Date: Fri, 11 Oct 2002 19:03:23 +1000 (EST) From: Bruce Evans <bde@zeta.org.au> To: David Schultz <dschultz@uclink.Berkeley.EDU> Cc: Peter Jeremy <peter.jeremy@alcatel.com.au>, The Anarcat <anarcat@anarcat.ath.cx>, FreeBSD Security Issues <FreeBSD-security@FreeBSD.ORG> Subject: Re: access() is a security hole? Message-ID: <20021011185423.B12227-100000@gamplex.bde.org> In-Reply-To: <20021010193137.GA13547@HAL9000.homeunix.com>
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On Thu, 10 Oct 2002, David Schultz wrote:
> Thus spake Peter Jeremy <peter.jeremy@alcatel.com.au>:
> > On 2002-Oct-08 17:23:35 -0400, The Anarcat <anarcat@anarcat.ath.cx> wrote:
> > >Also, this means that the stat() manpage should also contains a
> > >similar section about its non-fd incarnations.
> >
> > I disagree. access(2) is specifically designed to allow setuid/setgid
> > programs to validate access rights based on the real uid/gid - but is
> > virtually impossible to use safely for this task because of the
> > inherent race conditions.
>
> No, access(2) is designed to allow NON-setuid programs to easily
> do sanity checks without opening a file or device right away.
> There's still a race condition, but it isn't typically a security
> threat when all you're trying to do is prevent the user from
> shooting himself in the foot. To use access() in a setuid program
> is usually an error.
No, it was designed to be useful to setuid programs. Whether it
actually is useful is arguable. From the V7 manual:
"The user and group IDs with respect to which permission is checked
are the real UID and GID of the process, so that this call is useful
to set-UID programs".
Setuid programs should only use access() to check whether they will
have permission after they set[ug]id() to the real [ug]id. Non-setuid
programs mostly don't need such checks. They can just try the operation.
Bruce
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