[RFC][PATCH 2/2 v3] security: Add task_settimerslack/task_gettimerslack LSM hook

[John Stultz <john.stultz@linaro.org>]

As requested, this patch implements a task_settimerslack and
task_gettimerslack LSM hooks so that the /proc/<tid>/timerslack_ns
interface can have finer grained security policies applied to it.

I've kept the CAP_SYS_NICE check in the timerslack_ns_write/show
functions, as hiding it in the LSM hook seems too opaque, and doesn't
seem like a widely enough adopted practice.

Don't really know what I'm doing here, so close review would be
appreciated!

Cc: Kees Cook <keescook@chromium.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
CC: Arjan van de Ven <arjan@linux.intel.com>
Cc: Oren Laadan <orenl@cellrox.com>
Cc: Ruchi Kandoi <kandoiruchi@google.com>
Cc: Rom Lemarchand <romlem@android.com>
Cc: Todd Kjos <tkjos@google.com>
Cc: Colin Cross <ccross@android.com>
Cc: Nick Kralevich <nnk@google.com>
Cc: Dmitry Shmidt <dimitrysh@google.com>
Cc: Elliott Hughes <enh@google.com>
Cc: Android Kernel Team <kernel-team@android.com>
Cc: linux-security-module@vger.kernel.org
Cc: selinux@tycho.nsa.gov
Signed-off-by: John Stultz <john.stultz@linaro.org>
---
v2:
 * Initial swing at adding settimerslack LSM hook
v3:
 * Fix current/p switchup bug noted by NickK
 * Add gettimerslack hook suggested by NickK

 fs/proc/base.c            | 10 ++++++++++
 include/linux/lsm_hooks.h | 13 +++++++++++++
 include/linux/security.h  | 12 ++++++++++++
 security/security.c       | 14 ++++++++++++++
 security/selinux/hooks.c  | 12 ++++++++++++
 5 files changed, 61 insertions(+)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index c94abae..cc66aa8 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2286,6 +2286,12 @@ static ssize_t timerslack_ns_write(struct file *file, const char __user *buf,
 		goto out;
 	}
 
+	err = security_task_settimerslack(p, slack_ns);
+	if (err) {
+		count = err;
+		goto out;
+	}
+
 	task_lock(p);
 	if (slack_ns == 0)
 		p->timer_slack_ns = p->default_timer_slack_ns;
@@ -2314,6 +2320,10 @@ static int timerslack_ns_show(struct seq_file *m, void *v)
 		goto out;
 	}
 
+	ret = security_task_gettimerslack(p);
+	if (ret)
+		goto out;
+
 	task_lock(p);
 	seq_printf(m, "%llu\n", p->timer_slack_ns);
 	task_unlock(p);
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 7ae3976..290483e 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -627,6 +627,15 @@
  *	Check permission before moving memory owned by process @p.
  *	@p contains the task_struct for process.
  *	Return 0 if permission is granted.
+ * @task_settimerslack:
+ *	Check permission before setting timerslack value of @p to @slack.
+ *	@p contains the task_struct of a process.
+ *	@slack contains the new slack value.
+ *	Return 0 if permission is granted.
+ * @task_gettimerslack:
+ *	Check permission before returning the timerslack value of @p.
+ *	@p contains the task_struct of a process.
+ *	Return 0 if permission is granted.
  * @task_kill:
  *	Check permission before sending signal @sig to @p.  @info can be NULL,
  *	the constant 1, or a pointer to a siginfo structure.  If @info is 1 or
@@ -1473,6 +1482,8 @@ union security_list_options {
 	int (*task_setscheduler)(struct task_struct *p);
 	int (*task_getscheduler)(struct task_struct *p);
 	int (*task_movememory)(struct task_struct *p);
+	int (*task_settimerslack)(struct task_struct *p, u64 slack);
+	int (*task_gettimerslack)(struct task_struct *p);
 	int (*task_kill)(struct task_struct *p, struct siginfo *info,
 				int sig, u32 secid);
 	int (*task_wait)(struct task_struct *p);
@@ -1732,6 +1743,8 @@ struct security_hook_heads {
 	struct list_head task_setscheduler;
 	struct list_head task_getscheduler;
 	struct list_head task_movememory;
+	struct list_head task_settimerslack;
+	struct list_head task_gettimerslack;
 	struct list_head task_kill;
 	struct list_head task_wait;
 	struct list_head task_prctl;
diff --git a/include/linux/security.h b/include/linux/security.h
index 14df373..ab70f47 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -325,6 +325,8 @@ int security_task_setrlimit(struct task_struct *p, unsigned int resource,
 int security_task_setscheduler(struct task_struct *p);
 int security_task_getscheduler(struct task_struct *p);
 int security_task_movememory(struct task_struct *p);
+int security_task_settimerslack(struct task_struct *p, u64 slack);
+int security_task_gettimerslack(struct task_struct *p);
 int security_task_kill(struct task_struct *p, struct siginfo *info,
 			int sig, u32 secid);
 int security_task_wait(struct task_struct *p);
@@ -950,6 +952,16 @@ static inline int security_task_movememory(struct task_struct *p)
 	return 0;
 }
 
+static inline int security_task_settimerslack(struct task_struct *p, u64 slack)
+{
+	return 0;
+}
+
+static inline int security_task_gettimerslack(struct task_struct *p)
+{
+	return 0;
+}
+
 static inline int security_task_kill(struct task_struct *p,
 				     struct siginfo *info, int sig,
 				     u32 secid)
diff --git a/security/security.c b/security/security.c
index 7095693..4ced236 100644
--- a/security/security.c
+++ b/security/security.c
@@ -977,6 +977,16 @@ int security_task_movememory(struct task_struct *p)
 	return call_int_hook(task_movememory, 0, p);
 }
 
+int security_task_settimerslack(struct task_struct *p, u64 slack)
+{
+	return call_int_hook(task_settimerslack, 0, p, slack);
+}
+
+int security_task_gettimerslack(struct task_struct *p)
+{
+	return call_int_hook(task_gettimerslack, 0, p);
+}
+
 int security_task_kill(struct task_struct *p, struct siginfo *info,
 			int sig, u32 secid)
 {
@@ -1720,6 +1730,10 @@ struct security_hook_heads security_hook_heads = {
 		LIST_HEAD_INIT(security_hook_heads.task_getscheduler),
 	.task_movememory =
 		LIST_HEAD_INIT(security_hook_heads.task_movememory),
+	.task_settimerslack =
+		LIST_HEAD_INIT(security_hook_heads.task_settimerslack),
+	.task_gettimerslack =
+		LIST_HEAD_INIT(security_hook_heads.task_gettimerslack),
 	.task_kill =	LIST_HEAD_INIT(security_hook_heads.task_kill),
 	.task_wait =	LIST_HEAD_INIT(security_hook_heads.task_wait),
 	.task_prctl =	LIST_HEAD_INIT(security_hook_heads.task_prctl),
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index a86d537..aac9599 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -3849,6 +3849,16 @@ static int selinux_task_movememory(struct task_struct *p)
 	return current_has_perm(p, PROCESS__SETSCHED);
 }
 
+static int selinux_task_settimerslack(struct task_struct *p, u64 slack)
+{
+	return current_has_perm(p, PROCESS__SETSCHED);
+}
+
+static int selinux_task_gettimerslack(struct task_struct *p)
+{
+	return current_has_perm(p, PROCESS__GETSCHED);
+}
+
 static int selinux_task_kill(struct task_struct *p, struct siginfo *info,
 				int sig, u32 secid)
 {
@@ -6092,6 +6102,8 @@ static struct security_hook_list selinux_hooks[] = {
 	LSM_HOOK_INIT(task_setscheduler, selinux_task_setscheduler),
 	LSM_HOOK_INIT(task_getscheduler, selinux_task_getscheduler),
 	LSM_HOOK_INIT(task_movememory, selinux_task_movememory),
+	LSM_HOOK_INIT(task_settimerslack, selinux_task_settimerslack),
+	LSM_HOOK_INIT(task_gettimerslack, selinux_task_gettimerslack),
 	LSM_HOOK_INIT(task_kill, selinux_task_kill),
 	LSM_HOOK_INIT(task_wait, selinux_task_wait),
 	LSM_HOOK_INIT(task_to_inode, selinux_task_to_inode),
-- 
1.9.1