linux-kernel - Re: [PATCH v2] vfs: Tighten up linkat(..., AT_EMPTY

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Date:	Thu, 22 Aug 2013 13:10:43 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Willy Tarreau <w@....eu>,
	"security@...nel.org" <security@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Brad Spengler <spender@...ecurity.net>
Subject: Re: [PATCH v2] vfs: Tighten up linkat(..., AT_EMPTY_PATH)

On Thu, Aug 22, 2013 at 12:23 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Aug 22, 2013 at 12:05 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>
>> Does this work for the procfs case?  As far as I understand it (which
>> isn't saying much), it goes through the symlink-following path.
>
> Right. The /proc case is still separate, and we really should do
> something about that too. But again, I don't think I_LINKABLE is the
> thing to use there either. We probably should tighten up the magic
> /proc follow-link a lot.
>
>> What if we added another field to struct nameidata that's indicates
>> what restrictions need to be enforced when following magical symlinks
>> and then enforcing them when nd_jump_link gets used.  (There are only
>> two of these, both in procfs.)
>
> Yes, I think that might be just the kind of thing to do. Except some
> tightening could well be quite regardless of any extra flags.
>

What's the point of nd_jump_link anyway?  The only way I can think of
for a magic symlink in /proc to point to another symlink is to open a
symlink with O_PATH | O_NOFOLLOW.  Actually trying to use the
resulting link in /proc results in -ELOOP.  (Even just trying to open
a normal symlink with O_NOFOLLOW and without O_PATH results in
-ELOOP.)

It might be a lot simpler to rig up nd_jump_link to immediately
terminate lookup and let the callers (or the outer level of lookup)
deal with it.  That way the checks could be (more) easily unified with
AT_EMPTY_PATH.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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