linux-kernel - Re: [PATCH 2/4] security: filesystem capabilities bugfix2

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Message-ID: <20080630194909.GE30669@us.ibm.com>
Date:	Mon, 30 Jun 2008 14:49:09 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	David Howells <dhowells@...hat.com>
Cc:	"Serge E. Hallyn" <serue@...ibm.com>,
	"Andrew G. Morgan" <morgan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Security Modules List 
	<linux-security-module@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] security: filesystem capabilities bugfix2

Quoting David Howells (dhowells@...hat.com):
> Serge E. Hallyn <serue@...ibm.com> wrote:
> 
> > > With Andrew's patch, capabilities are downgraded regardless of whether we
> > > have CAP_SETPCAP or not.  I guess that means that if you're tracing a
> > > binary whose filecaps say that it wants CAP_SETPCAP, then it retains
> > > CAP_SETPCAP.
> > 
> > I don't understand where that last sentence comes from.  Why would it
> > retain CAP_SETPCAP?
> 
> It seems I missed a bit out.  It should've read:
> 
> 	I guess that means that if you're tracing a binary that has
> 	CAP_SETPCAP already, and whose filecaps say that it wants CAP_SETPCAP,
> 	then it retains CAP_SETPCAP.
> 
> If the debugger has CAP_SYS_PTRACE, then it can attach to a binary that has
> CAP_SETPCAP according to cap_ptrace(), even if the debugger doesn't.

Ah.  Yes.  I think that's the desirable behavior in all proposals.

> > > I wonder if the tracing task should be examined here, and any capability the
> > > tracer isn't permitted should be denied the process doing the exec.
> > 
> > That sounds reasonable on its own, but it opens up a dangerous ability
> > for the partially-privileged tracer to manipulate the capability set for
> > the traced task.
> 
> Does it, though?  It would only reduce the capabilities of the inferior
> process; it wouldn't allow the inferior process or the debugger to get
> additional capabilities, apart from what's available under CAP_SETPCAP.

And the uids won't change unless capable(CAP_SETUID)...  so I think
you're right, it does sound safe.

thanks,
-serge
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