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

[David Howells <dhowells@redhat.com>]

Serge E. Hallyn <serue@us.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.

> > 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.

David