staging: comedi: fix memory leak for saved channel list

When `do_cmd_ioctl()` allocates memory for the kernel copy of a channel
list, it frees any previously allocated channel list in
`async->cmd.chanlist` and replaces it with the new one.  However, if the
device is ever removed (or "detached") the cleanup code in
`cleanup_device()` in "drivers.c" does not free this memory so it is
lost.

A sensible place to free the kernel copy of the channel list is in
`do_become_nonbusy()` as at that point the comedi asynchronous command
associated with the channel list is no longer valid.  Free the channel
list in `do_become_nonbusy()` instead of `do_cmd_ioctl()` and clear the
pointer to prevent it being freed more than once.

Note that `cleanup_device()` could be called at an inappropriate time
while the comedi device is open, but that's a separate bug not related
to this this patch.

Cc: stable@vger.kernel.org
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/drivers/staging/comedi/comedi_fops.c b/drivers/staging/comedi/comedi_fops.c
index f496f4d96c83..c2a32cf95a82 100644
--- a/drivers/staging/comedi/comedi_fops.c
+++ b/drivers/staging/comedi/comedi_fops.c
@@ -1195,7 +1195,6 @@ static int do_cmd_ioctl(struct comedi_device *dev,
                 goto cleanup;
         }
 
-        kfree(async->cmd.chanlist);
         async->cmd = cmd;
         async->cmd.data = NULL;
         /* load channel/gain list */
@@ -2033,6 +2032,8 @@ static void do_become_nonbusy(struct comedi_device *dev,
         if (async) {
                 comedi_reset_async_buf(async);
                 async->inttrig = NULL;
+                kfree(async->cmd.chanlist);
+                async->cmd.chanlist = NULL;
         } else {
                 dev_err(dev->class_dev,
                         "BUG: (?) do_become_nonbusy called with async=NULL\n");