fuse doesn't use security_inode_init_security?

[Stephen Smalley]

On Fri, May 1, 2020 at 3:54 AM Miklos Szeredi <miklos at szeredi.hu> wrote:
>
> On Fri, May 1, 2020 at 8:55 AM Chirantan Ekbote <chirantan at chromium.org> wrote:
> >
> > Hello,
> >
> > I noticed that the fuse module doesn't currently call
> > security_inode_init_security and I was wondering if there is a
> > specific reason for that.  I found a patch from 2013[1] that would
> > change fuse so that it would call that function but it doesn't appear
> > that the patch was merged.
> >
> > For background: I currently have a virtio-fs server with a guest VM
> > that wants to use selinux.  I was able to enable selinux support
> > without much issue by adding
> >
> >     fs_use_xattr virtiofs u:object_r:labeledfs:s0;
> >
> > to the selinux policy in the guest.  This works for the most part
> > except that `setfscreatecon` doesn't appear to work.  From what I can
> > tell, this ends up writing to `/proc/[pid]/attr/fscreate` and the
> > attributes actually get set via the `inode_init_security` lsm hook in
> > selinux.  However, since fuse doesn't call
> > `security_inode_init_security` the hook never runs so the
> > file/directory doesn't have the right attributes.
> >
> > Is it safe to just call `security_inode_init_security` whenever fuse
> > creates a new inode?  How does this affect non-virtiofs fuse servers?
>
> Not sure,  Adding more Cc's.
>
> I know there's a deadlock scenario with getxattr called on root inode
> before mount returns, which causes a deadlock unless mount is run in
> the background.  Current libfuse doesn't handle this, but I think some
> fuse fs work around this by not using libfuse, or at least have some
> special setup code (glusterfs? ceph-fuse? not sure...).  I also don't
> know whether the ->inode_init_security hook is related to this or not.

(cc selinux list)

security_inode_init_security() calls the initxattrs callback to
actually set each xattr in the backing store (if any), so unless you
have a way to pass that to the daemon along with the create request
the attribute won't be persisted with the file.  Setting the xattrs is
supposed to be atomic with the file creation, not a separate
setxattr() operation after creating the file, similar to ACL
inheritance on new files.

Also possibly related
https://lore.kernel.org/selinux/6df9b58c-fe9b-28f3-c151-f77aa6dd67e7@tycho.nsa.gov/.