From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-13.0 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 98B9FC433E1 for ; Wed, 29 Jul 2020 07:02:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 82E582070B for ; Wed, 29 Jul 2020 07:02:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727811AbgG2HCy (ORCPT ); Wed, 29 Jul 2020 03:02:54 -0400 Received: from mx2.suse.de ([195.135.220.15]:37728 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726314AbgG2HCx (ORCPT ); Wed, 29 Jul 2020 03:02:53 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id E8C71AB55; Wed, 29 Jul 2020 07:03:02 +0000 (UTC) From: Jiri Slaby To: b.zolnierkie@samsung.com Cc: linux-kernel@vger.kernel.org, Jiri Slaby , =?UTF-8?q?=E5=BC=A0=E4=BA=91=E6=B5=B7?= , Yang Yingliang , Kyungtae Kim , Linus Torvalds , Greg KH , Solar Designer , "Srivatsa S. Bhat" , Anthony Liguori , Security Officers , linux-distros@vs.openwall.org, dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org Subject: [PATCH] vgacon: fix out of bounds write to the scrollback buffer Date: Wed, 29 Jul 2020 09:02:49 +0200 Message-Id: <20200729070249.20892-1-jslaby@suse.cz> X-Mailer: git-send-email 2.28.0 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The current vgacon's scroll up implementation uses a circural buffer in vgacon_scrollback_cur. It always advances tail to prepare it for the next write and caps it to zero if the next ->vc_size_row bytes won't fit. But when we change the VT size (e.g. by VT_RESIZE) in the meantime, the new line might not fit to the end of the scrollback buffer in the next attempt to scroll. This leads to various crashes as vgacon_scrollback_update writes out of the buffer: BUG: unable to handle page fault for address: ffffc900001752a0 #PF: supervisor write access in kernel mode #PF: error_code(0x0002) - not-present page RIP: 0010:mutex_unlock+0x13/0x30 ... Call Trace: n_tty_write+0x1a0/0x4d0 tty_write+0x1a0/0x2e0 Or to KASAN reports: BUG: KASAN: slab-out-of-bounds in vgacon_scroll+0x57a/0x8ed So check whether the line fits in the buffer and wrap if needed. Do it before the loop as console_sem is held and ->vc_size_row cannot change during the execution of vgacon_scrollback_cur. If it does change, we need to ensure it does not change elsewhere, not here. Also, we do not split the write of a line into chunks as that would break the consumers of the buffer. They expect ->cnt, ->tail and ->size to be in harmony and advanced by ->vc_size_row. I found few reports of this in the past, some with patches included, some even 2 years old: https://lore.kernel.org/lkml/CAEAjamsJnG-=TSOwgRbbb3B9Z-PA63oWmNPoKYWQ=Z=+X49akg@mail.gmail.com/ https://lore.kernel.org/lkml/1589336932-35508-1-git-send-email-yangyingliang@huawei.com/ This fixes CVE-2020-14331. Big thanks to guys mentioned in the Reported-and-debugged-by lines below who actually found the root cause. Signed-off-by: Jiri Slaby Reported-and-debugged-by: 张云海 Reported-and-debugged-by: Yang Yingliang Reported-by: Kyungtae Kim Fixes: 15bdab959c9b ([PATCH] vgacon: Add support for soft scrollback) Cc: Linus Torvalds Cc: Greg KH Cc: Solar Designer Cc: "Srivatsa S. Bhat" Cc: Anthony Liguori Cc: Security Officers Cc: linux-distros@vs.openwall.org Cc: Yang Yingliang Cc: Bartlomiej Zolnierkiewicz Cc: dri-devel@lists.freedesktop.org Cc: linux-fbdev@vger.kernel.org --- drivers/video/console/vgacon.c | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/drivers/video/console/vgacon.c b/drivers/video/console/vgacon.c index f0f3d573f848..13194bb246f8 100644 --- a/drivers/video/console/vgacon.c +++ b/drivers/video/console/vgacon.c @@ -250,6 +250,11 @@ static void vgacon_scrollback_update(struct vc_data *c, int t, int count) p = (void *) (c->vc_origin + t * c->vc_size_row); + /* vc_size_row might have changed by VT_RESIZE in the meantime */ + if ((vgacon_scrollback_cur->tail + c->vc_size_row) >= + vgacon_scrollback_cur->size) + vgacon_scrollback_cur->tail = 0; + while (count--) { scr_memcpyw(vgacon_scrollback_cur->data + vgacon_scrollback_cur->tail, -- 2.28.0