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

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