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

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Date:	Wed, 07 Feb 2007 16:12:18 -0500
From:	Stephen Smalley <sds@...ho.nsa.gov>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Andrew Morton <akpm@...l.org>, Ingo Molnar <mingo@...e.hu>,
	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	selinux@...ho.nsa.gov, jmorris@...ei.org
Subject: Re: [PATCH 2/2] sysctl: Restore the selinux path based label
	lookup for sysctls.

On Wed, 2007-02-07 at 13:24 -0500, Stephen Smalley wrote:
> On Tue, 2007-02-06 at 14:21 -0700, Eric W. Biederman wrote:
> > This time instead of generating the generating the paths from proc_dir_entries
> > generate the labels from the names in the sysctl ctl_tables themselves.  This
> > removes an unnecessary layer of indirection, allows this to work even when
> > procfs support is not compiled into the kernel, and especially allows it
> > to work now that ctl_tables no longer have a proc_dir_entry field.
> 
> Thanks, looks sane.
> 
> > I continue passing "proc" into genfs sid although that is complete nonsense
> > to allow existing selinux policies to work without modification.
> > 
> > Signed-off-by: Eric W. Biederman <ebiederm@...ssion.com>
> 
> Acked-by:  Stephen Smalley <sds@...ho.nsa.gov>

Hmmm...but in testing the patch, I don't seem to (consistently) reach
these checks when accessing via /proc/sys.  I see that you are caching
the mode information and using it in some cases rather than calling the
sysctl_perm function.

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.
  
> 
> > ---
> >  security/selinux/hooks.c |   43 +++++++++++++++++++++++++++++++++++++++++--
> >  1 files changed, 41 insertions(+), 2 deletions(-)
> > 
> > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > index 3a36057..c17a8dd 100644
> > --- a/security/selinux/hooks.c
> > +++ b/security/selinux/hooks.c
> > @@ -1424,6 +1424,41 @@ static int selinux_capable(struct task_struct *tsk, int cap)
> >  	return task_has_capability(tsk,cap);
> >  }
> >  
> > +static int selinux_sysctl_get_sid(ctl_table *table, u16 tclass, u32 *sid)
> > +{
> > +	int buflen, rc;
> > +	char *buffer, *path, *end;
> > +
> > +	rc = -ENOMEM;
> > +	buffer = (char*)__get_free_page(GFP_KERNEL);
> > +	if (!buffer)
> > +		goto out;
> > +
> > +	buflen = PAGE_SIZE;
> > +	end = buffer+buflen;
> > +	*--end = '\0';
> > +	buflen--;
> > +	path = end-1;
> > +	*path = '/';
> > +	while (table) {
> > +		const char *name = table->procname;
> > +		size_t namelen = strlen(name);
> > +		buflen -= namelen + 1;
> > +		if (buflen < 0)
> > +			goto out_free;
> > +		end -= namelen;
> > +		memcpy(end, name, namelen);
> > +		*--end = '/';
> > +		path = end;
> > +		table = table->parent;
> > +	}
> > +	rc = security_genfs_sid("proc", path, tclass, sid);
> > +out_free:
> > +	free_page((unsigned long)buffer);
> > +out:
> > +	return rc;
> > +}
> > +
> >  static int selinux_sysctl(ctl_table *table, int op)
> >  {
> >  	int error = 0;
> > @@ -1438,8 +1473,12 @@ static int selinux_sysctl(ctl_table *table, int op)
> >  
> >  	tsec = current->security;
> >  
> > -	/* Use the well-defined sysctl SID. */
> > -	tsid = SECINITSID_SYSCTL;
> > +	rc = selinux_sysctl_get_sid(table, (op == 0001) ?
> > +				    SECCLASS_DIR : SECCLASS_FILE, &tsid);
> > +	if (rc) {
> > +		/* Default to the well-defined sysctl SID. */
> > +		tsid = SECINITSID_SYSCTL;
> > +	}
> >  
> >  	/* The op values are "defined" in sysctl.c, thereby creating
> >  	 * a bad coupling between this module and sysctl.c */
-- 
Stephen Smalley
National Security Agency

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