linux-kernel - Re: O_NOLINK for open()

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Message-Id: <E1IValy-0000fU-Sd@be1.lrz>
Date:	Thu, 13 Sep 2007 00:33:26 +0200
From:	Bodo Eggert <7eggert@....de>
To:	Brent Casavant <bcasavan@....com>, linux-kernel@...r.kernel.org
Subject: Re: O_NOLINK for open()

Brent Casavant <bcasavan@....com> wrote:

[...]
> I could mmap a temporary tmpfs file (tmpfs so that if there is a
> machine crash no sensitive data persists) which is created with
> permissions of 0, immediately unlink it, and pass the file
> descriptor through an AF_UNIX socket.  This does open up a very
> small window of vulnerability if another process is able to chmod
> the file and open it before the unlink.

If the process can chmod the file, it can ptrace the daemon, too.
Or, using CAP_DAC_OVERRIDE, it can patch the daemon.

Both will void any security.

> However, it occurs to me that this problem goes away if there were
> a method create a file in an unlinked state to begin with.  However
> there does not appear to be any such mechanism in Linux's open()
> interface.

Having no window for creating stale temp files is nice to have. We only
need a clever fool to implement it.-) But since it's hard to get killed
just in the right moment for having a stale temp file, there is very low
interest for this feature.
-- 
You know you're in trouble when packet floods are competing to flood you.
        -- grc.com

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