Re: [PATCH] staging: comedi: fix memory leak / bad pointer freeing for chanlist

[Ian Abbott]

On 20/10/14 22:38, Hartley Sweeten wrote:
> On Monday, October 20, 2014 7:11 AM, Ian Abbott wrote:
> > As a follow-up to commit 6cab7a37f5c04 ("staging: comedi: (regression)
> > channel list must be set for COMEDI_CMD ioctl"), Hartley Sweeten pointed
> > out another couple of bugs stemming from commit 6cab7a37f5c04 ("staging:
> > comedi: comedi_fops: introduce __comedi_get_user_chanlist()").
> >
> > Firstly, `do_cmdtest_ioctl()` never frees the kernel copy of the user
> > chanlist allocated by `__comedi_get_user_chanlist()`, so that memory is
> > leaked.  Fix it by freeing the allocated kernel memory pointed to by
> > `cmd.chanlist` before that pointer is overwritten with its original
> > pointer to user memory before `cmd` is copied back to user-space.
> >
> > Secondly, if `__comedi_get_user_chanlist()` returns an error,
> > `cmd->chanlist` is left unchanged and in fact will be a pointer to user
> > memory.  This causes `do_cmd_ioctl()` to `goto cleanup` and call
> > `do_become_nonbusy()` which would attempt to free the memory pointed to
> > by the user-space pointer.  Fix it by setting `cmd->chanlist` to NULL at
> > the start of `__comedi_get_user_chanlist()`.
> >
> > Fixes: c6cd0eefb27b ("staging: comedi: comedi_fops: introduce __comedi_get_user_chanlist()")
> > Reported-by: H Hartley Sweeten <hsweeten@xxxxxxxxxxxxxxxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx> # 3.15.y 3.16.y 3.17.y: 6cab7a37f5c04
> > Cc: <stable@xxxxxxxxxxxxxxx> # 3.15.y 3.16.y 3.17.y
> > ---
> > Greg, this patch applies to your "staging-linus" branch.
> > ---
> >   drivers/staging/comedi/comedi_fops.c | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/drivers/staging/comedi/comedi_fops.c b/drivers/staging/comedi/comedi_fops.c
> > index a9b7fe5..a1788e8 100644
> > --- a/drivers/staging/comedi/comedi_fops.c
> > +++ b/drivers/staging/comedi/comedi_fops.c
> > @@ -1462,6 +1462,7 @@ static int __comedi_get_user_chanlist(struct comedi_device *dev,
> >   	unsigned int *chanlist;
> >   	int ret;
> >
> > +	cmd->chanlist = NULL;
> >   	chanlist = memdup_user(user_chanlist,
> >   			       cmd->chanlist_len * sizeof(unsigned int));
> >   	if (IS_ERR(chanlist))
>
> I don't think this covers all the problems in your second issue above.
>
> It does fix the attempt to free the memory pointed to by the user-space pointer
> if the memdub_user() fails.
>
> But in do_cmd_ioctl() we still have an issue if the s->do_cmdtest() fails or
> the CMDF_BOGUS flag is set. There we restore the users cmd.chanlist
> pointer to allow copy_to_user() to return the cmd/ But the kernel allocated
> chanlist is not freed. The goto cleanup will again try to free the user-space
> pointer.

No, 'async->cmd' is copied (via struct assignment) to local variable 
'cmd' and it is the local 'cmd.chanlist' pointer that is set to the 
original user-space pointer and the local 'cmd' that is copied back to 
user-space.  'async->cmd.chanlist' still points to the kernel copy of 
the chanlist that gets cleaned up by 'goto cleanup' / 'do_become_nonbusy().

--
-=( Ian Abbott @ MEV Ltd.    E-mail: <abbotti@xxxxxxxxx> )=-
-=(                          Web: http://www.mev.co.uk/  )=-
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