On Sun, Nov 5, 2023 at 4:32 PM Paolo Bonzini <pbonzini@xxxxxxxxxx> 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@xxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
Reviewed-by: Fuad Tabba <tabba@xxxxxxxxxx>
Tested-by: Fuad Tabba <tabba@xxxxxxxxxx>
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
>
>
