[RFC PATCH v2 5/9] fs: add anon_inode_getfile_secure() similar to anon_inode_getfd_secure()

[Paul Moore]

On Thu, Aug 12, 2021 at 5:32 AM Mickaël Salaün <mic at digikod.net> wrote:
> On 11/08/2021 22:48, Paul Moore wrote:
> > Extending the secure anonymous inode support to other subsystems
> > requires that we have a secure anon_inode_getfile() variant in
> > addition to the existing secure anon_inode_getfd() variant.
> >
> > Thankfully we can reuse the existing __anon_inode_getfile() function
> > and just wrap it with the proper arguments.
> >
> > Signed-off-by: Paul Moore <paul at paul-moore.com>
> >
> > ---
> > v2:
> > - no change
> > v1:
> > - initial draft
> > ---
> >  fs/anon_inodes.c            |   29 +++++++++++++++++++++++++++++
> >  include/linux/anon_inodes.h |    4 ++++
> >  2 files changed, 33 insertions(+)
> >
> > diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
> > index a280156138ed..e0c3e33c4177 100644
> > --- a/fs/anon_inodes.c
> > +++ b/fs/anon_inodes.c
> > @@ -148,6 +148,35 @@ struct file *anon_inode_getfile(const char *name,
> >  }
> >  EXPORT_SYMBOL_GPL(anon_inode_getfile);
> >
> > +/**
> > + * anon_inode_getfile_secure - Like anon_inode_getfile(), but creates a new
> > + *                             !S_PRIVATE anon inode rather than reuse the
> > + *                             singleton anon inode and calls the
> > + *                             inode_init_security_anon() LSM hook.  This
> > + *                             allows for both the inode to have its own
> > + *                             security context and for the LSM to enforce
> > + *                             policy on the inode's creation.
> > + *
> > + * @name:    [in]    name of the "class" of the new file
> > + * @fops:    [in]    file operations for the new file
> > + * @priv:    [in]    private data for the new file (will be file's private_data)
> > + * @flags:   [in]    flags
> > + * @context_inode:
> > + *           [in]    the logical relationship with the new inode (optional)
> > + *
> > + * The LSM may use @context_inode in inode_init_security_anon(), but a
> > + * reference to it is not held.  Returns the newly created file* or an error
> > + * pointer.  See the anon_inode_getfile() documentation for more information.
> > + */
> > +struct file *anon_inode_getfile_secure(const char *name,
> > +                                    const struct file_operations *fops,
> > +                                    void *priv, int flags,
> > +                                    const struct inode *context_inode)
> > +{
> > +     return __anon_inode_getfile(name, fops, priv, flags,
> > +                                 context_inode, true);
>
> This is not directly related to this patch but why using the "secure"
> boolean in __anon_inode_getfile() and __anon_inode_getfd() instead of
> checking that context_inode is not NULL? This would simplify the code,
> remove this anon_inode_getfile_secure() wrapper and avoid potential
> inconsistencies.

The issue is that it is acceptable for the context_inode to be either
valid or NULL for callers who request the "secure" code path.

Look at the SELinux implementation of the anonymous inode hook in
selinux_inode_init_security_anon() and you will see that in cases
where the context_inode is valid we simply inherit the label from the
given inode, whereas if context_inode is NULL we do a type transition
using the requesting task and the anonymous inode's "name".

-- 
paul moore
www.paul-moore.com