Re: [PATCH] fork: fail on non-zero higher 32 bits of args.exit_signal

[Christian Brauner <christian.brauner@ubuntu.com>]

On Tue, Sep 10, 2019 at 04:39:44PM +0200, Oleg Nesterov wrote:
> On 09/10, Christian Brauner wrote:
> > On Tue, Sep 10, 2019 at 03:10:48PM +0200, Christian Brauner wrote:
> > > On Tue, Sep 10, 2019 at 03:09:35PM +0200, Christian Brauner wrote:
> > > > On Tue, Sep 10, 2019 at 02:44:41PM +0200, Oleg Nesterov wrote:
> > > > > On 09/10, Eugene Syromiatnikov wrote:
> > > > > >
> > > > > > --- a/kernel/fork.c
> > > > > > +++ b/kernel/fork.c
> > > > > > @@ -2562,6 +2562,9 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
> > > > > >  	if (copy_from_user(&args, uargs, size))
> > > > > >  		return -EFAULT;
> > > > > >  
> > > > > > +	if (unlikely(((unsigned int)args.exit_signal) != args.exit_signal))
> > > > > > +		return -EINVAL;
> > > > > 
> > > > > Hmm. Unless I am totally confused you found a serious bug...
> > > > > 
> > > > > Without CLONE_THREAD/CLONE_PARENT copy_process() blindly does
> > > > > 
> > > > > 	p->exit_signal = args->exit_signal;
> > > > > 
> > > > > the valid_signal(sig) check in do_notify_parent() mostly saves us, but we
> > > > > must not allow child->exit_signal < 0, if nothing else this breaks
> > > > > thread_group_leader().
> > > > > 
> > > > > And afaics this patch doesn't fix this? I think we need the valid_signal()
> > > > > check...
> > > > 
> > > > Thanks for sending this patch so quickly after our conversation
> > > > yesterday, Eugene!
> > > > We definitely want valid_signal() to verify the signal is ok.
> > 
> > So we could do your check in copy_clone_args_from_user(), and then we do
> > another valid_signal() check in clone3_args_valid()? We could do the
> > latter in copy_clone_args_from_user() too but it's nicer to do it along
> > the other checks in clone3_args_valid().
> 
> I am fine either way. Sure, we can add valid_signal() into clone3_args_valid(),
> but then I'd ask to simplify the "overflow" check above. Something like
> 
> 	if (args.exit_signal > UINT_MAX)
> 		return -EINVAL;
> 
> looks much more readable to me.
> 
> 
> Or we can simply do
> 
> 	if (args.exit_signal & ~((u64)CSIGNAL))
> 		return -EINVAL;
> 
> in copy_clone_args_from_user() and forget about all problems.

Both are fine with me. The latter might have the advantage that we catch
both legacy clone and clone3. I think Eugene prefers this as well.

Christian