[PATCH 4.9 66/72] usb: gadget: f_fs: Fix use-after-free in ffs_free_inst

From: Greg Kroah-Hartman
Date: Sun Nov 19 2017 - 09:52:13 EST

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


From: Andrew Gabbasov <andrew_gabbasov@xxxxxxxxxx>

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@xxxxxxxxxx>
Acked-by: Felipe Balbi <felipe.balbi@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

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
@@ -3698,6 +3698,7 @@ 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) &&