Re: [PATCH v13 2/4] fs: add LSM-supporting anon-inode interface

[Paul Moore <paul@paul-moore.com>]

On Wed, Nov 11, 2020 at 8:54 PM Lokesh Gidra <lokeshgidra@google.com> wrote:
> From: Daniel Colascione <dancol@google.com>
>
> 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 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@google.com>
>
> [Delete obsolete comments to alloc_anon_inode()]
> [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()]
> [Fix error handling in __anon_inode_getfile()]

Lokesh, I'm assuming you made the changes in the brackets above?  If
so they should include your initials or some other means of
attributing them to you, e.g. "[LG: Fix error ...]".

> Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
> Reviewed-by: Eric Biggers <ebiggers@google.com>
> ---
>  fs/anon_inodes.c            | 150 ++++++++++++++++++++++++++----------
>  fs/libfs.c                  |   5 --
>  include/linux/anon_inodes.h |   5 ++
>  3 files changed, 115 insertions(+), 45 deletions(-)
>
> diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
> index 89714308c25b..023337d65a03 100644
> --- a/fs/anon_inodes.c
> +++ b/fs/anon_inodes.c
> @@ -55,61 +55,79 @@ 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 file *file;
> +       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);
> +       if (error) {
> +               iput(inode);
> +               return ERR_PTR(error);
> +       }
> +       return inode;
> +}
>
> -       if (IS_ERR(anon_inode_inode))
> -               return ERR_PTR(-ENODEV);
> +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)

Is it necessary to pass both the context_inode pointer and the secure
boolean?  It seems like if context_inode is non-NULL then one could
assume that a secure anonymous inode was requested; is there ever
going to be a case where this is not true?

> +{
> +       struct inode *inode;
> +       struct file *file;
>
>         if (fops->owner && !try_module_get(fops->owner))
>                 return ERR_PTR(-ENOENT);
>
> -       /*
> -        * We know the anon_inode inode count is always greater than zero,
> -        * so ihold() is safe.
> -        */
> -       ihold(anon_inode_inode);
> -       file = alloc_file_pseudo(anon_inode_inode, anon_inode_mnt, name,
> +       if (secure) {
> +               inode = anon_inode_make_secure_inode(name, context_inode);
> +               if (IS_ERR(inode)) {
> +                       file = ERR_CAST(inode);
> +                       goto err;
> +               }
> +       } else {
> +               inode = anon_inode_inode;
> +               if (IS_ERR(inode)) {
> +                       file = ERR_PTR(-ENODEV);
> +                       goto err;
> +               }
> +               /*
> +                * We know the anon_inode inode count is always
> +                * greater than zero, so ihold() is safe.
> +                */
> +               ihold(inode);
> +       }
> +
> +       file = alloc_file_pseudo(inode, anon_inode_mnt, name,
>                                  flags & (O_ACCMODE | O_NONBLOCK), fops);
>         if (IS_ERR(file))
> -               goto err;
> +               goto err_iput;
>
> -       file->f_mapping = anon_inode_inode->i_mapping;
> +       file->f_mapping = inode->i_mapping;
>
>         file->private_data = priv;
>
>         return file;
>
> +err_iput:
> +       iput(inode);
>  err:
> -       iput(anon_inode_inode);
>         module_put(fops->owner);
>         return file;
>  }
> -EXPORT_SYMBOL_GPL(anon_inode_getfile);

--
paul moore
www.paul-moore.com