AppArmor: contexts used in attaching policy to system objects

AppArmor contexts attach profiles and state to tasks, files, etc. when
a direct profile reference is not sufficient.
Signed-off-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: James Morris <jmorris@namei.org>

security/apparmor/context.c

+/*
+ * AppArmor security module
+ *
+ * This file contains AppArmor functions used to manipulate object security
+ * contexts.
+ *
+ * Copyright (C) 1998-2008 Novell/SUSE
+ * Copyright 2009-2010 Canonical Ltd.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation, version 2 of the
+ * License.
+ *
+ *
+ * AppArmor sets confinement on every task, via the the aa_task_cxt and
+ * the aa_task_cxt.profile, both of which are required and are not allowed
+ * to be NULL.  The aa_task_cxt is not reference counted and is unique
+ * to each cred (which is reference count).  The profile pointed to by
+ * the task_cxt is reference counted.
+ *
+ * TODO
+ * If a task uses change_hat it currently does not return to the old
+ * cred or task context but instead creates a new one.  Ideally the task
+ * should return to the previous cred if it has not been modified.
+ *
+ */
+
+#include "include/context.h"
+#include "include/policy.h"
+
+/**
+ * aa_alloc_task_context - allocate a new task_cxt
+ * @flags: gfp flags for allocation
+ *
+ * Returns: allocated buffer or NULL on failure
+ */
+struct aa_task_cxt *aa_alloc_task_context(gfp_t flags)
+{
+	return kzalloc(sizeof(struct aa_task_cxt), flags);
+}
+
+/**
+ * aa_free_task_context - free a task_cxt
+ * @cxt: task_cxt to free (MAYBE NULL)
+ */
+void aa_free_task_context(struct aa_task_cxt *cxt)
+{
+	if (cxt) {
+		aa_put_profile(cxt->profile);
+		aa_put_profile(cxt->previous);
+		aa_put_profile(cxt->onexec);
+
+		kzfree(cxt);
+	}
+}
+
+/**
+ * aa_dup_task_context - duplicate a task context, incrementing reference counts
+ * @new: a blank task context      (NOT NULL)
+ * @old: the task context to copy  (NOT NULL)
+ */
+void aa_dup_task_context(struct aa_task_cxt *new, const struct aa_task_cxt *old)
+{
+	*new = *old;
+	aa_get_profile(new->profile);
+	aa_get_profile(new->previous);
+	aa_get_profile(new->onexec);
+}
+
+/**
+ * aa_replace_current_profile - replace the current tasks profiles
+ * @profile: new profile  (NOT NULL)
+ *
+ * Returns: 0 or error on failure
+ */
+int aa_replace_current_profile(struct aa_profile *profile)
+{
+	struct aa_task_cxt *cxt = current_cred()->security;
+	struct cred *new;
+	BUG_ON(!profile);
+
+	if (cxt->profile == profile)
+		return 0;
+
+	new  = prepare_creds();
+	if (!new)
+		return -ENOMEM;
+
+	cxt = new->security;
+	if (unconfined(profile) || (cxt->profile->ns != profile->ns)) {
+		/* if switching to unconfined or a different profile namespace
+		 * clear out context state
+		 */
+		aa_put_profile(cxt->previous);
+		aa_put_profile(cxt->onexec);
+		cxt->previous = NULL;
+		cxt->onexec = NULL;
+		cxt->token = 0;
+	}
+	/* be careful switching cxt->profile, when racing replacement it
+	 * is possible that cxt->profile->replacedby is the reference keeping
+	 * @profile valid, so make sure to get its reference before dropping
+	 * the reference on cxt->profile */
+	aa_get_profile(profile);
+	aa_put_profile(cxt->profile);
+	cxt->profile = profile;
+
+	commit_creds(new);
+	return 0;
+}
+
+/**
+ * aa_set_current_onexec - set the tasks change_profile to happen onexec
+ * @profile: system profile to set at exec  (MAYBE NULL to clear value)
+ *
+ * Returns: 0 or error on failure
+ */
+int aa_set_current_onexec(struct aa_profile *profile)
+{
+	struct aa_task_cxt *cxt;
+	struct cred *new = prepare_creds();
+	if (!new)
+		return -ENOMEM;
+
+	cxt = new->security;
+	aa_get_profile(profile);
+	aa_put_profile(cxt->onexec);
+	cxt->onexec = profile;
+
+	commit_creds(new);
+	return 0;
+}
+
+/**
+ * aa_set_current_hat - set the current tasks hat
+ * @profile: profile to set as the current hat  (NOT NULL)
+ * @token: token value that must be specified to change from the hat
+ *
+ * Do switch of tasks hat.  If the task is currently in a hat
+ * validate the token to match.
+ *
+ * Returns: 0 or error on failure
+ */
+int aa_set_current_hat(struct aa_profile *profile, u64 token)
+{
+	struct aa_task_cxt *cxt;
+	struct cred *new = prepare_creds();
+	if (!new)
+		return -ENOMEM;
+	BUG_ON(!profile);
+
+	cxt = new->security;
+	if (!cxt->previous) {
+		/* transfer refcount */
+		cxt->previous = cxt->profile;
+		cxt->token = token;
+	} else if (cxt->token == token) {
+		aa_put_profile(cxt->profile);
+	} else {
+		/* previous_profile && cxt->token != token */
+		abort_creds(new);
+		return -EACCES;
+	}
+	cxt->profile = aa_get_profile(aa_newest_version(profile));
+	/* clear exec on switching context */
+	aa_put_profile(cxt->onexec);
+	cxt->onexec = NULL;
+
+	commit_creds(new);
+	return 0;
+}
+
+/**
+ * aa_restore_previous_profile - exit from hat context restoring the profile
+ * @token: the token that must be matched to exit hat context
+ *
+ * Attempt to return out of a hat to the previous profile.  The token
+ * must match the stored token value.
+ *
+ * Returns: 0 or error of failure
+ */
+int aa_restore_previous_profile(u64 token)
+{
+	struct aa_task_cxt *cxt;
+	struct cred *new = prepare_creds();
+	if (!new)
+		return -ENOMEM;
+
+	cxt = new->security;
+	if (cxt->token != token) {
+		abort_creds(new);
+		return -EACCES;
+	}
+	/* ignore restores when there is no saved profile */
+	if (!cxt->previous) {
+		abort_creds(new);
+		return 0;
+	}
+
+	aa_put_profile(cxt->profile);
+	cxt->profile = aa_newest_version(cxt->previous);
+	BUG_ON(!cxt->profile);
+	if (unlikely(cxt->profile != cxt->previous)) {
+		aa_get_profile(cxt->profile);
+		aa_put_profile(cxt->previous);
+	}
+	/* clear exec && prev information when restoring to previous context */
+	cxt->previous = NULL;
+	cxt->token = 0;
+	aa_put_profile(cxt->onexec);
+	cxt->onexec = NULL;
+
+	commit_creds(new);
+	return 0;
+}

security/apparmor/include/context.h

+/*
+ * AppArmor security module
+ *
+ * This file contains AppArmor contexts used to associate "labels" to objects.
+ *
+ * Copyright (C) 1998-2008 Novell/SUSE
+ * Copyright 2009-2010 Canonical Ltd.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation, version 2 of the
+ * License.
+ */
+
+#ifndef __AA_CONTEXT_H
+#define __AA_CONTEXT_H
+
+#include <linux/cred.h>
+#include <linux/slab.h>
+#include <linux/sched.h>
+
+#include "policy.h"
+
+/* struct aa_file_cxt - the AppArmor context the file was opened in
+ * @perms: the permission the file was opened with
+ *
+ * The file_cxt could currently be directly stored in file->f_security
+ * as the profile reference is now stored in the f_cred.  However the
+ * cxt struct will expand in the future so we keep the struct.
+ */
+struct aa_file_cxt {
+	u16 allow;
+};
+
+/**
+ * aa_alloc_file_context - allocate file_cxt
+ * @gfp: gfp flags for allocation
+ *
+ * Returns: file_cxt or NULL on failure
+ */
+static inline struct aa_file_cxt *aa_alloc_file_context(gfp_t gfp)
+{
+	return kzalloc(sizeof(struct aa_file_cxt), gfp);
+}
+
+/**
+ * aa_free_file_context - free a file_cxt
+ * @cxt: file_cxt to free  (MAYBE_NULL)
+ */
+static inline void aa_free_file_context(struct aa_file_cxt *cxt)
+{
+	if (cxt)
+		kzfree(cxt);
+}
+
+/**
+ * struct aa_task_cxt - primary label for confined tasks
+ * @profile: the current profile   (NOT NULL)
+ * @exec: profile to transition to on next exec  (MAYBE NULL)
+ * @previous: profile the task may return to     (MAYBE NULL)
+ * @token: magic value the task must know for returning to @previous_profile
+ *
+ * Contains the task's current profile (which could change due to
+ * change_hat).  Plus the hat_magic needed during change_hat.
+ *
+ * TODO: make so a task can be confined by a stack of contexts
+ */
+struct aa_task_cxt {
+	struct aa_profile *profile;
+	struct aa_profile *onexec;
+	struct aa_profile *previous;
+	u64 token;
+};
+
+struct aa_task_cxt *aa_alloc_task_context(gfp_t flags);
+void aa_free_task_context(struct aa_task_cxt *cxt);
+void aa_dup_task_context(struct aa_task_cxt *new,
+			 const struct aa_task_cxt *old);
+int aa_replace_current_profile(struct aa_profile *profile);
+int aa_set_current_onexec(struct aa_profile *profile);
+int aa_set_current_hat(struct aa_profile *profile, u64 token);
+int aa_restore_previous_profile(u64 cookie);
+
+/**
+ * __aa_task_is_confined - determine if @task has any confinement
+ * @task: task to check confinement of  (NOT NULL)
+ *
+ * If @task != current needs to be called in RCU safe critical section
+ */
+static inline bool __aa_task_is_confined(struct task_struct *task)
+{
+	struct aa_task_cxt *cxt = __task_cred(task)->security;
+
+	BUG_ON(!cxt || !cxt->profile);
+	if (unconfined(aa_newest_version(cxt->profile)))
+		return 0;
+
+	return 1;
+}
+
+/**
+ * aa_cred_profile - obtain cred's profiles
+ * @cred: cred to obtain profiles from  (NOT NULL)
+ *
+ * Returns: confining profile
+ *
+ * does NOT increment reference count
+ */
+static inline struct aa_profile *aa_cred_profile(const struct cred *cred)
+{
+	struct aa_task_cxt *cxt = cred->security;
+	BUG_ON(!cxt || !cxt->profile);
+	return aa_newest_version(cxt->profile);
+}
+
+/**
+ * __aa_current_profile - find the current tasks confining profile
+ *
+ * Returns: up to date confining profile or the ns unconfined profile (NOT NULL)
+ *
+ * This fn will not update the tasks cred to the most up to date version
+ * of the profile so it is safe to call when inside of locks.
+ */
+static inline struct aa_profile *__aa_current_profile(void)
+{
+	return aa_cred_profile(current_cred());
+}
+
+/**
+ * aa_current_profile - find the current tasks confining profile and do updates
+ *
+ * Returns: up to date confining profile or the ns unconfined profile (NOT NULL)
+ *
+ * This fn will update the tasks cred structure if the profile has been
+ * replaced.  Not safe to call inside locks
+ */
+static inline struct aa_profile *aa_current_profile(void)
+{
+	const struct aa_task_cxt *cxt = current_cred()->security;
+	struct aa_profile *profile;
+	BUG_ON(!cxt || !cxt->profile);
+
+	profile = aa_newest_version(cxt->profile);
+	/*
+	 * Whether or not replacement succeeds, use newest profile so
+	 * there is no need to update it after replacement.
+	 */
+	if (unlikely((cxt->profile != profile)))
+		aa_replace_current_profile(profile);
+
+	return profile;
+}
+
+#endif /* __AA_CONTEXT_H */