usb: gadget: f_fs: Prevent race during ffs_ep0_queue_wait

While performing fast composition switch, there is a possibility that the
process of ffs_ep0_write/ffs_ep0_read get into a race condition
due to ep0req being freed up from functionfs_unbind.

Consider the scenario that the ffs_ep0_write calls the ffs_ep0_queue_wait
by taking a lock &ffs->ev.waitq.lock. However, the functionfs_unbind isn't
bounded so it can go ahead and mark the ep0req to NULL, and since there
is no NULL check in ffs_ep0_queue_wait we will end up in use-after-free.

Fix this by making a serialized execution between the two functions using
a mutex_lock(ffs->mutex).

Fixes: ddf8abd25994 ("USB: f_fs: the FunctionFS driver")
Signed-off-by: Udipto Goswami <quic_ugoswami@quicinc.com>
Tested-by: Krishna Kurapati <quic_kriskura@quicinc.com>
Link: https://lore.kernel.org/r/20221215052906.8993-2-quic_ugoswami@quicinc.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
index 73dc10a..bb6fe1f 100644 (file)
--- a/drivers/usb/gadget/function/f_fs.c
+++ b/drivers/usb/gadget/function/f_fs.c
@@ -279,6 +279,9 @@ static int __ffs_ep0_queue_wait(struct ffs_data *ffs, char *data, size_t len)
        struct usb_request *req = ffs->ep0req;
        int ret;
 
+       if (!req)
+               return -EINVAL;
+
        req->zero     = len < le16_to_cpu(ffs->ev.setup.wLength);
 
        spin_unlock_irq(&ffs->ev.waitq.lock);
@@ -1892,10 +1895,12 @@ static void functionfs_unbind(struct ffs_data *ffs)
        ENTER();
 
        if (!WARN_ON(!ffs->gadget)) {
+               mutex_lock(&ffs->mutex);
                usb_ep_free_request(ffs->gadget->ep0, ffs->ep0req);
                ffs->ep0req = NULL;
                ffs->gadget = NULL;
                clear_bit(FFS_FL_BOUND, &ffs->flags);
+               mutex_unlock(&ffs->mutex);
                ffs_data_put(ffs);
        }
 }