[PATCH] Fix keyctl usage of strnlen_user()

In the small window between strnlen_user() and copy_from_user() userspace
could alter the terminating `\0' character.

Signed-off-by: Davi Arnaut <[e-mail]>
Cc: David Howells <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

security/keys/keyctl.c

···
 	description = kmalloc(dlen + 1, GFP_KERNEL);
 	if (!description)
 		goto error;
+	description[dlen] = '\0';
 
 	ret = -EFAULT;
-	if (copy_from_user(description, _description, dlen + 1) != 0)
+	if (copy_from_user(description, _description, dlen) != 0)
 		goto error2;
 
 	/* pull the payload in if one was supplied */
···
 	description = kmalloc(dlen + 1, GFP_KERNEL);
 	if (!description)
 		goto error;
+	description[dlen] = '\0';
 
 	ret = -EFAULT;
-	if (copy_from_user(description, _description, dlen + 1) != 0)
+	if (copy_from_user(description, _description, dlen) != 0)
 		goto error2;
 
 	/* pull the callout info into kernel space */
···
 		callout_info = kmalloc(dlen + 1, GFP_KERNEL);
 		if (!callout_info)
 			goto error2;
+		callout_info[dlen] = '\0';
 
 		ret = -EFAULT;
-		if (copy_from_user(callout_info, _callout_info, dlen + 1) != 0)
+		if (copy_from_user(callout_info, _callout_info, dlen) != 0)
 			goto error3;
 	}
 
···
 		name = kmalloc(nlen + 1, GFP_KERNEL);
 		if (!name)
 			goto error;
+		name[nlen] = '\0';
 
 		ret = -EFAULT;
-		if (copy_from_user(name, _name, nlen + 1) != 0)
+		if (copy_from_user(name, _name, nlen) != 0)
 			goto error2;
 	}
 
···
 	description = kmalloc(dlen + 1, GFP_KERNEL);
 	if (!description)
 		goto error;
+	description[dlen] = '\0';
 
 	ret = -EFAULT;
-	if (copy_from_user(description, _description, dlen + 1) != 0)
+	if (copy_from_user(description, _description, dlen) != 0)
 		goto error2;
 
 	/* get the keyring at which to begin the search */