Re: [RFC] The reflink(2) system call v4.

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

On Tue, 2009-05-12 at 14:04 -0400, Stephen Smalley wrote:
> On Tue, 2009-05-12 at 11:03 -0700, Joel Becker wrote:
> > On Tue, May 12, 2009 at 01:32:47PM -0400, Stephen Smalley wrote:
> > > On Tue, 2009-05-12 at 10:22 -0700, Joel Becker wrote:
> > > > On Tue, May 12, 2009 at 08:18:34AM -0400, Stephen Smalley wrote:
> > > > > Is preserve_security supposed to also control the preservation of the
> > > > > SELinux security attribute (security.selinux extended attribute)?  I'd
> > > > > expect that either we preserve all the security-relevant attributes or
> > > > > none of them.  And if that is the case, then SELinux has to know about
> > > > > preserve_security in order to know what the security context of the new
> > > > > inode will be.  
> > > > 
> > > > 	Thank you Stephen, you read my mind.  In the ocfs2 case, we're 
> > > > expecting to just reflink the extended attribute structures verbatim in
> > > > the preserve_security case.
> > > 
> > > And in the preserve_security==0 case, you'll be calling
> > > security_inode_init_security() in order to get the attribute name/value
> > > pair to assign to the new inode just as in the normal file creation
> > > case?
> > 
> > 	Oh, absolutely.
> > 	As an aside, do inodes ever have more than one security.*
> > attribute?  It would appear that security_inode_init_security() just
> > returns one attribute, but what if I had a system running under SMACK
> > and then changed to SELinux?  Would my (existing) inode then have
> > security.smack and security.selinux attributes?
> 
> No, there would be no security.selinux attribute and the file would be
> treated as having a well-defined 'unlabeled' attribute by SELinux.  Not
> something you have to worry about.
> 
> > > > > Also, if you are going to automatically degrade reflink(2) behavior
> > > > > based on the owner_or_cap test, then you ought to allow the same to be
> > > > > true if the security module vetoes the attempt to preserve attributes.
> > > > > Either DAC or MAC logic may say that security attributes cannot be
> > > > > preserved.  Your current logic will only allow graceful degradation in
> > > > > the DAC case, but the MAC case will remain a hard failure.
> > > > 
> > > > 	I did not think of this, and its a very good point as well.  I'm
> > > > not sure how to have the return value of security_inode_reflink()
> > > > distinguish between "disallow the reflink" and "disallow
> > > > preserve_security".  But since !preserve_security requires read access
> > > > only, perhaps we move security_inode_reflink up higher and say:
> > > > 
> > > > 	error = security_inode_reflink(old_dentry, dir);
> > > > 	if (error)
> > > > 		preserve_security = 0;
> > > > 
> > > > Here security_inode_reflink() does not need new_dentry, because it isn't
> > > > setting a security context.  If it's ok with the reflink, we'll be
> > > > copying the extended attribute.  If it's not OK, it falls through to the
> > > > inode_permission(inode, MAY_READ) check, which will check for plain old
> > > > read access.
> > > > 	What do we think?
> > > 
> > > I'd rather have two hooks, one to allow the security module to override
> > > preserve_security and one to allow the security module to deny the
> > > operation altogether.  The former hook only needs to be called if
> > > preserve_security is not already cleared by the DAC logic.  The latter
> > > hook needs to know the final verdict on preserve_security in order to
> > > determine the right set of checks to apply, which isn't necessarily
> > > limited to only checking read access.
> > 
> > 	Ok, is that two hooks or one hook with specific error returns?
> > I don't care, it's up to the LSM group.  I just can't come up with a
> > good distinguishing set of names if its two hooks :-)
> 
> I suppose you could coalesce them into a single hook ala:
> 	error = security_inode_reflink(old_dentry, dir, &preserve_security);
> 	if (error)
> 		return (error);

On second thought (agreeing with Andy about making the interface
explicit wrt preserve_security), I don't expect us to ever override
preserve_security from SELinux, so you can just pass it in by value.

-- 
Stephen Smalley
National Security Agency