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, 14 insertions, 8 deletions

diff --git a/kernel/futex.c b/kernel/futex.c
index 28421d8..4fe790e 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -439,15 +439,20 @@ static void free_pi_state(struct futex_pi_state *pi_state)
 static struct task_struct * futex_find_get_task(pid_t pid)
 {
 	struct task_struct *p;
-	uid_t euid = current_euid();
+	const struct cred *cred = current_cred(), *pcred;
 
 	rcu_read_lock();
 	p = find_task_by_vpid(pid);
-	if (!p || (euid != p->cred->euid &&
-		   euid != p->cred->uid))
+	if (!p) {
 		p = ERR_PTR(-ESRCH);
-	else
-		get_task_struct(p);
+	} else {
+		pcred = __task_cred(p);
+		if (cred->euid != pcred->euid &&
+		    cred->euid != pcred->uid)
+			p = ERR_PTR(-ESRCH);
+		else
+			get_task_struct(p);
+	}
 
 	rcu_read_unlock();
 
@@ -1831,7 +1836,7 @@ sys_get_robust_list(int pid, struct robust_list_head __user * __user *head_ptr,
 {
 	struct robust_list_head __user *head;
 	unsigned long ret;
-	uid_t euid = current_euid();
+	const struct cred *cred = current_cred(), *pcred;
 
 	if (!futex_cmpxchg_enabled)
 		return -ENOSYS;
@@ -1847,8 +1852,9 @@ sys_get_robust_list(int pid, struct robust_list_head __user * __user *head_ptr,
 		if (!p)
 			goto err_unlock;
 		ret = -EPERM;
-		if (euid != p->cred->euid &&
-		    euid != p->cred->uid &&
+		pcred = __task_cred(p);
+		if (cred->euid != pcred->euid &&
+		    cred->euid != pcred->uid &&
 		    !capable(CAP_SYS_PTRACE))
 			goto err_unlock;
 		head = p->robust_list;