[PATCH v8 0/3] SELinux support for anonymous inodes and UFFD

[PATCH v8 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.

With SELinux managed userfaultfd, an admin can control creation and
movement of the file descriptors. 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.

Inside the kernel, a new anon_inode interface, anon_inode_getfd_secure,
allows callers to opt into this SELinux management. In this new "secure"
mode, anon_inodes create 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.

Changed from the sixth version of the patch:

  - Removed definition of anon_inode_getfile_secure() as there are no
    callers.
  - Simplified function description of anon_inode_getfd_secure().
  - Elaborated more on the purpose of 'context_inode' in commit message.

Changed from the seventh version of the patch:

  - Fixed error handling in _anon_inode_getfile().
  - Fixed minor comment and indentation related issues.

[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                    | 147 ++++++++++++++++++++--------
 fs/userfaultfd.c                    |  19 ++--
 include/linux/anon_inodes.h         |   8 ++
 include/linux/lsm_hook_defs.h       |   2 +
 include/linux/lsm_hooks.h           |   9 ++
 include/linux/security.h            |  10 ++
 security/security.c                 |   8 ++
 security/selinux/hooks.c            |  53 ++++++++++
 security/selinux/include/classmap.h |   2 +
 9 files changed, 209 insertions(+), 49 deletions(-)

-- 
2.28.0.297.g1956fa8f8d-goog

[PATCH v8 1/3] Add a new LSM-supporting anonymous inode interface

[Lokesh Gidra]

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 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 <dancol@google.com>

[Fix comment documenting return values of inode_init_security_anon()]
[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 <lokeshgidra@google.com>
---
 fs/anon_inodes.c              | 147 +++++++++++++++++++++++++---------
 include/linux/anon_inodes.h   |   8 ++
 include/linux/lsm_hook_defs.h |   2 +
 include/linux/lsm_hooks.h     |   9 +++
 include/linux/security.h      |  10 +++
 security/security.c           |   8 ++
 6 files changed, 144 insertions(+), 40 deletions(-)

diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
index 89714308c25b..c3f16deda211 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 +176,45 @@ 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 adds the @context_inode argument to
+ * allow security modules to control creation of the new file. Once the
+ * security module makes the decision, this inode is no longer needed
+ * and hence 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/include/linux/anon_inodes.h b/include/linux/anon_inodes.h
index d0d7d96261ad..6ab840ee93bc 100644
--- a/include/linux/anon_inodes.h
+++ b/include/linux/anon_inodes.h
@@ -10,12 +10,20 @@
 #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);
 
+
 #endif /* _LINUX_ANON_INODES_H */
 
diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 2a8c74d99015..35ff75c43de4 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 9e2e3e63719d..586186f1184b 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -233,6 +233,15 @@
  *	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 the incore security field for the new anonymous inode
+ *      and return whether the inode creation is permitted by the security
+ *      module or not.
+ *      @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..7c6b3dcf4721 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);
@@ -732,6 +735,13 @@ static inline int security_inode_init_security(struct inode *inode,
 	return 0;
 }
 
+static inline int security_inode_init_security_anon(struct inode *inode,
+						    const struct qstr *name,
+						    const struct inode *context_inode)
+{
+	return 0;
+}
+
 static inline int security_old_inode_init_security(struct inode *inode,
 						   struct inode *dir,
 						   const struct qstr *qstr,
diff --git a/security/security.c b/security/security.c
index 70a7ad357bc6..2c4b121a01b9 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1057,6 +1057,14 @@ 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)
-- 
2.28.0.297.g1956fa8f8d-goog

[PATCH v8 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 anon_inode_getfd_secure()
function.

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>
---
 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 a340986aa92e..b83f56e5ef40 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);
@@ -6987,6 +7039,7 @@ 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 }
   };
 
-- 
2.28.0.297.g1956fa8f8d-goog

[PATCH v8 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 | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 0e4a3837da52..918535b49475 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -978,14 +978,14 @@ 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 +995,7 @@ 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 +1106,7 @@ 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 +1166,7 @@ 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 +1174,7 @@ 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 +1996,8 @@ 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);
-- 
2.28.0.297.g1956fa8f8d-goog

Re: [PATCH v8 2/3] Teach SELinux about anonymous inodes

[Stephen Smalley]

On Thu, Aug 27, 2020 at 2:35 AM Lokesh Gidra <lokeshgidra@google.com> wrote:
>
> 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 anon_inode_getfd_secure()
> function.
>
> 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>
> ---
>  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 a340986aa92e..b83f56e5ef40 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))

This should use selinux_initialized(&selinux_state) instead.

Re: [PATCH v8 2/3] Teach SELinux about anonymous inodes

[Lokesh Gidra]

On Mon, Aug 31, 2020 at 11:05 AM Stephen Smalley
<stephen.smalley.work@gmail.com> wrote:
>
> On Thu, Aug 27, 2020 at 2:35 AM Lokesh Gidra <lokeshgidra@google.com> wrote:
> >
> > 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 anon_inode_getfd_secure()
> > function.
> >
> > 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>
> > ---
> >  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 a340986aa92e..b83f56e5ef40 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))
>
> This should use selinux_initialized(&selinux_state) instead.

Thanks for the review. I'll make the change in the next version.

Kindly have a look at other patches in the series as well.

Re: [PATCH v8 1/3] Add a new LSM-supporting anonymous inode interface

[Christian Brauner]

On Wed, Aug 26, 2020 at 11:35:20PM -0700, Lokesh Gidra 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 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 <dancol@google.com>
> 
> [Fix comment documenting return values of inode_init_security_anon()]
> [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 <lokeshgidra@google.com>
> ---
>  fs/anon_inodes.c              | 147 +++++++++++++++++++++++++---------
>  include/linux/anon_inodes.h   |   8 ++
>  include/linux/lsm_hook_defs.h |   2 +
>  include/linux/lsm_hooks.h     |   9 +++
>  include/linux/security.h      |  10 +++
>  security/security.c           |   8 ++
>  6 files changed, 144 insertions(+), 40 deletions(-)
> 
> diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
> index 89714308c25b..c3f16deda211 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;
> +}

Hey,

Iiuc, this makes each newly created anon inode fd correspond to a unique
file and to a unique inode:

fd1 -> file1 -> inode1
fd2 -> file2 -> inode2

Whereas before we had every anon inode fd correspond to a unique file
but all files map to the _same_ inode:

fd1 -> file1 -> inode
fd2 -> file2 -> inode

The old behavior of hooking up a each anon inode fd to the same inode
prevented having an evict method attached to the inode. Because it was
shared that wasn't possible but also simply because that inode never got
evicted anyway. That surely was intended but it's a bummer to some
extent.
With the new model you also can't have an evict method because now you
have a separate inode for each file.

I'm probably going to get killed for suggesting this but:
If we're going to expand the anonymous inode infrastructure anyway is
there a way we can make it so that we have a way to allocate a single
inode for multiple anonymous inode fds and have callers opt-in to this
behavior. We'd need a way to find this inode again, obviously.

This would allow for some features on top of anonymous inode fds that
can refer to the same object, i.e. anonymous inode fds that currently
stash away the same object in f->private_data.
In such a model we could allow such anonymous inode fds to stash away
objects in inode->i_private instead of f->private_data and attach an
evict method to it. This would e.g. allow a process to be killed when
the last pidfd to it is closed or a seccomp notifier fd to notify when
the filter is released without having to do separate reference counting.

This would need a way to lookup that inode by the object that is stashed
away in it of course which could probably be done by an idr or an
xarray or something cleverer. It would obviously only affect a subset of
anonymous inode fds so any other anonymous inode fds wouldn't be
impacted since they can still use the single-anon-inode interface.

Christian

Re: [PATCH v8 1/3] Add a new LSM-supporting anonymous inode interface

[Lokesh Gidra]

On Tue, Sep 1, 2020 at 5:41 AM Christian Brauner
<christian.brauner@ubuntu.com> wrote:
>
> On Wed, Aug 26, 2020 at 11:35:20PM -0700, Lokesh Gidra 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 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 <dancol@google.com>
> >
> > [Fix comment documenting return values of inode_init_security_anon()]
> > [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 <lokeshgidra@google.com>
> > ---
> >  fs/anon_inodes.c              | 147 +++++++++++++++++++++++++---------
> >  include/linux/anon_inodes.h   |   8 ++
> >  include/linux/lsm_hook_defs.h |   2 +
> >  include/linux/lsm_hooks.h     |   9 +++
> >  include/linux/security.h      |  10 +++
> >  security/security.c           |   8 ++
> >  6 files changed, 144 insertions(+), 40 deletions(-)
> >
> > diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
> > index 89714308c25b..c3f16deda211 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;
> > +}
>
> Hey,
>
> Iiuc, this makes each newly created anon inode fd correspond to a unique
> file and to a unique inode:
>
> fd1 -> file1 -> inode1
> fd2 -> file2 -> inode2
>
Not every anon inode. Just the ones created through
anon_inode_getfd_secure() API.

> Whereas before we had every anon inode fd correspond to a unique file
> but all files map to the _same_ inode:
>
> fd1 -> file1 -> inode
> fd2 -> file2 -> inode
>
Thils is still the case if anon_inode_getfile() and/or
anon_inode_getfd() APIs are used.

> The old behavior of hooking up a each anon inode fd to the same inode
> prevented having an evict method attached to the inode. Because it was
> shared that wasn't possible but also simply because that inode never got
> evicted anyway. That surely was intended but it's a bummer to some
> extent.
> With the new model you also can't have an evict method because now you
> have a separate inode for each file.
>
> I'm probably going to get killed for suggesting this but:
> If we're going to expand the anonymous inode infrastructure anyway is
> there a way we can make it so that we have a way to allocate a single
> inode for multiple anonymous inode fds and have callers opt-in to this
> behavior. We'd need a way to find this inode again, obviously.
>
> This would allow for some features on top of anonymous inode fds that
> can refer to the same object, i.e. anonymous inode fds that currently
> stash away the same object in f->private_data.
> In such a model we could allow such anonymous inode fds to stash away
> objects in inode->i_private instead of f->private_data and attach an
> evict method to it. This would e.g. allow a process to be killed when
> the last pidfd to it is closed or a seccomp notifier fd to notify when
> the filter is released without having to do separate reference counting.
>
I didn't fully understand the example you gave and the role that evict
method will play in it. Can you please elaborate a bit more.

But, I'd like to point you to a previous discussion between Daniel
Colascione (the original contributor of this patch series) and Stephan
Smalley on the topic of inodes
https://lore.kernel.org/lkml/CAKOZuesUVSYJ6EjHFL3QyiWKVmyhm1fLp5Bm_SHjB3_s1gn08A@mail.gmail.com/

I agree with Daniel (see his replies in the thread link above) that a
separate inode per anon inode fd keeps the design simple, particularly
from the security context perspective.

> This would need a way to lookup that inode by the object that is stashed
> away in it of course which could probably be done by an idr or an
> xarray or something cleverer. It would obviously only affect a subset of
> anonymous inode fds so any other anonymous inode fds wouldn't be
> impacted since they can still use the single-anon-inode interface.
>
> Christian