Re: Thoughts on credential switching

[Theodore Ts'o <tytso@mit.edu>]

On Thu, Mar 27, 2014 at 07:08:02AM -0700, Jeff Layton wrote:
> I had some time to think about this last night...
> 
> While using a fd to pass around credentials is convenient, the danger
> is that it's pretty opaque. You have a fd that you know has creds
> attached to it, but it's hard to be certain what is going to change.

I don't think that's a particularly tough problem.  In general, the fd
isn't something that you would want to pass around, and so the process
which generated it will know exactly what it contained.

> Perhaps we can use the flags field for that. So, assuming we have a fd
> with the creds attached, we could do something like:
> 
>     err = switch_creds(fd, SC_FSUID|SC_FSGID|SC_GROUPS);
> 
> ...then the switch_creds syscall could be set up to fail if the new
> credentials had other fields that didn't match those in the current
> task credentials. So if (for instance) the cred->euid were
> different between the two, the above could fail with -EINVAL or
> something.

Huh?  The whole *point* is that the creds value will be different, of
course they won't match!  I would think this would be over
complicating the interface.


A couple of other things.  What I would suggest is that we create a
few new fd flags, to join FD_CLOEXEC:

FD_NOPROCFS	disallow being able to open the inode via /proc/<pid>/fd
		(but in the case of a creds fd, for bonus points, the
		target of the pseudo-symlink could be something like:
		"uid: 15806 gid: 100: grps: 27, 50" to aid in debugging
		a userspace file server).  This also answers Jeff's concern
		if for some reason --- I don't know how --- a process
		doesn't know what the contents of the creds fd that
		it created itself.

FD_NOPASSFD	disallow being able to pass the fd via a unix domain socket

FD_LOCKFLAGS	if this bit is set, disallow any further changes of FD_CLOEXEC,
		FD_NOPROCFS, FD_NOPASSFD, and FD_LOCKFLAGS flags.

Some of the functionality requested by the folks suggesting the "SEAL"
API would also be covered by these fd flags.

In order to solve some potential race concerns, a credsfd must be
created with FD_CLOEXEC and FD_NOPROCFS enabled.

Why is this important even if the anon_inode is owned by root with a
mode of 0?  Because if the system is set up to use SELinux or full
Posix capabilities, merely having the a uid of 0 is not special, and
we don't want to allow a process with uid of 0 to be able modify the
mode with the /proc/<pid>/fd/<FD> and then proceed to open the inode
using open.  This way, instead of adding special case code to prevent
this from happening, we can add a more general facility which can be
used to solve a few other problems.

Regards,

						- Ted