[PATCH 4.14 12/31] usb: gadget: f_fs: Fix use-after-free in ffs_free_inst

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Andrew Gabbasov <andrew_gabbasov@mentor.com>

commit cdafb6d8b8da7fde266f79b3287ac221aa841879 upstream.

KASAN enabled configuration reports an error

BUG: KASAN: use-after-free in ffs_free_inst+... [usb_f_fs] at addr ...
Write of size 8 by task ...

This is observed after "ffs-test" is run and interrupted. If after that
functionfs is unmounted and g_ffs module is unloaded, that use-after-free
occurs during g_ffs module removal.

Although the report indicates ffs_free_inst() function, the actual
use-after-free condition occurs in _ffs_free_dev() function, which
is probably inlined into ffs_free_inst().

This happens due to keeping the ffs_data reference in device structure
during functionfs unmounting, while ffs_data itself is freed as no longer
needed. The fix is to clear that reference in ffs_closed() function,
which is a counterpart of ffs_ready(), where the reference is stored.

Fixes: 3262ad824307 ("usb: gadget: f_fs: Stop ffs_closed NULL pointer dereference")
Signed-off-by: Andrew Gabbasov <andrew_gabbasov@mentor.com>
Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/usb/gadget/function/f_fs.c |    1 +
 1 file changed, 1 insertion(+)

--- a/drivers/usb/gadget/function/f_fs.c
+++ b/drivers/usb/gadget/function/f_fs.c
@@ -3677,6 +3677,7 @@ static void ffs_closed(struct ffs_data *
 		goto done;
 
 	ffs_obj->desc_ready = false;
+	ffs_obj->ffs_data = NULL;
 
 	if (test_and_clear_bit(FFS_FL_CALL_CLOSED_CALLBACK, &ffs->flags) &&
 	    ffs_obj->ffs_closed_callback)