[v11,2/4] fs: add LSM-supporting anon-inode interface

Commit Message

Lokesh Gidra Nov. 5, 2020, 9:33 p.m. UTC

From: Daniel Colascione <[e-mail]>

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 <[e-mail]>

[Update 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()]

Signed-off-by: Lokesh Gidra <[e-mail]>
---
 fs/anon_inodes.c            | 149 ++++++++++++++++++++++++++----------
 fs/libfs.c                  |   6 +-
 include/linux/anon_inodes.h |   5 ++
 3 files changed, 117 insertions(+), 43 deletions(-)

Comments

Eric Biggers Nov. 5, 2020, 10:24 p.m. UTC | #1

On Thu, Nov 05, 2020 at 01:33:22PM -0800, Lokesh Gidra wrote:
> +/**
> + * Like anon_inode_getfd(), but creates a new !S_PRIVATE anon inode rather than
> + * reuse the singleton anon inode, and call the 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.  An optional @context_inode argument is also
> + * added to provide the logical relationship with the new inode.  The LSM may use
> + * @context_inode in init_security_anon(), but a 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);
> +}
> +EXPORT_SYMBOL_GPL(anon_inode_getfd_secure);

inode_init_security_anon(), not init_security_anon().  Also please use a
consistent line width (preferably 80 characters).

> diff --git a/fs/libfs.c b/fs/libfs.c
> index fc34361c1489..5b12228ecc81 100644
> --- a/fs/libfs.c
> +++ b/fs/libfs.c
> @@ -1213,9 +1213,9 @@ static int anon_set_page_dirty(struct page *page)
>  };
>  
>  /*
> - * A single inode exists for all anon_inode files. Contrary to pipes,
> - * anon_inode inodes have no associated per-instance data, so we need
> - * only allocate one of them.
> + * A single inode exists for all anon_inode files, except for the secure ones.
> + * Contrary to pipes and secure anon_inode inodes, ordinary anon_inode inodes
> + * have no associated per-instance data, so we need only allocate one of them.
>   */
>  struct inode *alloc_anon_inode(struct super_block *s)
>  {

This comment is still wrong, and the first sentence contradicts the second one.
There are a lot of callers of alloc_anon_inode() and none of them use the
singleton inode, since alloc_anon_inode() doesn't actually use it.  The
singleton inode is only used by anon_inode_getfile() and anon_inode_getfd(),
which already have comments describing how they use a singleton inode.

IMO, just deleting this comment would be much better than either the original
version or your proposed version.

> diff --git a/include/linux/anon_inodes.h b/include/linux/anon_inodes.h
> index d0d7d96261ad..6cf447cfceed 100644
> --- a/include/linux/anon_inodes.h
> +++ b/include/linux/anon_inodes.h
> @@ -10,10 +10,15 @@
>  #define _LINUX_ANON_INODES_H
>  
>  struct file_operations;
> +struct inode;
>  
>  struct file *anon_inode_getfile(const char *name,
>  				const struct file_operations *fops,
>  				void *priv, int flags);
> +int anon_inode_getfd_secure(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);
>  

Keeping declarations in the same order as the definitions can be helpful.

- Eric

Patch

diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
index 89714308c25b..fc935acb90d6 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)
+{
+	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);
 
 /**
- * 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
+ * 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
@@ -118,12 +136,23 @@  EXPORT_SYMBOL_GPL(anon_inode_getfile);
  *
  * 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_getfd() will share a single inode,
+ * 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 new descriptor or an error code.
+ * setup.  Returns the newly created file* or an error pointer.
  */
-int anon_inode_getfd(const char *name, const struct file_operations *fops,
-		     void *priv, int flags)
+struct file *anon_inode_getfile(const char *name,
+				const struct file_operations *fops,
+				void *priv, int flags)
+{
+	return __anon_inode_getfile(name, fops, priv, flags, NULL, false);
+}
+EXPORT_SYMBOL_GPL(anon_inode_getfile);
+
+static int __anon_inode_getfd(const char *name,
+			      const struct file_operations *fops,
+			      void *priv, int flags,
+			      const struct inode *context_inode,
+			      bool secure)
 {
 	int error, fd;
 	struct file *file;
@@ -133,7 +162,8 @@  int anon_inode_getfd(const char *name, const struct file_operations *fops,
 		return error;
 	fd = error;
 
-	file = anon_inode_getfile(name, fops, priv, flags);
+	file = __anon_inode_getfile(name, fops, priv, flags, context_inode,
+				    secure);
 	if (IS_ERR(file)) {
 		error = PTR_ERR(file);
 		goto err_put_unused_fd;
@@ -146,8 +176,47 @@  int anon_inode_getfd(const char *name, const struct file_operations *fops,
 	put_unused_fd(fd);
 	return error;
 }
+
+/**
+ * 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_getfd() will use the same singleton inode, reducing
+ * memory use and avoiding code duplication for the file/inode/dentry
+ * setup.  Returns a newly created file descriptor or an error code.
+ */
+int anon_inode_getfd(const char *name, const struct file_operations *fops,
+		     void *priv, int flags)
+{
+	return __anon_inode_getfd(name, fops, priv, flags, NULL, false);
+}
 EXPORT_SYMBOL_GPL(anon_inode_getfd);
 
+/**
+ * Like anon_inode_getfd(), but creates a new !S_PRIVATE anon inode rather than
+ * reuse the singleton anon inode, and call the 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.  An optional @context_inode argument is also
+ * added to provide the logical relationship with the new inode.  The LSM may use
+ * @context_inode in init_security_anon(), but a 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);
+}
+EXPORT_SYMBOL_GPL(anon_inode_getfd_secure);
+
 static int __init anon_inode_init(void)
 {
 	anon_inode_mnt = kern_mount(&anon_inode_fs_type);
diff --git a/fs/libfs.c b/fs/libfs.c
index fc34361c1489..5b12228ecc81 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -1213,9 +1213,9 @@  static int anon_set_page_dirty(struct page *page)
 };
 
 /*
- * A single inode exists for all anon_inode files. Contrary to pipes,
- * anon_inode inodes have no associated per-instance data, so we need
- * only allocate one of them.
+ * A single inode exists for all anon_inode files, except for the secure ones.
+ * Contrary to pipes and secure anon_inode inodes, ordinary anon_inode inodes
+ * have no associated per-instance data, so we need only allocate one of them.
  */
 struct inode *alloc_anon_inode(struct super_block *s)
 {
diff --git a/include/linux/anon_inodes.h b/include/linux/anon_inodes.h
index d0d7d96261ad..6cf447cfceed 100644
--- a/include/linux/anon_inodes.h
+++ b/include/linux/anon_inodes.h
@@ -10,10 +10,15 @@ 
 #define _LINUX_ANON_INODES_H
 
 struct file_operations;
+struct inode;
 
 struct file *anon_inode_getfile(const char *name,
 				const struct file_operations *fops,
 				void *priv, int flags);
+int anon_inode_getfd_secure(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);