Re: [PATCH] sanitize task->comm to avoid leaking escape codes

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 29 Jun 2010 08:09:52 -0700
Kees Cook <kees.cook@canonical.com> wrote:

> On Tue, Jun 29, 2010 at 11:45:14AM +0300, Alexey Dobriyan wrote:
> > On Tue, Jun 29, 2010 at 12:03 AM, Kees Cook <kees.cook@canonical.com> wrote:
> > > On Mon, Jun 28, 2010 at 01:00:28PM -0700, Andrew Morton wrote:
> > >> Surely it would be better to fix the tools which display this info
> > >> rather than making the kernel tell fibs.
> > >
> > > The strncpy in get_task_comm() is totally wrong -- it's testing the length
> > > of task->comm.
> > 
> > It also fills not just any buffer but buffer which is TASK_COMM_LEN byte wide.
> > 
> > > Why should get_task_comm not take a destination buffer length argument?
> > 
> > If you pass too small, you needlessly truncate output.
> 
> If you pass too small a buffer, get_task_comm will happily write all over
> the caller's stack past the end of the buffer if the contents of task->comm
> are large enough:
> 
>         strncpy(buf, tsk->comm, sizeof(tsk->comm));
> 
> The "n" argument to get_task_comm's use of strncpy is totally wrong --
> it needs to be the size of the destination, not the size of the source.
> Luckily, everyone using get_task_comm currently uses buffers that are
> sizeof(task->comm).

It's not "totally wrong" at all.  get_task_comm() *requires* that it be
passed a buffer of at least TASK_COMM_LEN bytes.  sizeof(tsk->comm)
equals TASK_COMM_LEN and always will do so.  We could replace the
sizeof with TASK_COMM_LEN for cosmetic reasons but that's utter
nitpicking.  But then, the comment right there says "buf must be at
least sizeof(tsk->comm) in size".  That's so simple that even a kernel
developer could understand it?

Do we need a runtime check every time to make sure that some developer
didn't misunderstand such a simple thing?  Seems pretty pointless -
there are a zillion such runtime checks we could add.  It'd be better
to do

#define get_task_comm(buf, tsk) {			\
	BUILD_BUG_ON(sizeof(buf) < TASK_COMM_LEN);	\
	__get_task_comm(buf, tsk);			\
}

and save the runtime bloat.  But again, what was special about this
particular programmer error?  There are five or six instances of
strcpy(foo, current->comm).  Do we need runtime checks there as well??