Re: [PATCH 2/2] sysctl: Restore the selinux path based label lookup for sysctls.

[Stephen Smalley <sds@tycho.nsa.gov>]

On Thu, 2007-02-08 at 10:53 -0700, Eric W. Biederman wrote:
> Stephen Smalley <sds@tycho.nsa.gov> writes:
> 
> >
> > Hmmm...turns out to not be quite enough, as the /proc/sys inodes aren't
> > truly private to the fs, so we can run into them in a variety of
> > security hooks beyond just the inode hooks, such as
> > security_file_permission (when reading and writing them via the vfs
> > helpers), security_sb_mount (when mounting other filesystems on
> > directories in proc like binfmt_misc), and deeper within the security
> > module itself (as in flush_unauthorized_files upon inheritance across
> > execve).  So I think we have to add an IS_PRIVATE() guard within
> > SELinux, as below.  Note however that the use of the private flag here
> > could be confusing, as these inodes are _not_ private to the fs, are
> > exposed to userspace, and security modules must implement the sysctl
> > hook to get any access control over them.
> 
> Agreed, the naming is confusing, and using private here doesn't quite
> feel right.
> 
> A practical question is: Will we ever encounter these inodes
> in the inode_init() path from superblock_init?

Possibly, during setup upon initial policy load (initiated by /sbin/init
these days) from selinux_complete_init, as early userspace may have
already been accessing them.

>   If all of the accesses
> that we care about go through inode_doinit_with_dentry we can just
> walk the dcache to get the names, and that should work for the normal
> proc case as well.

Walking the proc_dir_entry tree (or the ctl_table tree) is preferable as
it is a stable, user-immutable representation.  Also avoids taking the
dcache lock.

> A somewhat related question: How do you handle security labels for
> sysfs?  No fine grained security yet.

Right, they are all mapped to a single label presently.  I was thinking
of handling that from userspace after introducing a setxattr handler for
sysfs and a way to preserve the SID on the entry (likely caching it in
the sysfs_dirent and propagating that to the inode when the inode is
populated from the sysfs_dirent).  Then early userspace could walk sysfs
and apply finer-grained labeling from a configuration.

> If it doesn't look easy to solve this another way I will certainly
> go with marking the inodes private.

-- 
Stephen Smalley
National Security Agency