[PATCH v6 0/3] SELinux support for anonymous inodes and UFFD

[PATCH v6 0/3] SELinux support for anonymous inodes and UFFD

[Lokesh Gidra]

Userfaultfd in unprivileged contexts could be potentially very
useful. We'd like to harden userfaultfd to make such unprivileged use
less risky. This patch series allows SELinux to manage userfaultfd
file descriptors and in the future, other kinds of
anonymous-inode-based file descriptor.  SELinux policy authors can
apply policy types to anonymous inodes by providing name-based
transition rules keyed off the anonymous inode internal name (
"[userfaultfd]" in the case of userfaultfd(2) file descriptors) and
applying policy to the new SIDs thus produced.

Inside the kernel, a pair of new anon_inodes interface,
anon_inode_getfile_secure and anon_inode_getfd_secure, allow callers
to opt into this SELinux management. In this new "secure" mode,
anon_inodes creates new ephemeral inodes for anonymous file objects
instead of reusing the normal anon_inodes singleton dummy inode. A new
LSM hook gives security modules an opportunity to configure and veto
these ephemeral inodes.

This patch series is one of two fork of [1] and is an
alternative to [2].

The primary difference between the two patch series is that this
partch series creates a unique inode for each "secure" anonymous
inode, while the other patch series ([2]) continues using the
singleton dummy anonymous inode and adds a way to attach SELinux
security information directly to file objects.

I prefer the approach in this patch series because 1) it's a smaller
patch than [2], and 2) it produces a more regular security
architecture: in this patch series, secure anonymous inodes aren't
S_PRIVATE and they maintain the SELinux property that the label for a
file is in its inode. We do need an additional inode per anonymous
file, but per-struct-file inode creation doesn't seem to be a problem
for pipes and sockets.

The previous version of this feature ([1]) created a new SELinux
security class for userfaultfd file descriptors. This version adopts
the generic transition-based approach of [2].

This patch series also differs from [2] in that it doesn't affect all
anonymous inodes right away --- instead requiring anon_inodes callers
to opt in --- but this difference isn't one of basic approach. The
important question to resolve is whether we should be creating new
inodes or enhancing per-file data.

Changes from the first version of the patch:

  - Removed some error checks
  - Defined a new anon_inode SELinux class to resolve the
    ambiguity in [3]
  - Inherit sclass as well as descriptor from context inode

Changes from the second version of the patch:

  - Fixed example policy in the commit message to reflect the use of
    the new anon_inode class.

Changes from the third version of the patch:

  - Dropped the fops parameter to the LSM hook
  - Documented hook parameters
  - Fixed incorrect class used for SELinux transition
  - Removed stray UFFD changed early in the series
  - Removed a redundant ERR_PTR(PTR_ERR())

Changes from the fourth version of the patch:

  - Removed an unused parameter from an internal function
  - Fixed function documentation

Changes from the fifth version of the patch:

  - Fixed function documentation in fs/anon_inodes.c and
    include/linux/lsm_hooks.h
  - Used anon_inode_getfd_secure() in userfaultfd() syscall and removed
    owner from userfaultfd_ctx.

[1] https://lore.kernel.org/lkml/20200211225547.235083-1-dancol@google.com/
[2] https://lore.kernel.org/linux-fsdevel/20200213194157.5877-1-sds@tycho.nsa.gov/
[3] https://lore.kernel.org/lkml/23f725ca-5b5a-5938-fcc8-5bbbfc9ba9bc@tycho.nsa.gov/

Daniel Colascione (3):
  Add a new LSM-supporting anonymous inode interface
  Teach SELinux about anonymous inodes
  Wire UFFD up to SELinux

 fs/anon_inodes.c                    | 193 ++++++++++++++++++++++------
 fs/userfaultfd.c                    |  23 ++--
 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 ++
 security/selinux/hooks.c            |  53 ++++++++
 security/selinux/include/classmap.h |   2 +
 9 files changed, 255 insertions(+), 50 deletions(-)

-- 
2.28.0.236.gb10cc79966-goog

[PATCH v6 1/3] Add a new LSM-supporting anonymous inode interface

[Lokesh Gidra]

From: Daniel Colascione <dancol@google.com>

This change adds two new functions, anon_inode_getfile_secure and
anon_inode_getfd_secure, that create anonymous-node files with
individual non-S_PRIVATE inodes 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 functions accept an optional context_inode parameter that
callers can use to provide additional contextual information to
security modules, e.g., indicating that one anonymous struct file is a
logical child of another, allowing a security model to propagate
security information from one to the other.

Signed-off-by: Daniel Colascione <dancol@google.com>

[Fix comment documenting return values of inode_init_security_anon()]
[Add context_inode description in comments to anon_inode_getfd_secure()
 and anon_inode_getfile_secure()]
[Make _anon_inode_getfile() static]
[Use correct error cast in _anon_inode_getfile()]

Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
---
 fs/anon_inodes.c              | 193 ++++++++++++++++++++++++++--------
 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, 186 insertions(+), 41 deletions(-)

diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
index 89714308c25b..4e09915fe847 100644
--- a/fs/anon_inodes.c
+++ b/fs/anon_inodes.c
@@ -55,61 +55,109 @@ 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);
+	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);
+		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;
+	}
 
-	/*
-	 * 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,
+	file = alloc_file_pseudo(inode, anon_inode_mnt, name,
 				 flags & (O_ACCMODE | O_NONBLOCK), fops);
 	if (IS_ERR(file))
 		goto err;
 
-	file->f_mapping = anon_inode_inode->i_mapping;
+	file->f_mapping = inode->i_mapping;
 
 	file->private_data = priv;
 
 	return file;
 
 err:
-	iput(anon_inode_inode);
+	iput(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_secure - creates a new file instance by hooking
+ *                             it up to a new anonymous inode and a
+ *                             dentry that describe the "class" of the
+ *                             file.  Make it possible to use security
+ *                             modules to control access to the
+ *                             new 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
+ * @context_inode [in]  inode for additional contextual info to security modules
+ *
+ * Creates a new file by hooking it on an unspecified inode. This is
+ * useful for files that do not need to have a full-fledged filesystem
+ * to operate correctly.  All the files created with
+ * anon_inode_getfile_secure() will have distinct inodes, avoiding
+ * code duplication for the file/inode/dentry setup.  Returns the
+ * newly created file* or an error pointer.
+ */
+struct file *anon_inode_getfile_secure(const char *name,
+				       const struct file_operations *fops,
+				       void *priv, int flags,
+				       const struct inode *context_inode)
+{
+	return _anon_inode_getfile(
+		name, fops, priv, flags, context_inode, true);
+}
+EXPORT_SYMBOL_GPL(anon_inode_getfile_secure);
+
+/**
+ * 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 +166,23 @@ 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 +192,8 @@ 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;
@@ -146,6 +206,58 @@ int anon_inode_getfd(const char *name, const struct file_operations *fops,
 	put_unused_fd(fd);
 	return error;
 }
+
+/**
+ * anon_inode_getfd_secure - creates a new file instance by hooking it
+ *                           up to a new anonymous inode and a dentry
+ *                           that describe the "class" of the file.
+ *                           Make it possible to use security modules
+ *                           to control access to the new 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
+ * @context_inode [in]  inode for additional contextual info to security modules
+ *
+ * Creates a new file by hooking it on an unspecified inode. This is
+ * useful for files that do not need to have a full-fledged filesystem
+ * to operate correctly.  All the files created with
+ * anon_inode_getfile_secure() will have distinct inodes, avoiding
+ * code duplication for the file/inode/dentry setup.  Returns a newly
+ * created file descriptor or an error code.
+ */
+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);
+
+/**
+ * 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
+ * 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);
 
 static int __init anon_inode_init(void)
@@ -162,4 +274,3 @@ static int __init anon_inode_init(void)
 }
 
 fs_initcall(anon_inode_init);
-
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);
 
 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);
 
+
 #endif /* _LINUX_ANON_INODES_H */
 
diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index af998f93d256..613a3d6306c2 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -113,6 +113,8 @@ LSM_HOOK(void, LSM_RET_VOID, inode_free_security, struct inode *inode)
 LSM_HOOK(int, 0, inode_init_security, struct inode *inode,
 	 struct inode *dir, const struct qstr *qstr, const char **name,
 	 void **value, size_t *len)
+LSM_HOOK(int, 0, inode_init_security_anon, struct inode *inode,
+	 const struct qstr *name, const struct inode *context_inode)
 LSM_HOOK(int, 0, inode_create, struct inode *dir, struct dentry *dentry,
 	 umode_t mode)
 LSM_HOOK(int, 0, inode_link, struct dentry *old_dentry, struct inode *dir,
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 95b7c1d32062..22847380c26c 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -233,6 +233,13 @@
  *	Returns 0 if @name and @value have been successfully set,
  *	-EOPNOTSUPP if no security attribute is needed, or
  *	-ENOMEM on memory allocation failure.
+ * @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.
  * @inode_create:
  *	Check permission to create a regular file.
  *	@dir contains inode structure of the parent of the new file.
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);
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);
+}
+
 int security_old_inode_init_security(struct inode *inode, struct inode *dir,
 				     const struct qstr *qstr, const char **name,
 				     void **value, size_t *len)
-- 
2.28.0.236.gb10cc79966-goog

[PATCH v6 2/3] Teach SELinux about anonymous inodes

[Lokesh Gidra]

From: Daniel Colascione <dancol@google.com>

This change uses the anon_inodes and LSM infrastructure introduced in
the previous patch to give SELinux the ability to control
anonymous-inode files that are created using the new _secure()
anon_inodes functions.

A SELinux policy author detects and controls these anonymous inodes by
adding a name-based type_transition rule that assigns a new security
type to anonymous-inode files created in some domain. The name used
for the name-based transition is the name associated with the
anonymous inode for file listings --- e.g., "[userfaultfd]" or
"[perf_event]".

Example:

type uffd_t;
type_transition sysadm_t sysadm_t : anon_inode uffd_t "[userfaultfd]";
allow sysadm_t uffd_t:anon_inode { create };

(The next patch in this series is necessary for making userfaultfd
support this new interface.  The example above is just
for exposition.)

Signed-off-by: Daniel Colascione <dancol@google.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Acked-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: James Morris <jmorris@namei.org>
---
 security/selinux/hooks.c            | 53 +++++++++++++++++++++++++++++
 security/selinux/include/classmap.h |  2 ++
 2 files changed, 55 insertions(+)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index ca901025802a..5b403ad44aad 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2926,6 +2926,58 @@ static int selinux_inode_init_security(struct inode *inode, struct inode *dir,
 	return 0;
 }
 
+static int selinux_inode_init_security_anon(struct inode *inode,
+					    const struct qstr *name,
+					    const struct inode *context_inode)
+{
+	const struct task_security_struct *tsec = selinux_cred(current_cred());
+	struct common_audit_data ad;
+	struct inode_security_struct *isec;
+	int rc;
+
+	if (unlikely(!selinux_state.initialized))
+		return 0;
+
+	isec = selinux_inode(inode);
+
+	/*
+	 * We only get here once per ephemeral inode.  The inode has
+	 * been initialized via inode_alloc_security but is otherwise
+	 * untouched.
+	 */
+
+	if (context_inode) {
+		struct inode_security_struct *context_isec =
+			selinux_inode(context_inode);
+		isec->sclass = context_isec->sclass;
+		isec->sid = context_isec->sid;
+	} else {
+		isec->sclass = SECCLASS_ANON_INODE;
+		rc = security_transition_sid(
+			&selinux_state, tsec->sid, tsec->sid,
+			isec->sclass, name, &isec->sid);
+		if (rc)
+			return rc;
+	}
+
+	isec->initialized = LABEL_INITIALIZED;
+
+	/*
+	 * Now that we've initialized security, check whether we're
+	 * allowed to actually create this type of anonymous inode.
+	 */
+
+	ad.type = LSM_AUDIT_DATA_INODE;
+	ad.u.inode = inode;
+
+	return avc_has_perm(&selinux_state,
+			    tsec->sid,
+			    isec->sid,
+			    isec->sclass,
+			    FILE__CREATE,
+			    &ad);
+}
+
 static int selinux_inode_create(struct inode *dir, struct dentry *dentry, umode_t mode)
 {
 	return may_create(dir, dentry, SECCLASS_FILE);
@@ -6993,6 +7045,7 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = {
 
 	LSM_HOOK_INIT(inode_free_security, selinux_inode_free_security),
 	LSM_HOOK_INIT(inode_init_security, selinux_inode_init_security),
+	LSM_HOOK_INIT(inode_init_security_anon, selinux_inode_init_security_anon),
 	LSM_HOOK_INIT(inode_create, selinux_inode_create),
 	LSM_HOOK_INIT(inode_link, selinux_inode_link),
 	LSM_HOOK_INIT(inode_unlink, selinux_inode_unlink),
diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
index 40cebde62856..ba2e01a6955c 100644
--- a/security/selinux/include/classmap.h
+++ b/security/selinux/include/classmap.h
@@ -249,6 +249,8 @@ struct security_class_mapping secclass_map[] = {
 	  {"open", "cpu", "kernel", "tracepoint", "read", "write"} },
 	{ "lockdown",
 	  { "integrity", "confidentiality", NULL } },
+	{ "anon_inode",
+	  { COMMON_FILE_PERMS, NULL } },
 	{ NULL }
   };
 
-- 
2.28.0.236.gb10cc79966-goog

[PATCH v6 3/3] Wire UFFD up to SELinux

[Lokesh Gidra]

From: Daniel Colascione <dancol@google.com>

This change gives userfaultfd file descriptors a real security
context, allowing policy to act on them.

Signed-off-by: Daniel Colascione <dancol@google.com>

[Remove owner inode from userfaultfd_ctx]
[Use anon_inode_getfd_secure() instead of anon_inode_getfile_secure()
 in userfaultfd syscall]
[Use inode of file in userfaultfd_read() in resolve_userfault_fork()]

Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
---
 fs/userfaultfd.c | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 6e264dded46e..23c8618ebe35 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -978,14 +978,16 @@ static __poll_t userfaultfd_poll(struct file *file, poll_table *wait)
 
 static const struct file_operations userfaultfd_fops;
 
-static int resolve_userfault_fork(struct userfaultfd_ctx *ctx,
-				  struct userfaultfd_ctx *new,
+static int resolve_userfault_fork(struct userfaultfd_ctx *new,
+				  struct inode *inode,
 				  struct uffd_msg *msg)
 {
 	int fd;
 
-	fd = anon_inode_getfd("[userfaultfd]", &userfaultfd_fops, new,
-			      O_RDWR | (new->flags & UFFD_SHARED_FCNTL_FLAGS));
+	fd = anon_inode_getfd_secure(
+		"[userfaultfd]", &userfaultfd_fops, new,
+		O_RDWR | (new->flags & UFFD_SHARED_FCNTL_FLAGS),
+		inode);
 	if (fd < 0)
 		return fd;
 
@@ -995,7 +997,7 @@ static int resolve_userfault_fork(struct userfaultfd_ctx *ctx,
 }
 
 static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
-				    struct uffd_msg *msg)
+				    struct uffd_msg *msg, struct inode *inode)
 {
 	ssize_t ret;
 	DECLARE_WAITQUEUE(wait, current);
@@ -1106,7 +1108,7 @@ static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
 	spin_unlock_irq(&ctx->fd_wqh.lock);
 
 	if (!ret && msg->event == UFFD_EVENT_FORK) {
-		ret = resolve_userfault_fork(ctx, fork_nctx, msg);
+		ret = resolve_userfault_fork(fork_nctx, inode, msg);
 		spin_lock_irq(&ctx->event_wqh.lock);
 		if (!list_empty(&fork_event)) {
 			/*
@@ -1166,6 +1168,7 @@ static ssize_t userfaultfd_read(struct file *file, char __user *buf,
 	ssize_t _ret, ret = 0;
 	struct uffd_msg msg;
 	int no_wait = file->f_flags & O_NONBLOCK;
+	struct inode *inode = file_inode(file);
 
 	if (ctx->state == UFFD_STATE_WAIT_API)
 		return -EINVAL;
@@ -1173,7 +1176,7 @@ static ssize_t userfaultfd_read(struct file *file, char __user *buf,
 	for (;;) {
 		if (count < sizeof(msg))
 			return ret ? ret : -EINVAL;
-		_ret = userfaultfd_ctx_read(ctx, no_wait, &msg);
+		_ret = userfaultfd_ctx_read(ctx, no_wait, &msg, inode);
 		if (_ret < 0)
 			return ret ? ret : _ret;
 		if (copy_to_user((__u64 __user *) buf, &msg, sizeof(msg)))
@@ -1995,8 +1998,10 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
 	/* prevent the mm struct to be freed */
 	mmgrab(ctx->mm);
 
-	fd = anon_inode_getfd("[userfaultfd]", &userfaultfd_fops, ctx,
-			      O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS));
+	fd = anon_inode_getfd_secure("[userfaultfd]",
+		&userfaultfd_fops, ctx,
+		O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS),
+		NULL);
 	if (fd < 0) {
 		mmdrop(ctx->mm);
 		kmem_cache_free(userfaultfd_ctx_cachep, ctx);
-- 
2.28.0.236.gb10cc79966-goog

Re: [PATCH v6 1/3] Add a new LSM-supporting anonymous inode interface

[Al Viro]

On Fri, Aug 07, 2020 at 03:49:39PM -0700, Lokesh Gidra wrote:

> The new functions accept an optional context_inode parameter that
> callers can use to provide additional contextual information to
> security modules, e.g., indicating that one anonymous struct file is a
> logical child of another, allowing a security model to propagate
> security information from one to the other.

What the hell is "logical child" and what are the lifetime rules implied
by that relationship?

Re: [PATCH v6 1/3] Add a new LSM-supporting anonymous inode interface

[Lokesh Gidra]

On Fri, Aug 7, 2020 at 4:02 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> On Fri, Aug 07, 2020 at 03:49:39PM -0700, Lokesh Gidra wrote:
>
> > The new functions accept an optional context_inode parameter that
> > callers can use to provide additional contextual information to
> > security modules, e.g., indicating that one anonymous struct file is a
> > logical child of another, allowing a security model to propagate
> > security information from one to the other.
>
> What the hell is "logical child" and what are the lifetime rules implied
> by that relationship?

context_inode provides the security context required by the security
modules for granting/denying permission to create an anon inode of the
same type.

In case of userfaultfd, the relationship between the context_inode and
the created inode is described as that of ‘logical child’ because the
context_inode (userfaultfd inode of the parent process) provides the
security context required for creation of child process’ userfaultfd
inode. But there is no relationship beyond this point. Therefore, no
reference to context_inode is held anywhere.

Re: [PATCH v6 0/3] SELinux support for anonymous inodes and UFFD

[James Morris]

On Fri, 7 Aug 2020, Lokesh Gidra wrote:

> Userfaultfd in unprivileged contexts could be potentially very
> useful. We'd like to harden userfaultfd to make such unprivileged use
> less risky. This patch series allows SELinux to manage userfaultfd
> file descriptors and in the future, other kinds of
> anonymous-inode-based file descriptor.  SELinux policy authors can
> apply policy types to anonymous inodes by providing name-based
> transition rules keyed off the anonymous inode internal name (
> "[userfaultfd]" in the case of userfaultfd(2) file descriptors) and
> applying policy to the new SIDs thus produced.

Can you expand more on why this would be useful, e.g. use-cases?


-- 
James Morris
<jmorris@namei.org>

Re: [PATCH v6 0/3] SELinux support for anonymous inodes and UFFD

[Lokesh Gidra]

On Thu, Aug 20, 2020 at 11:36 AM James Morris <jmorris@namei.org> wrote:
>
> On Fri, 7 Aug 2020, Lokesh Gidra wrote:
>
> > Userfaultfd in unprivileged contexts could be potentially very
> > useful. We'd like to harden userfaultfd to make such unprivileged use
> > less risky. This patch series allows SELinux to manage userfaultfd
> > file descriptors and in the future, other kinds of
> > anonymous-inode-based file descriptor.  SELinux policy authors can
> > apply policy types to anonymous inodes by providing name-based
> > transition rules keyed off the anonymous inode internal name (
> > "[userfaultfd]" in the case of userfaultfd(2) file descriptors) and
> > applying policy to the new SIDs thus produced.
>
> Can you expand more on why this would be useful, e.g. use-cases?
>
With SELinux managed userfaultfd file descriptors, an administrator
can control creation and movement of them. In particular, handling of
a userfaultfd descriptor by a different process is essentially a
ptrace access into the process, without any of the
corresponding security_ptrace_access_check() checks. For privacy, the
admin may want to deny such accesses,
which is possible with SELinux support.

I'll add this use case in the cover letter too in the next version.