Smack: fix use-after-free in smk_write_relabel_self()

smk_write_relabel_self() frees memory from the task's credentials with
no locking, which can easily cause a use-after-free because multiple
tasks can share the same credentials structure.

Fix this by using prepare_creds() and commit_creds() to correctly modify
the task's credentials.

Reproducer for "BUG: KASAN: use-after-free in smk_write_relabel_self":

	#include <fcntl.h>
	#include <pthread.h>
	#include <unistd.h>

	static void *thrproc(void *arg)
	{
		int fd = open("/sys/fs/smackfs/relabel-self", O_WRONLY);
		for (;;) write(fd, "foo", 3);
	}

	int main()
	{
		pthread_t t;
		pthread_create(&t, NULL, thrproc, NULL);
		thrproc(NULL);
	}

Reported-by: syzbot+e6416dabb497a650da40@syzkaller.appspotmail.com
Fixes: 38416e53 ("Smack: limited capability for changing process label")
Cc: <stable@vger.kernel.org> # v4.4+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>

security/smack/smackfs.c

@@ -2720,7 +2720,6 @@ static int smk_open_relabel_self(struct inode *inode, struct file *file)
 static ssize_t smk_write_relabel_self(struct file *file, const char __user *buf,
 				size_t count, loff_t *ppos)
 {
-	struct task_smack *tsp = smack_cred(current_cred());
 	char *data;
 	int rc;
 	LIST_HEAD(list_tmp);
@@ -2745,11 +2744,21 @@ static ssize_t smk_write_relabel_self(struct file *file, const char __user *buf,
 	kfree(data);
 
 	if (!rc || (rc == -EINVAL && list_empty(&list_tmp))) {
+		struct cred *new;
+		struct task_smack *tsp;
+
+		new = prepare_creds();
+		if (!new) {
+			rc = -ENOMEM;
+			goto out;
+		}
+		tsp = smack_cred(new);
 		smk_destroy_label_list(&tsp->smk_relabel);
 		list_splice(&list_tmp, &tsp->smk_relabel);
+		commit_creds(new);
 		return count;
 	}
-
+out:
 	smk_destroy_label_list(&list_tmp);
 	return rc;
 }