USB: gadget: f_midi: fixing a possible double-free in f_midi

[Mark Salyzyn <salyzyn@android.com>]

From: "Yavuz, Tuba" <tuba@ece.ufl.edu>

cherry pick from commit 7fafcfdf6377b18b2a726ea554d6e593ba44349f
("USB: gadget: f_midi: fixing a possible double-free in f_midi")
Removing 'return err;' from conflict.

It looks like there is a possibility of a double-free vulnerability on an
error path of the f_midi_set_alt function in the f_midi driver. If the
path is feasible then free_ep_req gets called twice:

         req->complete = f_midi_complete;
         err = usb_ep_queue(midi->out_ep, req, GFP_ATOMIC);
            => ...
             usb_gadget_giveback_request
               =>
                 f_midi_complete (CALLBACK)
                   (inside f_midi_complete, for various cases of status)
                   free_ep_req(ep, req); // first kfree
         if (err) {
                 ERROR(midi, "%s: couldn't enqueue request: %d\n",
                             midi->out_ep->name, err);
                 free_ep_req(midi->out_ep, req); // second kfree
                 return err;
         }

The double-free possibility was introduced with commit ad0d1a058eac
("usb: gadget: f_midi: fix leak on failed to enqueue out requests").

Found by MOXCAFE tool.

Signed-off-by: Tuba Yavuz <tuba@ece.ufl.edu>
Fixes: ad0d1a058eac ("usb: gadget: f_midi: fix leak on failed to enqueue out requests")
Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Cc: stable <stable@vger.kernel.org> # 4.4.y
---
 drivers/usb/gadget/function/f_midi.c | 3 ++-
 drivers/usb/gadget/u_f.h             | 2 ++
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/gadget/function/f_midi.c b/drivers/usb/gadget/function/f_midi.c
index 5ead414586a1..e5c4a907e5d4 100644
--- a/drivers/usb/gadget/function/f_midi.c
+++ b/drivers/usb/gadget/function/f_midi.c
@@ -366,7 +366,8 @@ static int f_midi_set_alt(struct usb_function *f, unsigned intf, unsigned alt)
 		if (err) {
 			ERROR(midi, "%s queue req: %d\n",
 				    midi->out_ep->name, err);
-			free_ep_req(midi->out_ep, req);
+			if (req->buf != NULL)
+				free_ep_req(midi->out_ep, req);
 		}
 	}
 
diff --git a/drivers/usb/gadget/u_f.h b/drivers/usb/gadget/u_f.h
index 69a1d10df04f..3ee365fbc2e2 100644
--- a/drivers/usb/gadget/u_f.h
+++ b/drivers/usb/gadget/u_f.h
@@ -65,7 +65,9 @@ struct usb_request *alloc_ep_req(struct usb_ep *ep, size_t len, int default_len)
 /* Frees a usb_request previously allocated by alloc_ep_req() */
 static inline void free_ep_req(struct usb_ep *ep, struct usb_request *req)
 {
+	WARN_ON(req->buf == NULL);
 	kfree(req->buf);
+	req->buf = NULL;
 	usb_ep_free_request(ep, req);
 }
 
-- 
2.23.0.rc1.153.gdeed80330f-goog