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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no 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 73C16C432BE for ; Mon, 30 Aug 2021 18:38:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5697D60ED8 for ; Mon, 30 Aug 2021 18:38:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229982AbhH3Sjf (ORCPT ); Mon, 30 Aug 2021 14:39:35 -0400 Received: from smtp-relay-canonical-1.canonical.com ([185.125.188.121]:36162 "EHLO smtp-relay-canonical-1.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229738AbhH3Sje (ORCPT ); Mon, 30 Aug 2021 14:39:34 -0400 Received: from mussarela (201-69-234-220.dial-up.telesp.net.br [201.69.234.220]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (P-256) server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by smtp-relay-canonical-1.canonical.com (Postfix) with ESMTPSA id 093D340178; Mon, 30 Aug 2021 18:38:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=canonical.com; s=20210705; t=1630348719; bh=dP2bWFnr904Q3phxdSJFO6HdgfMSWA65sEUBMQrG98c=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:In-Reply-To; b=tEEdtAHGeXWPANZY9GSNFafDKT6z1GKu2nWOlfBIS5uxclCTIR+OK6BAg4Or0lddr 7eM/MOYbnRMI5cy14s8OTfzI3xxyCdrpWicFgGEmp7NF6tDKlRXWzrSj9R525VyKXY HWGPF9TAWc7iCr/1oDyPbxDz9t0FApF31/RyuLlsG7z8NijAQBbfUa5pwF/hIsltuK dCHv/HjFi6HCXmZgfdKFZPxT7KpfIKDHbyW/XA0MLwQnFn6HwuJhPQiLtu5SF28/gZ hvxsKGH9U8JRSo6Jkhe7JH+B0fo94WfYDbFxmE+l73jzJu78L+rEdssvswM0inoqzc 6xPnmnMin+MHg== Date: Mon, 30 Aug 2021 15:38:34 -0300 From: Thadeu Lima de Souza Cascardo To: Pavel Machek Cc: stable@vger.kernel.org, kernel list , daniel@iogearbox.net, john.fastabend@gmail.com, ast@kernel.org Subject: Re: CVE-2021-3600 aka bpf: Fix 32 bit src register truncation on div/mod Message-ID: References: <20210826102337.GA7362@duo.ucw.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210826102337.GA7362@duo.ucw.cz> Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org On Thu, Aug 26, 2021 at 12:23:37PM +0200, Pavel Machek wrote: > Hi! > > As far as I can tell, CVE-2021-3600 is still a problem for 4.14 and > 4.19. > > Unfortunately, those kernels lack BPF_JMP32 support, and that support > is too big and intrusive to backport. > > So I tried to come up with solution without JMP32 support... only to > end up with not seeing the bug in the affected code. > > Changelog says: > > bpf: Fix 32 bit src register truncation on div/mod > > While reviewing a different fix, John and I noticed an oddity in one of the > BPF program dumps that stood out, for example: > > # bpftool p d x i 13 > 0: (b7) r0 = 808464450 > 1: (b4) w4 = 808464432 > 2: (bc) w0 = w0 > 3: (15) if r0 == 0x0 goto pc+1 > 4: (9c) w4 %= w0 > [...] > > In line 2 we noticed that the mov32 would 32 bit truncate the original src > register for the div/mod operation. While for the two operations the dst > register is typically marked unknown e.g. from adjust_scalar_min_max_vals() > the src register is not, and thus verifier keeps tracking original bounds, > simplified: > > So this explains "mov32 w0, w0" is problematic, and fixes the bug by > replacing it with jmp32. Unfortunately, I can't do that in 4.19; plus > I don't really see how the bug is solved -- we avoided adding mov32 > sequence that triggers the problem, but the problematic sequence could > still be produced by the userspace, no? > > Does adjust_scalar_min_max_vals still need fixing? > > Do you have any hints how to solve this in 4.19? > > Best regards, > Pavel > -- > DENX Software Engineering GmbH, Managing Director: Wolfgang Denk > HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany Hi, Pavel. I have just sent the fixes for 4.14. I sent fixes for 4.19 last Friday. The problem here is that the verifier assumes the source register has a given value, but the fixups change that value to something else when it is truncated. The fixups run after the verifier, so a similar sequence produced by userspace will be correctly verified, so no fixes are necessary on adjust_scalar_min_max for this specific issue. The fixed-up code is not verified again. The challenge in providing those fixes to 4.14 and 4.19 is the absence of JMP32 in those kernels. So, AX was taken as a temporary, so it would still work on JITs. Cascardo.