Re: Thoughts on credential switching

[Jeff Layton]

On Wed, 26 Mar 2014 17:23:24 -0700
Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:

> Hi various people who care about user-space NFS servers and/or
> security-relevant APIs.
> 
> I propose the following set of new syscalls:
> 
> int credfd_create(unsigned int flags): returns a new credfd that
> corresponds to current's creds.
> 
> int credfd_activate(int fd, unsigned int flags): Change current's
> creds to match the creds stored in fd.  To be clear, this changes both
> the "subjective" and "objective" (aka real_cred and cred) because
> there aren't any real semantics for what happens when userspace code
> runs with real_cred != cred.
> 
> Rules:
> 
>  - credfd_activate fails (-EINVAL) if fd is not a credfd.
>  - credfd_activate fails (-EPERM) if the fd's userns doesn't match
> current's userns.  credfd_activate is not intended to be a substitute
> for setns.
>  - credfd_activate will fail (-EPERM) if LSM does not allow the
> switch.  This probably needs to be a new selinux action --
> dyntransition is too restrictive.
> 
> 
> Optional:
>  - credfd_create always sets cloexec, because the alternative is
> silly.
>  - credfd_activate fails (-EINVAL) if dumpable.  This is because we
> don't want a privileged daemon to be ptraced while impersonating
> someone else.
>  - optional: both credfd_create and credfd_activate fail if
> !ns_capable(CAP_SYS_ADMIN) or perhaps !capable(CAP_SETUID).
> 
> The first question: does this solve Ganesha's problem?
> 
> The second question: is this safe?  I can see two major concerns.  The
> bigger concern is that having these syscalls available will allow
> users to exploit things that were previously secure.  For example,
> maybe some configuration assumes that a task running as uid==1 can't
> switch to uid==2, even with uid 2's consent.  Similar issues happen
> with capabilities.  If CAP_SYS_ADMIN is not required, then this is no
> longer really true.
> 
> Alternatively, something running as uid == 0 with heavy capability
> restrictions in a mount namespace (but not a uid namespace) could pass
> a credfd out of the namespace.  This could break things like Docker
> pretty badly.  CAP_SYS_ADMIN guards against this to some extent.  But
> I think that Docker is already totally screwed if a Docker root task
> can receive an O_DIRECTORY or O_PATH fd out of the container, so it's
> not entirely clear that the situation is any worse, even without
> requiring CAP_SYS_ADMIN.
> 
> The second concern is that it may be difficult to use this correctly.
> There's a reason that real_cred and cred exist, but it's not really
> well set up for being used.
> 
> As a simple way to stay safe, Ganesha could only use credfds that have
> real_uid == 0.
> 
> --Andy


I still don't quite grok why having this special credfd_create call
buys you anything over simply doing what Al had originally suggested --
switch creds using all of the different syscalls and then simply caching
that in a "normal" fd:

    fd = open("/dev/null", O_PATH...);

...it seems to me that the credfd_activate call will still need to do
the same permission checking that all of the individual set*id() calls
require (and all of the other stuff like changing selinux contexts,
etc).

IOW, this fd is just a "handle" for passing around a struct cred, but I
don't see why having access to that handle would allow you to do
something you couldn't already do anyway.

Am I missing something obvious here?

-- 
Jeff Layton <jlayton@xxxxxxxxxx>
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