ksmbd: prevent out-of-bounds stream writes by validating *pos

ksmbd_vfs_stream_write() did not validate whether the write offset
(*pos) was within the bounds of the existing stream data length (v_len).
If *pos was greater than or equal to v_len, this could lead to an
out-of-bounds memory write.

This patch adds a check to ensure *pos is less than v_len before
proceeding. If the condition fails, -EINVAL is returned.

Cc: stable@vger.kernel.org
Signed-off-by: Norbert Szetei <norbert@doyensec.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: nszetei

fs/smb/server/vfs.c

@@ -426,6 +426,13 @@ 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) {