linux-kernel - Re: [PATCH 3/3] RFC: p9auth: add p9auth fs

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Message-ID: <20100428151015.GB4100@us.ibm.com>
Date:	Wed, 28 Apr 2010 10:10:15 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Ashwin Ganti <ashwin.ganti@...il.com>,
	David Howells <dhowells@...hat.com>, Greg KH <greg@...ah.com>,
	rsc@...ch.com, ericvh@...il.com,
	linux-security-module@...r.kernel.org,
	Ron Minnich <rminnich@...il.com>, jt.beard@...il.com,
	Andrew Morgan <morgan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Eric Paris <eparis@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Randy Dunlap <rdunlap@...otime.net>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Kyle Moffett <kyle@...fetthome.net>,
	Steve Grubb <sgrubb@...hat.com>
Subject: Re: [PATCH 3/3] RFC: p9auth: add p9auth fs

Quoting Oleg Nesterov (oleg@...hat.com):
> On 04/27, Serge E. Hallyn wrote:
> >
> > This introduces a Plan 9 style setuid capability filesystem.
> > See Documentation/p9auth.txt for a description of how to use this.
> 
> Can't comment these changes due to the lack of knowledge, just
> a couple of minor nits.

Thanks, Oleg.

> > +static ssize_t p9auth_use_write(struct file *file, const char __user *buffer,
> > +			       size_t count, loff_t *ppos)
> > +{
> > +	ssize_t retval = -ENOMEM;
> > +	char *user_buf;
> > +
> > +	if (mutex_lock_interruptible(&cap_mutex))
> > +		return -EINTR;
> 
> EINTR doesn't look exactly right here, especially if TIF_SIGPENDING is
> spurious. Probably ERESTARTNOINTR makes more sense. Or mutex_lock_killable().

Ashwin had had this as ERESTARTSYS I believe.  I'd read something about
userspace should only see -EINTR so I changed it.  Sounds like I need
to follow the caller chain some more and learn a thing or two, before
I repost.

> > +	user_buf = kzalloc(count+1, GFP_KERNEL);
> 
> Probably this is OK, but it looks a bit strange we do no check that
> count is not too large.

Yes, I should check that, thanks!

> > +	if (copy_from_user(user_buf, buffer, count)) {
> > +		retval = -EFAULT;
> > +		goto out;
> > +	}
> > +
> > +	retval = use_setuid_capability(user_buf);
> 
> It seems that use_setuid_capability() pathes assume that user_buf is
> null terminated? Say, parse_user_capability() does kstrdup(user_buf).

I kzalloc()d to count+1 before, and only copy_from_user() count bytes,
so the last byte should always be 0.

Thanks again,

-serge
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