Re: [PATCH v2] xattr: Enable security.capability in user namespaces

[Stefan Berger]

On 07/12/2017 01:53 PM, Vivek Goyal wrote:
> On Tue, Jul 11, 2017 at 11:05:11AM -0400, Stefan Berger wrote:
>
> [..]
> > @@ -301,14 +721,39 @@ ssize_t
> >   __vfs_getxattr(struct dentry *dentry, struct inode *inode, const char *name,
> >   	       void *value, size_t size)
> >   {
> > -	const struct xattr_handler *handler;
> > +	const struct xattr_handler *handler = NULL;
> > +	char *newname =  NULL;
> > +	int ret, userns_supt_xattr;
> > +	struct user_namespace *userns = current_user_ns();
> > +
> > +	userns_supt_xattr = (xattr_is_userns_supported(name, false) >= 0);
> > +
> Hi Stephan,
>
> > +	do {
> > +		kfree(newname);
> > +
> > +		newname = xattr_userns_name(name, userns);
> 					    ^^^
> Will name be pointing to a freed string in second iteration of loop.

Fixing for v3.

> > +		if (IS_ERR(newname))
> > +			return PTR_ERR(newname);
> > +
> > +		if (!handler) {
> > +			name = newname;
> Here we assign name and at the beginning of second iteration we free
> newname.
>
> Also I am not sure why do we do this assignment only if handler is NULL.

The handler shouldn't change but this optimization isn't helpful. Fixed 
through this patch:

https://github.com/stefanberger/linux/commit/10828401b29a13f8c56f8fad0c0fb2690e4af878


> BTW, I set cap_sys_admin on a file outside usernamespace and then launched
> user namespace (mapping 1000 to 0). And then tried to do getcap on file
> and I am not seeing security.capability set by host. Not sure what am I
> doing wrong. getxattr() seems to return -ENODATA. Still debugging it.

This was a regression due to the bug in the loop. I didn't have a test 
case (with runc) for it, now I do.

> Also, have we resovled the question of stacked filesystem like overlayfs.
> There we are switching creds to mounter's creds when doing operations on
> underlying filesystem. I am concenrned does that mean, we will get and
> return security.capability to caller in usernamespace instead of
> security.capability@uid=1000.

I would have to test this, otherwise I don't know. I'll try it out with 
Docker.

   Stefan

> Vivek
>
> > +			handler = xattr_resolve_name(inode, &name);
> > +			if (IS_ERR(handler)) {
> > +				ret = PTR_ERR(handler);
> > +				goto out;
> > +			}
> > +			if (!handler->get) {
> > +				ret = -EOPNOTSUPP;
> > +				goto out;
> > +			}
> > +		}
> > +		ret = handler->get(handler, dentry, inode, name, value, size);
> > +		userns = userns->parent;
> > +	} while ((ret == -ENODATA) && userns && userns_supt_xattr);
> >   
> > -	handler = xattr_resolve_name(inode, &name);
> > -	if (IS_ERR(handler))
> > -		return PTR_ERR(handler);
> > -	if (!handler->get)
> > -		return -EOPNOTSUPP;
> > -	return handler->get(handler, dentry, inode, name, value, size);
> > +out:
> > +	kfree(newname);
> > +	return ret;
> >   }
> >   EXPORT_SYMBOL(__vfs_getxattr);
> >   
> Thanks
> Vivek