linux-kernel - Re: [PATCH -v3 1/3] Capabilities: move cap_file

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Message-Id: <1248982958.11627.187.camel@moss-pluto.epoch.ncsc.mil>
Date:	Thu, 30 Jul 2009 15:42:38 -0400
From:	Stephen Smalley <sds@...ho.nsa.gov>
To:	Eric Paris <eparis@...hat.com>
Cc:	"Serge E. Hallyn" <serue@...ibm.com>, linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org, selinux@...ho.nsa.gov,
	jwcart2@...ho.nsa.gov, spender@...ecurity.net, dwalsh@...hat.com,
	cl@...ux-foundation.org, arjan@...radead.org,
	alan@...rguk.ukuu.org.uk, kees@...flux.net, csellers@...sys.com,
	penguin-kernel@...ove.sakura.ne.jp
Subject: Re: [PATCH -v3 1/3] Capabilities: move cap_file_mmap to commoncap.c

On Thu, 2009-07-30 at 13:50 -0400, Eric Paris wrote:
> On Thu, 2009-07-30 at 11:58 -0400, Stephen Smalley wrote:
> > On Thu, 2009-07-30 at 10:54 -0500, Serge E. Hallyn wrote:
> > > Quoting Eric Paris (eparis@...hat.com):
> > > > On Thu, 2009-07-30 at 00:14 -0500, Serge E. Hallyn wrote:
> > > > > Quoting Eric Paris (eparis@...hat.com):
> > > > > > Currently we duplicate the mmap_min_addr test in cap_file_mmap and in
> > > > > > security_file_mmap if !CONFIG_SECURITY.  This patch moves cap_file_mmap
> > > > > > into commoncap.c and then calls that function directly from
> > > > > > security_file_mmap ifndef CONFIG_SECURITY like all of the other capability
> > > > > > checks are done.
> > > > > 
> > > > > It also
> > > > > 
> > > > > 	1. changes the return value in error case from -EACCES to
> > > > > 	   -EPERM
> > > > > 	2. no onger sets PF_SUPERPRIV in t->flags if the capability
> > > > > 	   is used.
> > > > > 
> > > > > Do we care about these?
> > > > 
> > > > Personally, not really, but I'll gladly put them back if you care.   #2
> > > > seems more interesting to me than number 1.   I actually kinda like
> > > > getting EPERM from caps rather than EACCES since them I know if I was
> > > > denied by selinux or by caps.....
> > > > 
> > > > -Eric
> > > 
> > > Yup, I asked bc I didn't particularly care myself.
> > > 
> > > I think I agree with you about -EPERM being better anyway.  However I
> > > (now) think in this case PF_SUPERPRIV definately should be set, as this
> > > is a clear use of a capability to do something that couldn't have been
> > > done without it.
> > 
> > On a related but different note, we should consider all current uses of
> > cap_capable(), as they represent capability checks that will not be
> > subject to a further restrictive check by other security modules.  In
> > this case and in the vm_enough_memory case, that is intentional, but not
> > so clear for other uses in commoncap.c.
> 
> Most of commoncap.c is called either as a secondary hook from the active
> lsm (aka selinux calls the commoncap.c functions) or in the !
> CONFIG_SECURITY case.
> 
> I'll audit this afternoon to see which of them might not fit these
> rules....

That isn't what I meant.  Most of the commoncap functions call capable()
rather than directly calling cap_capable(), thereby causing:
- PF_SUPERPRIV to be set, and
- The primary security module (e.g. SELinux) to apply its own
restrictive check.

That is useful as it allows SELinux or AppArmor or TOMOYO to veto e.g.
CAP_SYS_PTRACE without replicating the same logic within its own hook.

The current exceptions are:
cap_inh_is_capped() called from cap_capset(),
cap_task_prctl() in the PR_SET_SECUREBITS case,
cap_vm_enough_memory(),
cap_file_mmap() after your patch.

The latter two are indeed cases where we made a conscious choice that
SELinux would not apply its capability check against policy.  But the
first two are unclear to me.

-- 
Stephen Smalley
National Security Agency

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