Re: [PATCH 14/34] fs: Rename anon_inode_getfile_secure() and anon_inode_getfd_secure()

[Fuad Tabba <tabba@google.com>]

On Sun, Nov 5, 2023 at 4:32 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> The call to the inode_init_security_anon() LSM hook is not the sole
> reason to use anon_inode_getfile_secure() or anon_inode_getfd_secure().
> For example, the functions also allow one to create a file with non-zero
> size, without needing a full-blown filesystem.  In this case, you don't
> need a "secure" version, just unique inodes; the current name of the
> functions is confusing and does not explain well the difference with
> the more "standard" anon_inode_getfile() and anon_inode_getfd().
>
> Of course, there is another side of the coin; neither io_uring nor
> userfaultfd strictly speaking need distinct inodes, and it is not
> that clear anymore that anon_inode_create_get{file,fd}() allow the LSM
> to intercept and block the inode's creation.  If one was so inclined,
> anon_inode_getfile_secure() and anon_inode_getfd_secure() could be kept,
> using the shared inode or a new one depending on CONFIG_SECURITY.
> However, this is probably overkill, and potentially a cause of bugs in
> different configurations.  Therefore, just add a comment to io_uring
> and userfaultfd explaining the choice of the function.
>
> While at it, remove the export for what is now anon_inode_create_getfd().
> There is no in-tree module that uses it, and the old name is gone anyway.
> If anybody actually needs the symbol, they can ask or they can just use
> anon_inode_create_getfile(), which will be exported very soon for use
> in KVM.
>
> Suggested-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---

Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>

Cheers,
/fuad


>  fs/anon_inodes.c            | 46 +++++++++++++++++++++++--------------
>  fs/userfaultfd.c            |  5 ++--
>  include/linux/anon_inodes.h |  4 ++--
>  io_uring/io_uring.c         |  3 ++-
>  4 files changed, 36 insertions(+), 22 deletions(-)
>
> diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
> index 24192a7667ed..3d4a27f8b4fe 100644
> --- a/fs/anon_inodes.c
> +++ b/fs/anon_inodes.c
> @@ -79,7 +79,7 @@ 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)
> +                                        bool make_inode)
>  {
>         struct inode *inode;
>         struct file *file;
> @@ -87,7 +87,7 @@ static struct file *__anon_inode_getfile(const char *name,
>         if (fops->owner && !try_module_get(fops->owner))
>                 return ERR_PTR(-ENOENT);
>
> -       if (secure) {
> +       if (make_inode) {
>                 inode = anon_inode_make_secure_inode(name, context_inode);
>                 if (IS_ERR(inode)) {
>                         file = ERR_CAST(inode);
> @@ -149,13 +149,10 @@ 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
> + * anon_inode_create_getfile - 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.
> + *                             inode_init_security_anon() LSM hook.
>   *
>   * @name:    [in]    name of the "class" of the new file
>   * @fops:    [in]    file operations for the new file
> @@ -164,11 +161,19 @@ EXPORT_SYMBOL_GPL(anon_inode_getfile);
>   * @context_inode:
>   *           [in]    the logical relationship with the new inode (optional)
>   *
> + * Create a new anonymous inode and file pair.  This can be done for two
> + * reasons:
> + * - for the inode to have its own security context, so that LSMs can enforce
> + *   policy on the inode's creation;
> + * - if the caller needs a unique inode, for example in order to customize
> + *   the size returned by fstat()
> + *
>   * 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.
> + * reference to it is not held.
> + *
> + * Returns the newly created file* or an error pointer.
>   */
> -struct file *anon_inode_getfile_secure(const char *name,
> +struct file *anon_inode_create_getfile(const char *name,
>                                        const struct file_operations *fops,
>                                        void *priv, int flags,
>                                        const struct inode *context_inode)
> @@ -181,7 +186,7 @@ static int __anon_inode_getfd(const char *name,
>                               const struct file_operations *fops,
>                               void *priv, int flags,
>                               const struct inode *context_inode,
> -                             bool secure)
> +                             bool make_inode)
>  {
>         int error, fd;
>         struct file *file;
> @@ -192,7 +197,7 @@ static int __anon_inode_getfd(const char *name,
>         fd = error;
>
>         file = __anon_inode_getfile(name, fops, priv, flags, context_inode,
> -                                   secure);
> +                                   make_inode);
>         if (IS_ERR(file)) {
>                 error = PTR_ERR(file);
>                 goto err_put_unused_fd;
> @@ -231,10 +236,9 @@ int anon_inode_getfd(const char *name, const struct file_operations *fops,
>  EXPORT_SYMBOL_GPL(anon_inode_getfd);
>
>  /**
> - * anon_inode_getfd_secure - Like anon_inode_getfd(), but creates a new
> + * anon_inode_create_getfd - Like anon_inode_getfd(), 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 the inode to have its
> - * own security context and for a LSM to reject creation of the inode.
> + * the inode_init_security_anon() LSM hook.
>   *
>   * @name:    [in]    name of the "class" of the new file
>   * @fops:    [in]    file operations for the new file
> @@ -243,16 +247,24 @@ EXPORT_SYMBOL_GPL(anon_inode_getfd);
>   * @context_inode:
>   *           [in]    the logical relationship with the new inode (optional)
>   *
> + * Create a new anonymous inode and file pair.  This can be done for two
> + * reasons:
> + * - for the inode to have its own security context, so that LSMs can enforce
> + *   policy on the inode's creation;
> + * - if the caller needs a unique inode, for example in order to customize
> + *   the size returned by fstat()
> + *
>   * The LSM may use @context_inode in inode_init_security_anon(), but a
>   * reference to it is not held.
> + *
> + * Returns a newly created file descriptor or an error code.
>   */
> -int anon_inode_getfd_secure(const char *name, const struct file_operations *fops,
> +int anon_inode_create_getfd(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);
>  }
> -EXPORT_SYMBOL_GPL(anon_inode_getfd_secure);
>
>  static int __init anon_inode_init(void)
>  {
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 56eaae9dac1a..7a1cf8bab5eb 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1033,7 +1033,7 @@ static int resolve_userfault_fork(struct userfaultfd_ctx *new,
>  {
>         int fd;
>
> -       fd = anon_inode_getfd_secure("[userfaultfd]", &userfaultfd_fops, new,
> +       fd = anon_inode_create_getfd("[userfaultfd]", &userfaultfd_fops, new,
>                         O_RDONLY | (new->flags & UFFD_SHARED_FCNTL_FLAGS), inode);
>         if (fd < 0)
>                 return fd;
> @@ -2205,7 +2205,8 @@ static int new_userfaultfd(int flags)
>         /* prevent the mm struct to be freed */
>         mmgrab(ctx->mm);
>
> -       fd = anon_inode_getfd_secure("[userfaultfd]", &userfaultfd_fops, ctx,
> +       /* Create a new inode so that the LSM can block the creation.  */
> +       fd = anon_inode_create_getfd("[userfaultfd]", &userfaultfd_fops, ctx,
>                         O_RDONLY | (flags & UFFD_SHARED_FCNTL_FLAGS), NULL);
>         if (fd < 0) {
>                 mmdrop(ctx->mm);
> diff --git a/include/linux/anon_inodes.h b/include/linux/anon_inodes.h
> index 5deaddbd7927..93a5f16d03f3 100644
> --- a/include/linux/anon_inodes.h
> +++ b/include/linux/anon_inodes.h
> @@ -15,13 +15,13 @@ struct inode;
>  struct file *anon_inode_getfile(const char *name,
>                                 const struct file_operations *fops,
>                                 void *priv, int flags);
> -struct file *anon_inode_getfile_secure(const char *name,
> +struct file *anon_inode_create_getfile(const char *name,
>                                        const struct file_operations *fops,
>                                        void *priv, int flags,
>                                        const struct inode *context_inode);
>  int anon_inode_getfd(const char *name, const struct file_operations *fops,
>                      void *priv, int flags);
> -int anon_inode_getfd_secure(const char *name,
> +int anon_inode_create_getfd(const char *name,
>                             const struct file_operations *fops,
>                             void *priv, int flags,
>                             const struct inode *context_inode);
> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
> index 8d1bc6cdfe71..22b98f47bb28 100644
> --- a/io_uring/io_uring.c
> +++ b/io_uring/io_uring.c
> @@ -3835,7 +3835,8 @@ static struct file *io_uring_get_file(struct io_ring_ctx *ctx)
>                 return ERR_PTR(ret);
>  #endif
>
> -       file = anon_inode_getfile_secure("[io_uring]", &io_uring_fops, ctx,
> +       /* Create a new inode so that the LSM can block the creation.  */
> +       file = anon_inode_create_getfile("[io_uring]", &io_uring_fops, ctx,
>                                          O_RDWR | O_CLOEXEC, NULL);
>  #if defined(CONFIG_UNIX)
>         if (IS_ERR(file)) {
> --
> 2.39.1
>
>