Re: [PATCH] staging: comedi: (regression) channel list must be set for COMEDI_CMD ioctl

From: Ian Abbott
Date: Thu Oct 09 2014 - 05:03:22 EST

On 09/10/14 00:13, Hartley Sweeten wrote:
On Wednesday, October 08, 2014 8:09 AM, Ian Abbott wrote:
`do_cmd_ioctl()`, the handler for the `COMEDI_CMD` ioctl can incorrectly
call the Comedi subdevice's `do_cmd()` handler with a NULL channel list
pointer. This is a regression as the `do_cmd()` handler has never been
expected to deal with that, leading to a kernel OOPS when it tries to
dereference it.

A NULL channel list pointer is allowed for the `COMEDI_CMDTEST` ioctl,
handled by `do_cmdtest_ioctl()` and the subdevice's `do_cmdtest()`
handler, but not for the `COMEDI_CMD` ioctl and its handlers.

Both `do_cmd_ioctl()` and `do_cmdtest_ioctl()` call
`__comedi_get_user_chanlist()` to copy the channel list from user memory
into dynamically allocated kernel memory and check it for consistency.
That function currently returns 0 if the `user_chanlist` parameter
(pointing to the channel list in user memory) is NULL. That's fine for
`do_cmdtest_ioctl()`, but `do_cmd_ioctl()` incorrectly assumes the
kernel copy of the channel list has been set-up correctly.

Fix it by not allowing the `user_chanlist` parameter to be NULL in
`__comedi_get_user_chanlist()`, and only calling it from
`do_cmdtest_ioctl()` if the parameter is non-NULL.

Thanks to Bernd Porr for reporting the bug via an initial patch sent

Fixes: c6cd0eefb27b ("staging: comedi: comedi_fops: introduce __comedi_get_user_chanlist()")
Reported-by: Bernd Porr <mail@xxxxxxxxxxxxxxx>
Signed-off-by: Ian Abbott <abbotti@xxxxxxxxx>
Cc: Bernd Porr <mail@xxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # 3.15.y 3.16.y 3.17.y
drivers/staging/comedi/comedi_fops.c | 15 +++++++--------
1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/drivers/staging/comedi/comedi_fops.c b/drivers/staging/comedi/comedi_fops.c
index 495969f..a9b7fe5 100644
--- a/drivers/staging/comedi/comedi_fops.c
+++ b/drivers/staging/comedi/comedi_fops.c
@@ -1462,10 +1462,6 @@ static int __comedi_get_user_chanlist(struct comedi_device *dev,
unsigned int *chanlist;
int ret;

- /* user_chanlist could be NULL for do_cmdtest ioctls */
- if (!user_chanlist)
- return 0;

I think you need a check here to avoid a NULL pointer getting
passed to the memdup_user().

If (!user_chanlist || cmd->chanlist_len == 0)
return -EINVAL;

chanlist = memdup_user(user_chanlist,
cmd->chanlist_len * sizeof(unsigned int));
if (IS_ERR(chanlist))

It's fine to pass a NULL pointer to memdup_user(). It won't succeed (returning ERR_PTR(-EFAULT)), but it's fine.

-=( Ian Abbott @ MEV Ltd. E-mail: <abbotti@xxxxxxxxx> )=-
-=( Web: )=-
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