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, 20 insertions, 12 deletions

diff --git a/arch/ia64/kernel/perfmon.c b/arch/ia64/kernel/perfmon.c
index dd38db4..0e49975 100644
--- a/arch/ia64/kernel/perfmon.c
+++ b/arch/ia64/kernel/perfmon.c
@@ -2399,25 +2399,33 @@ error_kmem:
 static int
 pfm_bad_permissions(struct task_struct *task)
 {
+	const struct cred *tcred;
 	uid_t uid = current_uid();
 	gid_t gid = current_gid();
+	int ret;
+
+	rcu_read_lock();
+	tcred = __task_cred(task);
 
 	/* inspired by ptrace_attach() */
 	DPRINT(("cur: uid=%d gid=%d task: euid=%d suid=%d uid=%d egid=%d sgid=%d\n",
 		uid,
 		gid,
-		task->euid,
-		task->suid,
-		task->uid,
-		task->egid,
-		task->sgid));
-
-	return (uid != task->euid)
-	    || (uid != task->suid)
-	    || (uid != task->uid)
-	    || (gid != task->egid)
-	    || (gid != task->sgid)
-	    || (gid != task->gid)) && !capable(CAP_SYS_PTRACE);
+		tcred->euid,
+		tcred->suid,
+		tcred->uid,
+		tcred->egid,
+		tcred->sgid));
+
+	ret = ((uid != tcred->euid)
+	       || (uid != tcred->suid)
+	       || (uid != tcred->uid)
+	       || (gid != tcred->egid)
+	       || (gid != tcred->sgid)
+	       || (gid != tcred->gid)) && !capable(CAP_SYS_PTRACE);
+
+	rcu_read_unlock();
+	return ret;
 }
 
 static int