[PATCH v3 2/3] LSM: module hierarchy in /proc/.../attr

[Casey Schaufler]

Subject: [PATCH v3 2/3] LSM: module hierarchy in /proc/.../attr

Back in 2007 I made what turned out to be a rather serious
mistake in the implementation of the Smack security module.
The SELinux module used an interface in /proc to manipulate
the security context on processes. Rather than use a similar
interface, I used the same interface. The AppArmor team did
likewise. Now /proc/.../attr/current will tell you the
security "context" of the process, but it will be different
depending on the security module you're using. That hasn't
been a problem to date, as you can only have one module
that supports process attributes at a time. We are coming
up on a change to that, where multiple modules with process
attributes can be supported. (Not included here)

This patch provides a subdirectory in /proc/.../attr for
each of the security modules that use the LSM hooks
getprocattr() and setprocattr(). Each of the interfaces
used by a module are presented in the subdirectory. The
old interfaces remain and work the same as before.
User space code can begin migrating to the subdirectory
interfaces in anticipation of the time when what comes
from /proc/self/attr/current might not be what a runtime
wants.

The original implementation is by Kees Cook. The code
has been changed a bit to reflect changes in the direction
of the multiple concurrent module work, to be independent
of it, and to bring it up to date with the current tree.

Signed-off-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>

---
 Documentation/security/LSM.txt | 26 +++++++++---
 fs/proc/base.c                 | 91 +++++++++++++++++++++++++++++++++++++-----
 fs/proc/internal.h             |  1 +
 include/linux/security.h       | 15 ++++---
 security/security.c            | 31 ++++++++++++--
 5 files changed, 140 insertions(+), 24 deletions(-)

diff --git a/Documentation/security/LSM.txt b/Documentation/security/LSM.txt
index 3db7e67..125c489 100644
--- a/Documentation/security/LSM.txt
+++ b/Documentation/security/LSM.txt
@@ -16,11 +16,25 @@ MAC extensions, other extensions can be built using the LSM to provide
 specific changes to system operation when these tweaks are not available
 in the core functionality of Linux itself.
 
-Without a specific LSM built into the kernel, the default LSM will be the
-Linux capabilities system. Most LSMs choose to extend the capabilities
-system, building their checks on top of the defined capability hooks.
-For more details on capabilities, see capabilities(7) in the Linux
-man-pages project.
+The Linux capabilities modules will always be included. For more details
+on capabilities, see capabilities(7) in the Linux man-pages project.
+This may be followed by any number of "minor" modules and at most one
+"major" module.
+
+A list of the active security modules can be found by reading
+/sys/kernel/security/lsm. This is a comma separated list, and
+will always include the capability module. The list reflects the
+order in which checks are made. The capability module will always
+be first, followed by any "minor" modules (e.g. Yama) and then
+the one "major" module (e.g. SELinux) if there is one configured.
+
+Process attributes associated with "major" security modules should
+be accessed and maintained using the special files in the module
+specific subdirectories in /proc/.../attr. The attributes related
+to Smack would be found in /proc/.../attr/smack while the attributes
+for SELinux would be in /proc/.../attr/selinux. Using the files
+found directly in /proc/.../attr (e.g. current) should be avoided.
+These files remain as legacy interfaces.
 
 Based on https://lkml.org/lkml/2007/10/26/215,
 a new LSM is accepted into the kernel when its intent (a description of
@@ -31,4 +45,4 @@ that end users and distros can make a more informed decision about which
 LSMs suit their requirements.
 
 For extensive documentation on the available LSM hook interfaces, please
-see include/linux/security.h.
+see include/linux/lsm_hooks.h.
diff --git a/fs/proc/base.c b/fs/proc/base.c
index a11eb71..182bc28 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -131,9 +131,13 @@ struct pid_entry {
 #define REG(NAME, MODE, fops)				\
 	NOD(NAME, (S_IFREG|(MODE)), NULL, &fops, {})
 #define ONE(NAME, MODE, show)				\
-	NOD(NAME, (S_IFREG|(MODE)), 			\
+	NOD(NAME, (S_IFREG|(MODE)),			\
 		NULL, &proc_single_file_operations,	\
 		{ .proc_show = show } )
+#define ATTR(LSM, NAME, MODE)				\
+	NOD(NAME, (S_IFREG|(MODE)),			\
+		NULL, &proc_pid_attr_operations,	\
+		{ .lsm = LSM })
 
 /*
  * Count the number of hardlinks for the pid_entry table, excluding the .
@@ -2433,7 +2437,7 @@ static ssize_t proc_pid_attr_read(struct file * file, char __user * buf,
 	if (!task)
 		return -ESRCH;
 
-	length = security_getprocattr(task,
+	length = security_getprocattr(task, PROC_I(inode)->op.lsm,
 				      (char*)file->f_path.dentry->d_name.name,
 				      &p);
 	put_task_struct(task);
@@ -2473,7 +2477,7 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
 	if (length < 0)
 		goto out_free;
 
-	length = security_setprocattr(task,
+	length = security_setprocattr(task, PROC_I(inode)->op.lsm,
 				      (char*)file->f_path.dentry->d_name.name,
 				      page, count);
 	mutex_unlock(&task->signal->cred_guard_mutex);
@@ -2491,13 +2495,82 @@ static const struct file_operations proc_pid_attr_operations = {
 	.llseek		= generic_file_llseek,
 };
 
+#define LSM_DIR_OPS(LSM) \
+static int proc_##LSM##_attr_dir_iterate(struct file *filp, \
+			     struct dir_context *ctx) \
+{ \
+	return proc_pident_readdir(filp, ctx, \
+				   LSM##_attr_dir_stuff, \
+				   ARRAY_SIZE(LSM##_attr_dir_stuff)); \
+} \
+\
+static const struct file_operations proc_##LSM##_attr_dir_ops = { \
+	.read		= generic_read_dir, \
+	.iterate	= proc_##LSM##_attr_dir_iterate, \
+	.llseek		= default_llseek, \
+}; \
+\
+static struct dentry *proc_##LSM##_attr_dir_lookup(struct inode *dir, \
+				struct dentry *dentry, unsigned int flags) \
+{ \
+	return proc_pident_lookup(dir, dentry, \
+				  LSM##_attr_dir_stuff, \
+				  ARRAY_SIZE(LSM##_attr_dir_stuff)); \
+} \
+\
+static const struct inode_operations proc_##LSM##_attr_dir_inode_ops = { \
+	.lookup		= proc_##LSM##_attr_dir_lookup, \
+	.getattr	= pid_getattr, \
+	.setattr	= proc_setattr, \
+}
+
+#ifdef CONFIG_SECURITY_SELINUX
+static const struct pid_entry selinux_attr_dir_stuff[] = {
+	ATTR("selinux", "current",	S_IRUGO|S_IWUGO),
+	ATTR("selinux", "prev",		S_IRUGO),
+	ATTR("selinux", "exec",		S_IRUGO|S_IWUGO),
+	ATTR("selinux", "fscreate",	S_IRUGO|S_IWUGO),
+	ATTR("selinux", "keycreate",	S_IRUGO|S_IWUGO),
+	ATTR("selinux", "sockcreate",	S_IRUGO|S_IWUGO),
+};
+LSM_DIR_OPS(selinux);
+#endif
+
+#ifdef CONFIG_SECURITY_SMACK
+static const struct pid_entry smack_attr_dir_stuff[] = {
+	ATTR("smack", "current",	S_IRUGO|S_IWUGO),
+};
+LSM_DIR_OPS(smack);
+#endif
+
+#ifdef CONFIG_SECURITY_APPARMOR
+static const struct pid_entry apparmor_attr_dir_stuff[] = {
+	ATTR("apparmor", "current",	S_IRUGO|S_IWUGO),
+	ATTR("apparmor", "prev",	S_IRUGO),
+	ATTR("apparmor", "exec",	S_IRUGO|S_IWUGO),
+};
+LSM_DIR_OPS(apparmor);
+#endif
+
 static const struct pid_entry attr_dir_stuff[] = {
-	REG("current",    S_IRUGO|S_IWUGO, proc_pid_attr_operations),
-	REG("prev",       S_IRUGO,	   proc_pid_attr_operations),
-	REG("exec",       S_IRUGO|S_IWUGO, proc_pid_attr_operations),
-	REG("fscreate",   S_IRUGO|S_IWUGO, proc_pid_attr_operations),
-	REG("keycreate",  S_IRUGO|S_IWUGO, proc_pid_attr_operations),
-	REG("sockcreate", S_IRUGO|S_IWUGO, proc_pid_attr_operations),
+	ATTR(NULL, "current",		S_IRUGO|S_IWUGO),
+	ATTR(NULL, "prev",		S_IRUGO),
+	ATTR(NULL, "exec",		S_IRUGO|S_IWUGO),
+	ATTR(NULL, "fscreate",		S_IRUGO|S_IWUGO),
+	ATTR(NULL, "keycreate",		S_IRUGO|S_IWUGO),
+	ATTR(NULL, "sockcreate",	S_IRUGO|S_IWUGO),
+#ifdef CONFIG_SECURITY_SELINUX
+	DIR("selinux",			S_IRUGO|S_IXUGO,
+	    proc_selinux_attr_dir_inode_ops, proc_selinux_attr_dir_ops),
+#endif
+#ifdef CONFIG_SECURITY_SMACK
+	DIR("smack",			S_IRUGO|S_IXUGO,
+	    proc_smack_attr_dir_inode_ops, proc_smack_attr_dir_ops),
+#endif
+#ifdef CONFIG_SECURITY_APPARMOR
+	DIR("apparmor",			S_IRUGO|S_IXUGO,
+	    proc_apparmor_attr_dir_inode_ops, proc_apparmor_attr_dir_ops),
+#endif
 };
 
 static int proc_attr_dir_readdir(struct file *file, struct dir_context *ctx)
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index aa27810..b607cd5 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -56,6 +56,7 @@ union proc_op {
 	int (*proc_show)(struct seq_file *m,
 		struct pid_namespace *ns, struct pid *pid,
 		struct task_struct *task);
+	const char *lsm;
 };
 
 struct proc_inode {
diff --git a/include/linux/security.h b/include/linux/security.h
index 14df373..383fcb0 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -355,8 +355,10 @@ int security_sem_semctl(struct sem_array *sma, int cmd);
 int security_sem_semop(struct sem_array *sma, struct sembuf *sops,
 			unsigned nsops, int alter);
 void security_d_instantiate(struct dentry *dentry, struct inode *inode);
-int security_getprocattr(struct task_struct *p, char *name, char **value);
-int security_setprocattr(struct task_struct *p, char *name, void *value, size_t size);
+int security_getprocattr(struct task_struct *p, const char *lsm, char *name,
+				char **value);
+int security_setprocattr(struct task_struct *p, const char *lsm, char *name,
+				void *value, size_t size);
 int security_netlink_send(struct sock *sk, struct sk_buff *skb);
 int security_ismaclabel(const char *name);
 int security_secid_to_secctx(u32 secid, char **secdata, u32 *seclen);
@@ -1075,15 +1077,18 @@ static inline int security_sem_semop(struct sem_array *sma,
 	return 0;
 }
 
-static inline void security_d_instantiate(struct dentry *dentry, struct inode *inode)
+static inline void security_d_instantiate(struct dentry *dentry,
+						struct inode *inode)
 { }
 
-static inline int security_getprocattr(struct task_struct *p, char *name, char **value)
+static inline int security_getprocattr(struct task_struct *p, const char *lsm,
+					char *name, char **value)
 {
 	return -EINVAL;
 }
 
-static inline int security_setprocattr(struct task_struct *p, char *name, void *value, size_t size)
+static inline int security_setprocattr(struct task_struct *p, const char *lsm,
+					char *name, void *value, size_t size)
 {
 	return -EINVAL;
 }
diff --git a/security/security.c b/security/security.c
index dd792d5..41ac80d 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1183,14 +1183,37 @@ void security_d_instantiate(struct dentry *dentry, struct inode *inode)
 }
 EXPORT_SYMBOL(security_d_instantiate);
 
-int security_getprocattr(struct task_struct *p, char *name, char **value)
+int security_getprocattr(struct task_struct *p, const char *lsm, char *name,
+				char **value)
 {
-	return call_int_hook(getprocattr, -EINVAL, p, name, value);
+	struct security_hook_list *hp;
+	int rc = -EINVAL;
+
+
+	list_for_each_entry(hp, &security_hook_heads.getprocattr, list) {
+		if (lsm != NULL && strcmp(lsm, hp->lsm))
+			continue;
+		rc = hp->hook.getprocattr(p, name, value);
+		if (rc != -ENOENT)
+			return rc;
+	}
+	return -EINVAL;
 }
 
-int security_setprocattr(struct task_struct *p, char *name, void *value, size_t size)
+int security_setprocattr(struct task_struct *p, const char *lsm, char *name,
+				void *value, size_t size)
 {
-	return call_int_hook(setprocattr, -EINVAL, p, name, value, size);
+	struct security_hook_list *hp;
+	int rc = -EINVAL;
+
+	list_for_each_entry(hp, &security_hook_heads.setprocattr, list) {
+		if (lsm != NULL && strcmp(lsm, hp->lsm))
+			continue;
+		rc = hp->hook.setprocattr(p, name, value, size);
+		if (rc != -ENOENT)
+			break;
+	}
+	return rc;
 }
 
 int security_netlink_send(struct sock *sk, struct sk_buff *skb)