Re: [PATCH v2 1/4] cn_proc.h: use TASK_COMM_LEN instread of 16 in struct proc_event

[Kees Cook]

On Thu, Oct 07, 2021 at 07:51:35AM -0700, Kees Cook wrote:
> On Thu, Oct 07, 2021 at 12:07:49PM +0000, Yafang Shao wrote:
> > struct comm_proc_event was introduced in commit
> > f786ecba4158 ("connector: add comm change event report to proc connector").
> > It seems that there is no strong reason we must define the comm as a
> > hardcode 16 bytes. So we can use TASK_COMM_LEN instead.
> > 
> > Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> > Cc: Vladimir Zapolskiy <vzapolskiy@xxxxxxxxx>
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> > Cc: Petr Mladek <pmladek@xxxxxxxx>
> > ---
> >  include/uapi/linux/cn_proc.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/include/uapi/linux/cn_proc.h b/include/uapi/linux/cn_proc.h
> > index db210625cee8..351d02786350 100644
> > --- a/include/uapi/linux/cn_proc.h
> > +++ b/include/uapi/linux/cn_proc.h
> > @@ -110,7 +110,7 @@ struct proc_event {
> >  		struct comm_proc_event {
> >  			__kernel_pid_t process_pid;
> >  			__kernel_pid_t process_tgid;
> > -			char           comm[16];
> > +			char           comm[TASK_COMM_LEN];
> >  		} comm;
> 
> Hrmm. This is UAPI -- we can't change it without potentially breaking
> things (i.e. userspace binaries have this size built in, so we can't
> just change the size). This will either need to stay truncated, or may
> need a new interface with a variable-sized structure...

Specifically, this is needed for this series:


diff --git a/drivers/connector/cn_proc.c b/drivers/connector/cn_proc.c
index 646ad385e490..34bcba25c488 100644
--- a/drivers/connector/cn_proc.c
+++ b/drivers/connector/cn_proc.c
@@ -230,7 +230,9 @@ void proc_comm_connector(struct task_struct *task)
 	ev->what = PROC_EVENT_COMM;
 	ev->event_data.comm.process_pid  = task->pid;
 	ev->event_data.comm.process_tgid = task->tgid;
-	get_task_comm(ev->event_data.comm.comm, task);
+	/* This may get truncated. */
+	__get_task_comm(ev->event_data.comm.comm,
+			sizeof(ev->event_data.comm.comm), task);
 
 	memcpy(&msg->id, &cn_proc_event_id, sizeof(msg->id));
 	msg->ack = 0; /* not used */

-- 
Kees Cook