Re: [PATCH] umh: fix UAF when the process is being killed

[Luis Chamberlain]

On Mon, Dec 05, 2022 at 07:38:21PM +0800, Schspa Shi wrote:
> 
> Schspa Shi <schspa@xxxxxxxxx> writes:
> 
> > When the process is killed, wait_for_completion_state will return with
> > -ERESTARTSYS, and the completion variable in the stack will be freed.

There is no free'ing here, it's just not availabel anymore, which is
different.

> > If the user-mode thread is complete at the same time, there will be a UAF.
> >
> > Please refer to the following scenarios.
> >             T1                                  T2
> > ------------------------------------------------------------------
> > call_usermodehelper_exec
> >                                    call_usermodehelper_exec_async
> >                                    << do something >>
> >                                    umh_complete(sub_info);
> >                                    comp = xchg(&sub_info->complete, NULL);
> >                                    /* we got the completion */
> >                                    << context switch >>
> >
> >     << Being killed >>
> > 	retval = wait_for_completion_state(sub_info->complete, state);
> > 	if (!retval)
> > 		goto wait_done;
> >
> > 	if (wait & UMH_KILLABLE) {
> > 		/* umh_complete() will see NULL and free sub_info */
> > 		if (xchg(&sub_info->complete, NULL))
> > 			goto unlock;
> >         << we can't got the completion >>

I'm sorry I don't understand what you tried to say here, we can't got?

> > 	}
> > 	....
> > unlock:
> > 	helper_unlock();
> > 	return retval;
> > }
> >
> > /**
> >  * the completion variable in stack is end of life cycle.
> >  * and maybe freed due to process is recycled.
> >  */
> >                                    --------UAF here----------
> >                                    if (comp)
> >                                        complete(comp);
> >
> > To fix it, we can put the completion variable in the subprocess_info
> > variable.
> >
> > Reported-by: syzbot+10d19d528d9755d9af22@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Reported-by: syzbot+70d5d5d83d03db2c813d@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Reported-by: syzbot+83cb0411d0fcf0a30fc1@xxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > Signed-off-by: Schspa Shi <schspa@xxxxxxxxx>
> > ---
> >  include/linux/umh.h | 1 +
> >  kernel/umh.c        | 6 +++---
> >  2 files changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/umh.h b/include/linux/umh.h
> > index 5d1f6129b847..801f7efbc825 100644
> > --- a/include/linux/umh.h
> > +++ b/include/linux/umh.h
> > @@ -20,6 +20,7 @@ struct file;
> >  struct subprocess_info {
> >  	struct work_struct work;
> >  	struct completion *complete;
> > +	struct completion done;
> >  	const char *path;
> >  	char **argv;
> >  	char **envp;
> > diff --git a/kernel/umh.c b/kernel/umh.c
> > index 850631518665..3ed39956c777 100644
> > --- a/kernel/umh.c
> > +++ b/kernel/umh.c
> > @@ -380,6 +380,7 @@ struct subprocess_info *call_usermodehelper_setup(const char *path, char **argv,
> >  	sub_info->cleanup = cleanup;
> >  	sub_info->init = init;
> >  	sub_info->data = data;
> > +	init_completion(&sub_info->done);
> >    out:
> >  	return sub_info;
> >  }
> > @@ -405,7 +406,6 @@ EXPORT_SYMBOL(call_usermodehelper_setup);
> >  int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
> >  {
> >  	unsigned int state = TASK_UNINTERRUPTIBLE;
> > -	DECLARE_COMPLETION_ONSTACK(done);
> >  	int retval = 0;
> >  
> >  	if (!sub_info->path) {
> > @@ -431,7 +431,7 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
> >  	 * This makes it possible to use umh_complete to free
> >  	 * the data structure in case of UMH_NO_WAIT.
> >  	 */
> > -	sub_info->complete = (wait == UMH_NO_WAIT) ? NULL : &done;
> > +	sub_info->complete = (wait == UMH_NO_WAIT) ? NULL : &sub_info->done;
> >  	sub_info->wait = wait;
> >  
> >  	queue_work(system_unbound_wq, &sub_info->work);
> > @@ -444,7 +444,7 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
> >  	if (wait & UMH_FREEZABLE)
> >  		state |= TASK_FREEZABLE;
> >  
> > -	retval = wait_for_completion_state(&done, state);
> > +	retval = wait_for_completion_state(sub_info->complete, state);
> >  	if (!retval)
> >  		goto wait_done;
> 
> Hi Luis Chamberlain:
> 
> Could you help to review this patch? I'm not sure why we define the
> amount of completion here on the stack. But this UAF can be fixed by
> moving the completion variable to the heap.

It would seem to me that if this is an issue other areas would have
similar races as well, so I was hard pressed about the approach / fix.

Wouldn't something like this be a bit more explicit about ensuring
we don't let the work item race beyond?

diff --git a/kernel/umh.c b/kernel/umh.c
index 850631518665..55fc698115a7 100644
--- a/kernel/umh.c
+++ b/kernel/umh.c
@@ -447,6 +447,8 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
 	retval = wait_for_completion_state(&done, state);
 	if (!retval)
 		goto wait_done;
+	else if (retval == -ERESTARTSYS)
+		cancel_work_sync(&sub_info->work);
 
 	if (wait & UMH_KILLABLE) {
 		/* umh_complete() will see NULL and free sub_info */