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

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

On Wed, 2007-02-07 at 18:57 -0700, Eric W. Biederman wrote:
> Stephen Smalley <sds@tycho.nsa.gov> writes:
> 
> >
> > One related but separate issue is that the /proc/sys inode labeling is
> > also affected by the sysctl patch series.  Those inodes used to be
> > labeled by selinux_proc_get_sid (from selinux_d_instantiate), but that
> > no longer works, so they now fall back to the superblock SID (generic
> > proc label).  That changes the inode permission checks on an attempt to
> > access a /proc/sys node and will likely cause denials under current
> > policy for confined domains since one wouldn't generally be writing to
> > the generic proc label. If you always called sysctl_perm from the proc
> > sysctl code, we could possibly dispense with inode permission checking
> > on those inodes, e.g. marking them private.
> 
> Like this?
> 
> It seems a little weird but I'm happy with it if you are.
> 
> Eric
> 
> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> index b9d59c0..7d6f7c7 100644
> --- a/fs/proc/proc_sysctl.c
> +++ b/fs/proc/proc_sysctl.c
> @@ -47,6 +47,7 @@ static struct inode *proc_sys_make_inode(struct inode *dir, struct ctl_table *ta
>  	inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME;
>  	inode->i_op = &proc_sys_inode_operations;
>  	inode->i_fop = &proc_sys_file_operations;
> +	inode->i_flags |= S_PRIVATE; /* tell selinux to ignore this inode */
>  	proc_sys_refresh_inode(inode, table);
>  out:
>  	return inode;

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.

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 65fb5e8..21bf2f0 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -1078,6 +1077,9 @@ static int inode_has_perm(struct task_st
 	struct inode_security_struct *isec;
 	struct avc_audit_data ad;
 
+	if (unlikely (IS_PRIVATE (inode)))
+		return 0;
+
 	tsec = tsk->security;
 	isec = inode->i_security;
 


-- 
Stephen Smalley
National Security Agency