linux-kernel - Re: [PATCH v3 1/2] pidfd: add P

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Message-ID: <20190729175620.uwszsvidnb2cmul4@brauner.io>
Date:   Mon, 29 Jul 2019 19:56:21 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Kees Cook <keescook@...omium.org>
Cc:     linux-kernel@...r.kernel.org, oleg@...hat.com,
        torvalds@...ux-foundation.org, arnd@...db.de,
        ebiederm@...ssion.com, joel@...lfernandes.org, tglx@...utronix.de,
        tj@...nel.org, dhowells@...hat.com, jannh@...gle.com,
        luto@...nel.org, akpm@...ux-foundation.org, cyphar@...har.com,
        viro@...iv.linux.org.uk, kernel-team@...roid.com
Subject: Re: [PATCH v3 1/2] pidfd: add P_PIDFD to waitid()

On Mon, Jul 29, 2019 at 09:24:38AM -0700, Kees Cook wrote:
> On Sun, Jul 28, 2019 at 12:22:29AM +0200, Christian Brauner wrote:
> > diff --git a/kernel/exit.c b/kernel/exit.c
> > index a75b6a7f458a..64bb6893a37d 100644
> > --- a/kernel/exit.c
> > +++ b/kernel/exit.c
> > @@ -1552,6 +1552,23 @@ static long do_wait(struct wait_opts *wo)
> >  	return retval;
> >  }
> >  
> > +static struct pid *pidfd_get_pid(unsigned int fd)
> > +{
> > +	struct fd f;
> > +	struct pid *pid;
> > +
> > +	f = fdget(fd);
> > +	if (!f.file)
> > +		return ERR_PTR(-EBADF);
> > +
> > +	pid = pidfd_pid(f.file);
> > +	if (!IS_ERR(pid))
> > +		get_pid(pid);
> > +
> > +	fdput(f);
> > +	return pid;
> > +}
> > +
> >  static long kernel_waitid(int which, pid_t upid, struct waitid_info *infop,
> >  			  int options, struct rusage *ru)
> >  {
> > @@ -1574,19 +1591,29 @@ static long kernel_waitid(int which, pid_t upid, struct waitid_info *infop,
> >  		type = PIDTYPE_PID;
> >  		if (upid <= 0)
> >  			return -EINVAL;
> > +
> > +		pid = find_get_pid(upid);
> >  		break;
> >  	case P_PGID:
> >  		type = PIDTYPE_PGID;
> >  		if (upid <= 0)
> >  			return -EINVAL;
> > +
> > +		pid = find_get_pid(upid);
> > +		break;
> > +	case P_PIDFD:
> > +		type = PIDTYPE_PID;
> > +		if (upid < 0)
> > +			return -EINVAL;
> > +
> > +		pid = pidfd_get_pid(upid);
> > +		if (IS_ERR(pid))
> > +			return PTR_ERR(pid);
> 
> I spent some time convincing myself that this early bail out was
> correct. It seems this path is only reachable in the EBADF case, so that
> makes sense. The other failure modes in this switch all give a NULL pid
> so that the final do_wait() returns ECHILD. So, as long as that's
> intentional (which I think it is), this all looks fine. :)

Yep, it is. I didn't want to refactor that out of do_wait(). :)

Thanks!
Christian

> 
> Reviewed-by: Kees Cook <keescook@...omium.org>