security,selinux,smack: kill security_task_wait hook

As reported by yangshukui, a permission denial from security_task_wait()
can lead to a soft lockup in zap_pid_ns_processes() since it only expects
sys_wait4() to return 0 or -ECHILD. Further, security_task_wait() can
in general lead to zombies; in the absence of some way to automatically
reparent a child process upon a denial, the hook is not useful.  Remove
the security hook and its implementations in SELinux and Smack.  Smack
already removed its check from its hook.

Reported-by: yangshukui <yangshukui@huawei.com>
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

include/linux/lsm_hooks.h

@@ -666,11 +666,6 @@
  *	@sig contains the signal value.
  *	@secid contains the sid of the process where the signal originated
  *	Return 0 if permission is granted.
- * @task_wait:
- *	Check permission before allowing a process to reap a child process @p
- *	and collect its status information.
- *	@p contains the task_struct for process.
- *	Return 0 if permission is granted.
  * @task_prctl:
  *	Check permission before performing a process control operation on the
  *	current process.
@@ -1507,7 +1502,6 @@ union security_list_options {
 	int (*task_movememory)(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);
 	int (*task_prctl)(int option, unsigned long arg2, unsigned long arg3,
 				unsigned long arg4, unsigned long arg5);
 	void (*task_to_inode)(struct task_struct *p, struct inode *inode);
@@ -1767,7 +1761,6 @@ struct security_hook_heads {
 	struct list_head task_getscheduler;
 	struct list_head task_movememory;
 	struct list_head task_kill;
-	struct list_head task_wait;
 	struct list_head task_prctl;
 	struct list_head task_to_inode;
 	struct list_head ipc_permission;

include/linux/security.h

@@ -332,7 +332,6 @@ int security_task_getscheduler(struct task_struct *p);
 int security_task_movememory(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);
 int security_task_prctl(int option, unsigned long arg2, unsigned long arg3,
 			unsigned long arg4, unsigned long arg5);
 void security_task_to_inode(struct task_struct *p, struct inode *inode);
@@ -980,11 +979,6 @@ static inline int security_task_kill(struct task_struct *p,
 	return 0;
 }
 
-static inline int security_task_wait(struct task_struct *p)
-{
-	return 0;
-}
-
 static inline int security_task_prctl(int option, unsigned long arg2,
 				      unsigned long arg3,
 				      unsigned long arg4,

kernel/exit.c

@@ -14,7 +14,6 @@
 #include <linux/tty.h>
 #include <linux/iocontext.h>
 #include <linux/key.h>
-#include <linux/security.h>
 #include <linux/cpu.h>
 #include <linux/acct.h>
 #include <linux/tsacct_kern.h>
@@ -1360,7 +1359,7 @@ static int wait_task_continued(struct wait_opts *wo, struct task_struct *p)
  * Returns nonzero for a final return, when we have unlocked tasklist_lock.
  * Returns zero if the search for a child should continue;
  * then ->notask_error is 0 if @p is an eligible child,
- * or another error from security_task_wait(), or still -ECHILD.
+ * or still -ECHILD.
  */
 static int wait_consider_task(struct wait_opts *wo, int ptrace,
 				struct task_struct *p)
@@ -1380,20 +1379,6 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace,
 	if (!ret)
 		return ret;
 
-	ret = security_task_wait(p);
-	if (unlikely(ret < 0)) {
-		/*
-		 * If we have not yet seen any eligible child,
-		 * then let this error code replace -ECHILD.
-		 * A permission error will give the user a clue
-		 * to look for security policy problems, rather
-		 * than for mysterious wait bugs.
-		 */
-		if (wo->notask_error)
-			wo->notask_error = ret;
-		return 0;
-	}
-
 	if (unlikely(exit_state == EXIT_TRACE)) {
 		/*
 		 * ptrace == 0 means we are the natural parent. In this case
@@ -1486,7 +1471,7 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace,
  * Returns nonzero for a final return, when we have unlocked tasklist_lock.
  * Returns zero if the search for a child should continue; then
  * ->notask_error is 0 if there were any eligible children,
- * or another error from security_task_wait(), or still -ECHILD.
+ * or still -ECHILD.
  */
 static int do_wait_thread(struct wait_opts *wo, struct task_struct *tsk)
 {

security/security.c

@@ -1025,11 +1025,6 @@ int security_task_kill(struct task_struct *p, struct siginfo *info,
 	return call_int_hook(task_kill, 0, p, info, sig, secid);
 }
 
-int security_task_wait(struct task_struct *p)
-{
-	return call_int_hook(task_wait, 0, p);
-}
-
 int security_task_prctl(int option, unsigned long arg2, unsigned long arg3,
 			 unsigned long arg4, unsigned long arg5)
 {
@@ -1769,7 +1764,6 @@ struct security_hook_heads security_hook_heads = {
 	.task_movememory =
 		LIST_HEAD_INIT(security_hook_heads.task_movememory),
 	.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),
 	.task_to_inode =
 		LIST_HEAD_INIT(security_hook_heads.task_to_inode),

security/selinux/hooks.c

@@ -3963,12 +3963,6 @@ static int selinux_task_kill(struct task_struct *p, struct siginfo *info,
 	return avc_has_perm(secid, task_sid(p), SECCLASS_PROCESS, perm, NULL);
 }
 
-static int selinux_task_wait(struct task_struct *p)
-{
-	return avc_has_perm(task_sid(p), current_sid(), SECCLASS_PROCESS,
-			    PROCESS__SIGCHLD, NULL);
-}
-
 static void selinux_task_to_inode(struct task_struct *p,
 				  struct inode *inode)
 {
@@ -6211,7 +6205,6 @@ static struct security_hook_list selinux_hooks[] = {
 	LSM_HOOK_INIT(task_getscheduler, selinux_task_getscheduler),
 	LSM_HOOK_INIT(task_movememory, selinux_task_movememory),
 	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),
 
 	LSM_HOOK_INIT(ipc_permission, selinux_ipc_permission),

security/smack/smack_lsm.c

@@ -2271,25 +2271,6 @@ static int smack_task_kill(struct task_struct *p, struct siginfo *info,
 	return rc;
 }
 
-/**
- * smack_task_wait - Smack access check for waiting
- * @p: task to wait for
- *
- * Returns 0
- */
-static int smack_task_wait(struct task_struct *p)
-{
-	/*
-	 * Allow the operation to succeed.
-	 * Zombies are bad.
-	 * In userless environments (e.g. phones) programs
-	 * get marked with SMACK64EXEC and even if the parent
-	 * and child shouldn't be talking the parent still
-	 * may expect to know when the child exits.
-	 */
-	return 0;
-}
-
 /**
  * smack_task_to_inode - copy task smack into the inode blob
  * @p: task to copy from
@@ -4658,7 +4639,6 @@ static struct security_hook_list smack_hooks[] = {
 	LSM_HOOK_INIT(task_getscheduler, smack_task_getscheduler),
 	LSM_HOOK_INIT(task_movememory, smack_task_movememory),
 	LSM_HOOK_INIT(task_kill, smack_task_kill),
-	LSM_HOOK_INIT(task_wait, smack_task_wait),
 	LSM_HOOK_INIT(task_to_inode, smack_task_to_inode),
 
 	LSM_HOOK_INIT(ipc_permission, smack_ipc_permission),