linux-kernel - Re: Security issues with local filesystem caching

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Date:	Fri, 27 Oct 2006 12:10:10 -0400
From:	Stephen Smalley <sds@...ho.nsa.gov>
To:	David Howells <dhowells@...hat.com>
Cc:	aviro@...hat.com, linux-kernel@...r.kernel.org,
	selinux@...ho.nsa.gov, chrisw@...s-sol.org, jmorris@...ei.org
Subject: Re: Security issues with local filesystem caching

On Fri, 2006-10-27 at 16:42 +0100, David Howells wrote:
> Stephen Smalley <sds@...ho.nsa.gov> wrote:
> 
> > We might want more information passed into the hook, like the cache
> > directory itself,
> 
> I can do that.  I have the cache directory path and the cache tag name both
> available as strings.
> 
> > 	int security_cache_set_context(struct vfsmount *mnt, struct dentry *dentry, u32 secid)
> > 	{
> 
> Where are you envisioning this going?  In SELinux, in the LSM core or in
> cachefiles?

Sorry, that should have been named selinux_cache_set_context(), a
SELinux-specific implementation of a security_cache_set_context() LSM
hook.  The hook would be called after the cache directory pathname has
been looked up by your module, yielding a (mnt, dentry) pair and after
the security context string has been mapped to a secid.  

>   I was also wondering if I could generalise it to handle all cache
> types, but the permissions checks are probably going to be quite different for
> each type.  For instance, CacheFiles uses files on a mounted fs, whilst CacheFS
> uses a block device.

So in the latter case, the daemon supplies the path of a block device
node?  I suppose the hook could internally check the type of inode to
decide what checks to apply, using the checks I previously sketched when
it is a directory and using a different set of checks for the block
device (substituting a write check against the block device for the
directory-specific checks).  The hook interface itself would look the
same IIUC, i.e. providing the (mnt, dentry) pair to which the path
resolved and the secid to which the context resolved.

> Also, with your multiple cache example, how would I bring each cachefilesd
> daemon up in a different context so that it could handle a different cache with
> a different context?

runcon will run a program in a specified context, so if you defined
cachesfilesd_internal_t and aachesfilesd_external_t domains in policy,
you could then do:
	runcon -t cachefilesd_internal_t -- /path/to/cachefilesd args...

-- 
Stephen Smalley
National Security Agency

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