ksmbd: fix stream write failure

If there is no stream data in file, v_len is zero.
So, If position(*pos) is zero, stream write will fail
due to stream write position validation check.
This patch reorganize stream write position validation.

Fixes: 0ca6df4 ("ksmbd: prevent out-of-bounds stream writes by validating *pos")
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: namjaejeon

fs/smb/server/vfs.c

@@ -409,10 +409,15 @@ static int ksmbd_vfs_stream_write(struct ksmbd_file *fp, char *buf, loff_t *pos,
 	ksmbd_debug(VFS, "write stream data pos : %llu, count : %zd\n",
 		    *pos, count);
 
+	if (*pos >= XATTR_SIZE_MAX) {
+		pr_err("stream write position %lld is out of bounds\n",	*pos);
+		return -EINVAL;
+	}
+
 	size = *pos + count;
 	if (size > XATTR_SIZE_MAX) {
 		size = XATTR_SIZE_MAX;
-		count = (*pos + count) - XATTR_SIZE_MAX;
+		count = XATTR_SIZE_MAX - *pos;
 	}
 
 	v_len = ksmbd_vfs_getcasexattr(idmap,
@@ -426,13 +431,6 @@ static int ksmbd_vfs_stream_write(struct ksmbd_file *fp, char *buf, loff_t *pos,
 		goto out;
 	}
 
-	if (v_len <= *pos) {
-		pr_err("stream write position %lld is out of bounds (stream length: %zd)\n",
-				*pos, v_len);
-		err = -EINVAL;
-		goto out;
-	}
-
 	if (v_len < size) {
 		wbuf = kvzalloc(size, KSMBD_DEFAULT_GFP);
 		if (!wbuf) {