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

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Message-ID: <20190910142707.GQ4960@asgard.redhat.com>
Date:   Tue, 10 Sep 2019 15:27:07 +0100
From:   Eugene Syromiatnikov <esyr@...hat.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
        Christian Brauner <christian@...uner.io>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        Eric Biederman <ebiederm@...ssion.com>
Subject: Re: [PATCH] fork: fail on non-zero higher 32 bits of args.exit_signal

On Tue, Sep 10, 2019 at 03:27:02PM +0200, 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().

There's also a discrepancy between CSIGNAL (0xff) and _NSIG, used
in valid_signal (which is between 32 and 128, depending on architecture),
it seems it doesn't break thread_group_leader, but definitely allows
passing some invalid signal numbers via legacy clone-like syscalls—I'm
not sure if that's important.

> Christian