CRED: Use RCU to access another task's creds and to release a task's own creds

Use RCU to access another task's creds and to release a task's own creds.
This means that it will be possible for the credentials of a task to be
replaced without another task (a) requiring a full lock to read them, and (b)
seeing deallocated memory.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: James Morris <jmorris@namei.org>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>

1 files changed, 31 insertions, 18 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index 80e8a6489f9..84989124baf 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -177,6 +177,11 @@ int next_signal(struct sigpending *pending, sigset_t *mask)
 	return sig;
 }
 
+/*
+ * allocate a new signal queue record
+ * - this may be called without locks if and only if t == current, otherwise an
+ *   appopriate lock must be held to protect t's user_struct
+ */
 static struct sigqueue *__sigqueue_alloc(struct task_struct *t, gfp_t flags,
 					 int override_rlimit)
 {
@@ -184,11 +189,12 @@ static struct sigqueue *__sigqueue_alloc(struct task_struct *t, gfp_t flags,
 	struct user_struct *user;
 
 	/*
-	 * In order to avoid problems with "switch_user()", we want to make
-	 * sure that the compiler doesn't re-load "t->user"
+	 * We won't get problems with the target's UID changing under us
+	 * because changing it requires RCU be used, and if t != current, the
+	 * caller must be holding the RCU readlock (by way of a spinlock) and
+	 * we use RCU protection here
 	 */
-	user = t->cred->user;
-	barrier();
+	user = __task_cred(t)->user;
 	atomic_inc(&user->sigpending);
 	if (override_rlimit ||
 	    atomic_read(&user->sigpending) <=
@@ -562,12 +568,13 @@ static int rm_from_queue(unsigned long mask, struct sigpending *s)
 
 /*
  * Bad permissions for sending the signal
+ * - the caller must hold at least the RCU read lock
  */
 static int check_kill_permission(int sig, struct siginfo *info,
 				 struct task_struct *t)
 {
+	const struct cred *cred = current_cred(), *tcred;
 	struct pid *sid;
-	uid_t uid, euid;
 	int error;
 
 	if (!valid_signal(sig))
@@ -580,10 +587,11 @@ static int check_kill_permission(int sig, struct siginfo *info,
 	if (error)
 		return error;
 
-	uid = current_uid();
-	euid = current_euid();
-	if ((euid ^ t->cred->suid) && (euid ^ t->cred->uid) &&
-	    (uid  ^ t->cred->suid) && (uid  ^ t->cred->uid) &&
+	tcred = __task_cred(t);
+	if ((cred->euid ^ tcred->suid) &&
+	    (cred->euid ^ tcred->uid) &&
+	    (cred->uid  ^ tcred->suid) &&
+	    (cred->uid  ^ tcred->uid) &&
 	    !capable(CAP_KILL)) {
 		switch (sig) {
 		case SIGCONT:
@@ -1011,6 +1019,10 @@ struct sighand_struct *lock_task_sighand(struct task_struct *tsk, unsigned long
 	return sighand;
 }
 
+/*
+ * send signal info to all the members of a group
+ * - the caller must hold the RCU read lock at least
+ */
 int group_send_sig_info(int sig, struct siginfo *info, struct task_struct *p)
 {
 	unsigned long flags;
@@ -1032,8 +1044,8 @@ int group_send_sig_info(int sig, struct siginfo *info, struct task_struct *p)
 /*
  * __kill_pgrp_info() sends a signal to a process group: this is what the tty
  * control characters do (^C, ^Z etc)
+ * - the caller must hold at least a readlock on tasklist_lock
  */
-
 int __kill_pgrp_info(int sig, struct siginfo *info, struct pid *pgrp)
 {
 	struct task_struct *p = NULL;
@@ -1089,6 +1101,7 @@ int kill_pid_info_as_uid(int sig, struct siginfo *info, struct pid *pid,
 {
 	int ret = -EINVAL;
 	struct task_struct *p;
+	const struct cred *pcred;
 
 	if (!valid_signal(sig))
 		return ret;
@@ -1099,9 +1112,11 @@ int kill_pid_info_as_uid(int sig, struct siginfo *info, struct pid *pid,
 		ret = -ESRCH;
 		goto out_unlock;
 	}
-	if ((info == SEND_SIG_NOINFO || (!is_si_special(info) && SI_FROMUSER(info)))
-	    && (euid != p->cred->suid) && (euid != p->cred->uid)
-	    && (uid != p->cred->suid) && (uid != p->cred->uid)) {
+	pcred = __task_cred(p);
+	if ((info == SEND_SIG_NOINFO ||
+	     (!is_si_special(info) && SI_FROMUSER(info))) &&
+	    euid != pcred->suid && euid != pcred->uid &&
+	    uid  != pcred->suid && uid  != pcred->uid) {
 		ret = -EPERM;
 		goto out_unlock;
 	}
@@ -1372,10 +1387,9 @@ int do_notify_parent(struct task_struct *tsk, int sig)
 	 */
 	rcu_read_lock();
 	info.si_pid = task_pid_nr_ns(tsk, tsk->parent->nsproxy->pid_ns);
+	info.si_uid = __task_cred(tsk)->uid;
 	rcu_read_unlock();
 
-	info.si_uid = tsk->cred->uid;
-
 	thread_group_cputime(tsk, &cputime);
 	info.si_utime = cputime_to_jiffies(cputime.utime);
 	info.si_stime = cputime_to_jiffies(cputime.stime);
@@ -1443,10 +1457,9 @@ static void do_notify_parent_cldstop(struct task_struct *tsk, int why)
 	 */
 	rcu_read_lock();
 	info.si_pid = task_pid_nr_ns(tsk, tsk->parent->nsproxy->pid_ns);
+	info.si_uid = __task_cred(tsk)->uid;
 	rcu_read_unlock();
 
-	info.si_uid = tsk->cred->uid;
-
 	info.si_utime = cputime_to_clock_t(tsk->utime);
 	info.si_stime = cputime_to_clock_t(tsk->stime);
 
@@ -1713,7 +1726,7 @@ static int ptrace_signal(int signr, siginfo_t *info,
 		info->si_errno = 0;
 		info->si_code = SI_USER;
 		info->si_pid = task_pid_vnr(current->parent);
-		info->si_uid = current->parent->cred->uid;
+		info->si_uid = task_uid(current->parent);
 	}
 
 	/* If the (new) signal is now blocked, requeue it.  */