Re: [PATCH v7 1/3] Add a new LSM-supporting anonymous inode interface

[Lokesh Gidra]

On Mon, Aug 24, 2020 at 8:50 PM Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
>
> On Fri, Aug 21, 2020 at 11:56:43AM -0700, Lokesh Gidra wrote:
> > From: Daniel Colascione <dancol@xxxxxxxxxx>
> >
> > This change adds a new function, anon_inode_getfd_secure, that creates
> > anonymous-node file with individual non-S_PRIVATE inode to which security
> > modules can apply policy. Existing callers continue using the original
> > singleton-inode kind of anonymous-inode file. We can transition anonymous
> > inode users to the new kind of anonymous inode in individual patches for
> > the sake of bisection and review.
> >
> > The new function accepts an optional context_inode parameter that
> > callers can use to provide additional contextual information to
> > security modules for granting/denying permission to create an anon inode
> > of the same type.
> >
> > For example, in case of userfaultfd, the created inode is a
> > 'logical child' of the context_inode (userfaultfd inode of the
> > parent process) in the sense that it provides the security context
> > required during creation of the child process' userfaultfd inode.
> >
> > Signed-off-by: Daniel Colascione <dancol@xxxxxxxxxx>
> >
> > [Fix comment documenting return values of inode_init_security_anon()]
> > [Add context_inode description in comments to anon_inode_getfd_secure()]
> > [Remove definition of anon_inode_getfile_secure() as there are no callers]
> > [Make _anon_inode_getfile() static]
> > [Use correct error cast in _anon_inode_getfile()]
> >
> > Signed-off-by: Lokesh Gidra <lokeshgidra@xxxxxxxxxx>
> > ---
> >  fs/anon_inodes.c              | 148 ++++++++++++++++++++++++----------
> >  include/linux/anon_inodes.h   |  13 +++
> >  include/linux/lsm_hook_defs.h |   2 +
> >  include/linux/lsm_hooks.h     |   7 ++
> >  include/linux/security.h      |   3 +
> >  security/security.c           |   9 +++
> >  6 files changed, 141 insertions(+), 41 deletions(-)
> >
> > diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
> > index 89714308c25b..2aa8b57be895 100644
> > --- a/fs/anon_inodes.c
> > +++ b/fs/anon_inodes.c
> > @@ -55,61 +55,78 @@ static struct file_system_type anon_inode_fs_type = {
> >       .kill_sb        = kill_anon_super,
> >  };
> >
> > -/**
> > - * anon_inode_getfile - creates a new file instance by hooking it up to an
> > - *                      anonymous inode, and a dentry that describe the "class"
> > - *                      of the file
> > - *
> > - * @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
> > - *
> > - * Creates a new file by hooking it on a single inode. This is useful for files
> > - * that do not need to have a full-fledged inode in order to operate correctly.
> > - * All the files created with anon_inode_getfile() will share a single inode,
> > - * hence saving memory and avoiding code duplication for the file/inode/dentry
> > - * setup.  Returns the newly created file* or an error pointer.
> > - */
> > -struct file *anon_inode_getfile(const char *name,
> > -                             const struct file_operations *fops,
> > -                             void *priv, int flags)
> > +static struct inode *anon_inode_make_secure_inode(
> > +     const char *name,
> > +     const struct inode *context_inode)
> > +{
> > +     struct inode *inode;
> > +     const struct qstr qname = QSTR_INIT(name, strlen(name));
> > +     int error;
> > +
> > +     inode = alloc_anon_inode(anon_inode_mnt->mnt_sb);
> > +     if (IS_ERR(inode))
> > +             return inode;
> > +     inode->i_flags &= ~S_PRIVATE;
> > +     error = security_inode_init_security_anon(
> > +             inode, &qname, context_inode);
>
> Weird indentation here.  The call to security_inode_init_security_anon() fits on
> one line.
>
> > +     if (error) {
> > +             iput(inode);
> > +             return ERR_PTR(error);
> > +     }
> > +     return inode;
> > +}
> > +
> > +static struct file *_anon_inode_getfile(const char *name,
> > +                                     const struct file_operations *fops,
> > +                                     void *priv, int flags,
> > +                                     const struct inode *context_inode,
> > +                                     bool secure)
> >  {
> > +     struct inode *inode;
> >       struct file *file;
> >
> > -     if (IS_ERR(anon_inode_inode))
> > -             return ERR_PTR(-ENODEV);
> > +     if (secure) {
> > +             inode = anon_inode_make_secure_inode(
> > +                     name, context_inode);
>
> Likewise here.  The call to anon_inode_make_secure_inode() fits on one line.
>
> > +             if (IS_ERR(inode))
> > +                     return ERR_CAST(inode);
> > +     } else {
> > +             inode = anon_inode_inode;
> > +             if (IS_ERR(inode))
> > +                     return ERR_PTR(-ENODEV);
> > +             /*
> > +              * We know the anon_inode inode count is always
> > +              * greater than zero, so ihold() is safe.
> > +              */
> > +             ihold(inode);
> > +     }
> >
> > -     if (fops->owner && !try_module_get(fops->owner))
> > -             return ERR_PTR(-ENOENT);
> > +     if (fops->owner && !try_module_get(fops->owner)) {
> > +             file = ERR_PTR(-ENOENT);
> > +             goto err;
> > +     }
>
> The error path here does module_put(fops->owner), even though a reference wasn't
> acquired.
>
> > +
> > +/**
> > + * anon_inode_getfd - creates a new file instance by hooking it up to
> > + *                    an anonymous inode and a dentry that describe
> > + *                    the "class" of the file
> > + *
> > + * @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
> > + *
> > + * Creates a new file by hooking it on a single inode. This is
> > + * useful for files that do not need to have a full-fledged inode in
> > + * order to operate correctly.  All the files created with
> > + * anon_inode_getfile() will use the same singleton inode, reducing
>
> This should say anon_inode_getfd(), not anon_inode_getfile().
>
> > +/**
> > + * Like anon_inode_getfd(), but adds the @context_inode argument to
> > + * allow security modules to control creation of the new file. Once the
> > + * security module makes the decision, this inode is no longer needed
> > + * and hence reference to it is not held.
> > + */
> > +int anon_inode_getfd_secure(const char *name, const struct file_operations *fops,
> > +                         void *priv, int flags,
> > +                         const struct inode *context_inode)
> > +{
> > +     return _anon_inode_getfd(name, fops, priv, flags,
> > +                              context_inode, true);
> > +}
>
> Weird indentation here again.  The call to _anon_inode_getfd() fits on one line.
>
> > @@ -162,4 +229,3 @@ static int __init anon_inode_init(void)
> >  }
> >
> >  fs_initcall(anon_inode_init);
> > -
>
> Unnecessary whitespace change.
>
> > diff --git a/include/linux/anon_inodes.h b/include/linux/anon_inodes.h
> > index d0d7d96261ad..67bd85d92dca 100644
> > --- a/include/linux/anon_inodes.h
> > +++ b/include/linux/anon_inodes.h
> > @@ -10,12 +10,25 @@
> >  #define _LINUX_ANON_INODES_H
> >
> >  struct file_operations;
> > +struct inode;
> > +
> > +struct file *anon_inode_getfile_secure(const char *name,
> > +                                    const struct file_operations *fops,
> > +                                    void *priv, int flags,
> > +                                    const struct inode *context_inode);
>
> This function isn't defined anywhere.
>
> > + * @inode_init_security_anon:
> > + *      Set up a secure anonymous inode.
> > + *      @inode contains the inode structure
> > + *      @name name of the anonymous inode class
> > + *      @context_inode optional related inode
> > + *   Returns 0 on success, -EACCESS if the security module denies the
> > + *   creation of this inode, or another -errno upon other errors.
>
> Is there a better name for this than "secure anonymous inode"?
> (What is meant by "secure"?)
>
> > diff --git a/include/linux/security.h b/include/linux/security.h
> > index 0a0a03b36a3b..95c133a8f8bb 100644
> > --- a/include/linux/security.h
> > +++ b/include/linux/security.h
> > @@ -322,6 +322,9 @@ void security_inode_free(struct inode *inode);
> >  int security_inode_init_security(struct inode *inode, struct inode *dir,
> >                                const struct qstr *qstr,
> >                                initxattrs initxattrs, void *fs_data);
> > +int security_inode_init_security_anon(struct inode *inode,
> > +                                   const struct qstr *name,
> > +                                   const struct inode *context_inode);
> >  int security_old_inode_init_security(struct inode *inode, struct inode *dir,
> >                                    const struct qstr *qstr, const char **name,
> >                                    void **value, size_t *len);
>
> This patch doesn't compile when !CONFIG_SECURITY because this file is missing a
> !CONFIG_SECURITY stub for security_inode_init_security_anon().
>
> > diff --git a/security/security.c b/security/security.c
> > index 70a7ad357bc6..149b3f024e2d 100644
> > --- a/security/security.c
> > +++ b/security/security.c
> > @@ -1057,6 +1057,15 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
> >  }
> >  EXPORT_SYMBOL(security_inode_init_security);
> >
> > +int
> > +security_inode_init_security_anon(struct inode *inode,
> > +                               const struct qstr *name,
> > +                               const struct inode *context_inode)
> > +{
> > +     return call_int_hook(inode_init_security_anon, 0, inode, name,
> > +                          context_inode);
> > +}
>
> Nit: everything else in this file has 'int' on the same line as the function
> name.
>
Thanks a lot for reviewing. I'll send another version with all these fixed.

> - Eric