linux-kernel - Re: [PATCH 1/2] fs: add link restrictions

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Date:	Sun, 1 Jul 2012 17:43:22 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, Eric Paris <eparis@...hat.com>,
	Matthew Wilcox <matthew@....cx>,
	Doug Ledford <dledford@...hat.com>,
	Joe Korty <joe.korty@...r.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Ingo Molnar <mingo@...e.hu>,
	David Howells <dhowells@...hat.com>,
	James Morris <james.l.morris@...cle.com>,
	linux-doc@...r.kernel.org,
	Dan Rosenberg <drosenberg@...curity.com>,
	kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH 1/2] fs: add link restrictions

On Sat, Jun 30, 2012 at 2:14 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Mon, Jun 25, 2012 at 02:05:26PM -0700, Kees Cook wrote:
>
>> +config PROTECTED_LINKS
>> +     bool "Evaluate vulnerable link conditions"
>> +     default y
>
> Remember Linus' rants about 'default y' in general?

I added these configs due to other people's requests. I am happy to
remove them all and have the sysctls start their life == 1. It would
eliminate all the #ifdef logic too.

>> +     /* Check parent directory mode and owner. */
>
> I suspect that we ought to simply pass that parent directory as argument - caller
> *does* have a reference to it, so we don't need to mess with ->d_lock, etc.

I don't see where the parent is held in either path_openat nor
path_lookupat. What should I be passing into may_follow_link() for the
parent?

>> +                     err = may_follow_link(&link);
>> +                     if (unlikely(err))
>> +                             break;
>
> No.  This is definitely wrong - you are leaking dentries and vfsmount here.

What should I do to avoid the leak? I thought it was avoiding the need
to call put_link because it aborts before calling follow_link.

>> +             error = may_follow_link(&link);
>> +             if (unlikely(error))
>> +                     break;
>
> Ditto.

Same thing here -- it aborts before the follow_link. I must be
misunderstanding something. What am I missing?

Thanks for the feedback! I'll clean up the other things you mentioned as well.

-Kees

-- 
Kees Cook
Chrome OS Security
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