Re: [PATCH RFC] user-namespaced file capabilities - now with more magic

[Mimi Zohar <zohar@linux.vnet.ibm.com>]

On Fri, 2016-05-20 at 14:59 -0500, Serge E. Hallyn wrote:
> Quoting Eric W. Biederman (ebiederm@xmission.com):
> > "Serge E. Hallyn" <serge@hallyn.com> writes:
> > 
> > > Quoting Eric W. Biederman (ebiederm@xmission.com):
> > >> Mimi Zohar <zohar@linux.vnet.ibm.com> writes:
> > >> 
> > >> > On Thu, 2016-05-19 at 22:40 -0500, Serge E. Hallyn wrote:
> > >> >> Quoting Mimi Zohar (zohar@linux.vnet.ibm.com):
> > >> >> > On Wed, 2016-05-18 at 16:57 -0500, Serge E. Hallyn wrote:
> > >> >
> > >> >> > > diff --git a/fs/xattr.c b/fs/xattr.c
> > >> >> > > index 4861322..5c0e7ae 100644
> > >> >> > > --- a/fs/xattr.c
> > >> >> > > +++ b/fs/xattr.c
> > >> >> > > @@ -94,11 +94,26 @@ int __vfs_setxattr_noperm(struct dentry *dentry, const char *name,
> > >> >> > >  {
> > >> >> > >  	struct inode *inode = dentry->d_inode;
> > >> >> > >  	int error = -EOPNOTSUPP;
> > >> >> > > +	void *wvalue = NULL;
> > >> >> > > +	size_t wsize = 0;
> > >> >> > >  	int issec = !strncmp(name, XATTR_SECURITY_PREFIX,
> > >> >> > >  				   XATTR_SECURITY_PREFIX_LEN);
> > >> >> > > 
> > >> >> > > -	if (issec)
> > >> >> > > +	if (issec) {
> > >> >> > >  		inode->i_flags &= ~S_NOSEC;
> > >> >> > > +		/* if root in a non-init user_ns tries to set
> > >> >> > > +		 * security.capability, write a security.nscapability
> > >> >> > > +		 * in its place */
> > >> >> > > +		if (!strcmp(name, "security.capability") &&
> > >> >> > > +				current_user_ns() != &init_user_ns) {
> > >> >> > > +			cap_setxattr_make_nscap(dentry, value, size, &wvalue, &wsize);
> > >> >> > > +			if (!wvalue)
> > >> >> > > +				return -EPERM;
> > >> >> > > +			value = wvalue;
> > >> >> > > +			size = wsize;
> > >> >> > > +			name = "security.nscapability";
> > >> >> > > +		}
> > >> >> > 
> > >> >> > The call to capable_wrt_inode_uidgid() is hidden behind
> > >> >> > cap_setxattr_make_nscap().  Does it make sense to call it here instead,
> > >> >> > before the security.capability test?  This would lay the foundation for
> > >> >> > doing something similar for IMA.
> > >> >> 
> > >> >> Might make sense to move that.  Though looking at it with fresh eyes I wonder
> > >> >> whether adding less code here at __vfs_setxattr_noperm(), i.e.
> > >> >> 
> > >> >> 		if (!cap_setxattr_makenscap(dentry, &value, &size, &name))
> > >> >> 			return -EPERM;
> > >> >> 
> > >> >> would be cleaner.
> > >> >
> > >> > Yes, it would be cleaner,  but I'm suggesting you do all the hard work
> > >> > making it generic.  Then the rest of us can follow your lead.  Its more
> > >> > likely that you'll get it right.  At a high level, it might look like:
> > >> >
> > >> >                /* Permit root in a non-init user_ns to modify the security
> > >> >                  * namespace xattr equivalents (eg. nscapability, ns_ima, etc). 
> > >> >                  */
> > >> >                 if ((current_user_ns() != &init_user_ns) &&
> > >> >                         capable_wrt_inode_uidgid(inode, CAP_SETFCAP)) {
> > >> >
> > >> > 			if  security..capability
> > >> > 				call capability  /* set nscapability? */
> > >> >
> > >> > 			else if security.ima 
> > >> > 				call ima 	/* set ns_ima? */
> > >> > 		}
> > >> 
> > >> Hmm.  I am confused about this part of the strategy.
> > >> 
> > >> I don't understand the capability vs nscapability distinction.  It seems
> > >> to add complexity without benefit.
> > >
> > > ...  Well, yes, we could simply make a new version of security.capability
> > > xattr, and make rootid == 0 mean it was written by the init_user_ns.  Is
> > > that what you mean?
> > 
> > Yes.
> > 
> > That would seem to simplify the logic to ensure the policy we enforce is
> > consistent with what is on disk.
> 
> I'll give that a shot.  I think the reason I did it this way was that I'm
> still kind of stuck in the not-magic way of thinking about it.  But yeah
> with the kernel magically writing inthe kuid there's probably no reason not
> to.

Totally confused.  Will this method allow multiple instances of the
xattr on disk? 

Mimi