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, 13 insertions, 9 deletions

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 49849d12dd1..b9d5f4e4f6a 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -115,7 +115,7 @@ int ptrace_check_attach(struct task_struct *child, int kill)
 
 int __ptrace_may_access(struct task_struct *task, unsigned int mode)
 {
-	struct cred *cred = current->cred, *tcred = task->cred;
+	const struct cred *cred = current_cred(), *tcred;
 
 	/* May we inspect the given task?
 	 * This check is used both for attaching with ptrace
@@ -125,19 +125,23 @@ int __ptrace_may_access(struct task_struct *task, unsigned int mode)
 	 * because setting up the necessary parent/child relationship
 	 * or halting the specified task is impossible.
 	 */
-	uid_t uid = cred->uid;
-	gid_t gid = cred->gid;
 	int dumpable = 0;
 	/* Don't let security modules deny introspection */
 	if (task == current)
 		return 0;
-	if ((uid != tcred->euid ||
-	     uid != tcred->suid ||
-	     uid != tcred->uid  ||
-	     gid != tcred->egid ||
-	     gid != tcred->sgid ||
-	     gid != tcred->gid) && !capable(CAP_SYS_PTRACE))
+	rcu_read_lock();
+	tcred = __task_cred(task);
+	if ((cred->uid != tcred->euid ||
+	     cred->uid != tcred->suid ||
+	     cred->uid != tcred->uid  ||
+	     cred->gid != tcred->egid ||
+	     cred->gid != tcred->sgid ||
+	     cred->gid != tcred->gid) &&
+	    !capable(CAP_SYS_PTRACE)) {
+		rcu_read_unlock();
 		return -EPERM;
+	}
+	rcu_read_unlock();
 	smp_rmb();
 	if (task->mm)
 		dumpable = get_dumpable(task->mm);