[PATCH] net/9p/usbg: fix descriptor cleanup use-after-free

[Chengfeng Ye <nicoyip.dev@gmail.com>]

usb9pfs_free_func() frees the f_usb9pfs object before calling
usb_free_all_descriptors().  The usb_function passed to the latter is
embedded in the freed object, so removing the function from a gadget
dereferences freed memory.

The failing teardown sequence is:

  configfs unlink
    usb9pfs_free_func()
      kfree(usb9pfs)
      usb_free_all_descriptors(f)
        dereference the embedded usb_function

KASAN reported:

  BUG: KASAN: slab-use-after-free in usb_free_all_descriptors+0x138/0x190
  Read of size 8 at addr ffff888106a73088 by task poc/95
  Call Trace:
   usb_free_all_descriptors+0x138/0x190
   config_usb_cfg_unlink+0x1f0/0x2f0
   configfs_unlink+0x321/0x6f0

Free the descriptors before freeing their containing object.

Fixes: a3be076dc174 ("net/9p/usbg: Add new usb gadget function transport")
Cc: stable@vger.kernel.org
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
---
 net/9p/trans_usbg.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/net/9p/trans_usbg.c b/net/9p/trans_usbg.c
index 419cda13a7b5..8c2f0d8592c8 100644
--- a/net/9p/trans_usbg.c
+++ b/net/9p/trans_usbg.c
@@ -725,8 +725,6 @@ static void usb9pfs_free_func(struct usb_function *f)
 	struct f_usb9pfs *usb9pfs = func_to_usb9pfs(f);
 	struct f_usb9pfs_opts *opts;
 
-	kfree(usb9pfs);
-
 	opts = container_of(f->fi, struct f_usb9pfs_opts, func_inst);
 
 	mutex_lock(&opts->lock);
@@ -734,6 +732,7 @@ static void usb9pfs_free_func(struct usb_function *f)
 	mutex_unlock(&opts->lock);
 
 	usb_free_all_descriptors(f);
+	kfree(usb9pfs);
 }
 
 static int usb9pfs_set_alt(struct usb_function *f,