linux-kernel - Re: [PATCH 3/4] seccomp: Add SECCOMP_USER_NOTIF_FLAG

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Message-ID: <20200126041439.liwfmb4h74zmhi76@yavin.dot.cyphar.com>
Date:   Sun, 26 Jan 2020 15:14:39 +1100
From:   Aleksa Sarai <asarai@...e.de>
To:     Aleksa Sarai <cyphar@...har.com>
Cc:     Sargun Dhillon <sargun@...gun.me>, linux-fsdevel@...r.kernel.org,
        linux-api@...r.kernel.org, containers@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, christian.brauner@...ntu.com
Subject: Re: [PATCH 3/4] seccomp: Add SECCOMP_USER_NOTIF_FLAG_PIDFD to get
 pidfd on listener trap

On 2020-01-26, Aleksa Sarai <cyphar@...har.com> wrote:
> On 2020-01-24, Sargun Dhillon <sargun@...gun.me> wrote:
> >  static long seccomp_notify_recv(struct seccomp_filter *filter,
> >  				void __user *buf)
> >  {
> >  	struct seccomp_knotif *knotif = NULL, *cur;
> >  	struct seccomp_notif unotif;
> > +	struct task_struct *group_leader;
> > +	bool send_pidfd;
> >  	ssize_t ret;
> >  
> > +	if (copy_from_user(&unotif, buf, sizeof(unotif)))
> > +		return -EFAULT;
> >  	/* Verify that we're not given garbage to keep struct extensible. */
> > -	ret = check_zeroed_user(buf, sizeof(unotif));
> > -	if (ret < 0)
> > -		return ret;
> > -	if (!ret)
> > +	if (unotif.id ||
> > +	    unotif.pid ||
> > +	    memchr_inv(&unotif.data, 0, sizeof(unotif.data)) ||
> > +	    unotif.pidfd)
> > +		return -EINVAL;
> 
> IMHO this check is more confusing than the original check_zeroed_user().
> Something like the following is simpler and less prone to forgetting to
> add a new field in the future:
> 
> 	if (memchr_inv(&unotif, 0, sizeof(unotif)))
> 		return -EINVAL;

Also the check in the patch doesn't ensure that any unnamed padding is
zeroed -- memchr_inv(&unotif, 0, sizeof(unotif)) does.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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