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

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Date:	Thu, 22 Aug 2013 13:22:17 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Willy Tarreau <w@....eu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"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 1:15 PM, Willy Tarreau <w@....eu> wrote:
> On Thu, Aug 22, 2013 at 01:10:43PM -0700, Andy Lutomirski wrote:
>> 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's not only that, it also supports sockets and pipes that you can access
> via /proc/pid/fd and not via a real symlink which would try to open eg
> "pipe:[23456]" instead of the real file. So you can't get rid of it
> without breaking existing apps (starting with your shell for which
> /dev/stdin is a link to /proc/self/fd/0 for example).
>

Let me rephrase that: why do we allow these types of lookup to recurse
like normal symlinks?  I'm proposing that these links immediately
terminate lookup and return back to user_path_at_empty, which can, in
turn, do an extra check to see if the inode that was found is one of
these magic inodes (or checks to see if nd_jump_link was called).
user_path_at_empty could then enforce LOOKUP_EMPTY-list restrictions
(or the caller could).

--Andy
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