 fifo_mem_release+0x1f4/0x210
 ? __sanitizer_cov_trace_pc+0x1d/0x50
 endpoint_dealloc+0x35/0x2b0
 cleanup_dev+0x90/0x120
 xillyusb_probe+0x59a/0x700
...

Freed by task 166:
 kasan_save_stack+0x1b/0x40
 kasan_set_track+0x1c/0x30
 kasan_set_free_info+0x20/0x30
 __kasan_slab_free+0x109/0x140
 kfree+0x117/0x4c0
 xillyusb_probe+0x606/0x700

Set 'xdev->msg_ep' to NULL after being freed in xillyusb_setup_base_eps()
to fix the UAF problem.

Fixes: a53d120 ("char: xillybus: Add driver for XillyUSB (Xillybus variant for USB)")
Cc: stable <[email protected]>
Acked-by: Eli Billauer <[email protected]>
Signed-off-by: Ziyang Xuan <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

drivers/char/xillybus/xillyusb.c

@@ -1912,6 +1912,7 @@ static int xillyusb_setup_base_eps(struct xillyusb_dev *xdev)
 
 dealloc:
 	endpoint_dealloc(xdev->msg_ep); /* Also frees FIFO mem if allocated */
+	xdev->msg_ep = NULL;
 	return -ENOMEM;
 }