FAILED: patch "[PATCH] Smack: fix use-after-free in smk_write_relabel_self()" failed to apply to 4.9-stable tree

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From beb4ee6770a89646659e6a2178538d2b13e2654e Mon Sep 17 00:00:00 2001
From: Eric Biggers <[email protected]>
Date: Wed, 8 Jul 2020 13:15:20 -0700
Subject: [PATCH] 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);
	}

index c21b656b3263..840a192e9337 100644
@@ -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;
 }