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Date: Mon, 8 Nov 2010 12:37:35 +0100
From: Sebastian Krahmer <>
Subject: Re: filesystem capabilities


To me it looks like an error condition
where you should die() if you see +s root AND fscaps applied.
From my tests, you cant get rid of the suid of 0,
whether you drop setuid(getuid()) after cap_set or before.
It seems like in that case the kernel handles you like
a suid binary with +s to nobody. You cant
get rid of the nobody-suid if called setuid(getuid()).

Even though all HOWTOs/READMEs tell you that root is
nothing special if using caps and dropping all caps
so it "is like nobody" this aint true. Having most of
the important files like /etc owned by uid 0 is like
implicit having CAP_DAC_OVERRIDE and thats basically
full root with full caps.

And to make things complete we not only have to deal with
libcap (which I personally prefer even if its "more complex to use"),
we also have cap-ng (
where a first look at the provided ping etc patches shows me
that again the return values are not checked for
capng_apply(). (My fault if it does and I overlooked it)
And the whole cap/priv framework must *always and always*
check return values as shown a dozens of times that
this gives cool root exploits.

And I dont want to mention the semantics change between
the kernel versions (2.6.25 etc).


On Mon, Nov 08, 2010 at 11:28:13AM +0100, Ludwig Nussel wrote:
> Solar Designer wrote:
> > There's a lot of talk lately regarding replacing the SUID bit on program
> > binaries in Linux distros with filesystem capabilities.  Specifically,
> > Fedora and Ubuntu are heading in that direction.
> There are requests for openSUSE too.
> > - Some currently-SUID programs are aware of them being (potentially)
> > SUID, and will drop the "more privileged" euid when it is no longer
> > needed, but they will probably not be aware of them possessing
> > capabilities.  This may result in larger parts of the programs
> > (sometimes orders of magnitude larger) running with elevated privileges
> > (or with allowed-to-be-elevated privileges, which is a privilege on its
> > own and is usable through vulnerabilities that allow for arbitrary code
> > execution).  Let's consider ping, which appears to be the classical
> > example of "where filesystem capabilities will help" (or so it is
> > claimed).  IIRC, it starts by acquiring a raw socket (NB: of a certain
> > somewhat-limited type), then drops root privs (if it was installed SUID
> > root and run by non-root), then proceeds to parse the command-line,
> > resolve the provided hostname, and so on.  If the SUID bit is replaced
> > with cap_net_raw+ep, as seen in Kees' example above, will ping know to
> > drop this capability?  Hardly.  Not without a source code patch.
> Exactly. A community submission of an fscaps enabled iputils package
> brought the issue to our attention. We were astonished that the
> prime example ping doesn't drop it's capabilities. So it actually
> shows why blindly applying fscaps doesn't help security at all.
> So we are going to treat fscaps just like setuid bits and require
> code review by the security team.
> > You also absolutely have to deal with passwd, which would be another
> > SUID root program.  Like we did:
> >
> > And with all others (e.g., our crontab/at and crond changes). :-)
> The next step would be to get rid of those setgid programs too then.
> A daemon controlled via unix domain sockets could do the job just as
> well I suppose.
> > Thanks for reading this far, and I'd appreciate any comments and/or
> > corrections.  Some of the info above might be outdated - e.g., I am not
> > sure of what current kernels require (or not) to drop capabilities.
> AFAICT there are no special capabilities needed.
> > (If they no longer require anything extra to drop CAP_SETUID, then
> > that's a security problem on its own - the "sendmail risk" is back.)
> Thanks for the reminder! I actually didn't know. Here's a link
> explaining sendmail's problem:
> Indeed I ran into the trap when trying to patch ping to drop
> capabilities. If a program is capability aware but still installed
> setuid it needs to setuid(getuid()) before dropping it's
> capabilities. Doing it the wrong way around keeps the POSIX saved
> uid and setuid(0) is still possible even though getuid() ==
> geteuid() and no capabilities granted.
> cu
> Ludwig
> -- 
>  (o_   Ludwig Nussel
>  //\   
>  V_/_
> SUSE LINUX Products GmbH, GF: Markus Rex, HRB 16746 (AG Nuernberg)

~ perl
~ $_='print"\$_=\47$_\47;eval"';eval
~ - SuSE Security Team
~ SUSE LINUX Products GmbH, GF: Markus Rex, HRB 16746 (AG Nuernberg)

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