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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id AF326C433F5 for ; Fri, 20 May 2022 18:19:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1352587AbiETSTb (ORCPT ); Fri, 20 May 2022 14:19:31 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44472 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1352592AbiETSTW (ORCPT ); Fri, 20 May 2022 14:19:22 -0400 Received: from mail-pl1-x630.google.com (mail-pl1-x630.google.com [IPv6:2607:f8b0:4864:20::630]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CF20F6B7CF for ; Fri, 20 May 2022 11:19:20 -0700 (PDT) Received: by mail-pl1-x630.google.com with SMTP id n18so8009171plg.5 for ; Fri, 20 May 2022 11:19:20 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=wgrcw3AWnI5MJ/JhXJfsPIWwbLCvXeysZUnODXN4Ko4=; b=guBYQwxKDovD2VNhx5zMFKN7LQYg5JtyW6Y7ASt3KAzn1dpSAkq5tqlNDPGO5pKpM/ cVEeZiHJOfMPHIE5gtS0Fh1czT/Q2fxIWf06Skm68gqzf3NqPGIkp0eqSO0igmbVtltz ggiokUQwWAKEDsTlQ1XDEYgg1ub5xkkVymSxo= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=wgrcw3AWnI5MJ/JhXJfsPIWwbLCvXeysZUnODXN4Ko4=; b=N0EohLT7ySVEYGFU8pcnrwy1+IZiAPBlz25ga5Ne8yt+cTVsqgbV+ZRiHFizyzBZ5q oTVeRD+j5/wxHEfAb1SfXGLLvCpVTIX2/yDIvlLWZUpJH85I9yB5yydAu5h0pZ7g9oLk +YdZJrLNpDo8SWCGXV4yPv7DAVvmE4EaYbPR5N+u7rI+GPgrV0tIwI8yZP8HzxZKJfM+ Dq0XduXjM2duxTs+QHYtZBS+AkQy9CIiXKDiJ0VZnOprkgEtnC5+eMwzztjYBroF+/fa hz75bE6YEnwgbqNhYIRYISKmaMhU2/Ih5SCcpmaPGvtMR7Ckg9KlHfB650F5/1IHAtTs E8Pg== X-Gm-Message-State: AOAM5311O1D+xSb5g6aOyCmjKLURNr4UtUegCY02ruA8VcTjd7ZgHEDq wOZuQrZPj3rKqWQNiYohK9rQi+vf5Pj7NA== X-Google-Smtp-Source: ABdhPJzej/3St1PJgS9g4UCEKxkkv9Qgf+QM2ka3RrR3JIjvpUTa9IxTy16SHJm8NjocE+FYSdN8dw== X-Received: by 2002:a17:90a:d713:b0:1df:d114:deef with SMTP id y19-20020a17090ad71300b001dfd114deefmr11291356pju.13.1653070760362; Fri, 20 May 2022 11:19:20 -0700 (PDT) Received: from www.outflux.net (smtp.outflux.net. [198.145.64.163]) by smtp.gmail.com with ESMTPSA id v22-20020aa78096000000b0050dc76281d0sm2183516pff.170.2022.05.20.11.19.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 20 May 2022 11:19:20 -0700 (PDT) Date: Fri, 20 May 2022 11:19:19 -0700 From: Kees Cook To: Sean Christopherson Cc: Paolo Bonzini , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , x86@kernel.org, "H. Peter Anvin" , kvm@vger.kernel.org, Joerg Roedel , linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org Subject: Re: [PATCH] KVM: x86/emulator: Bounds check reg nr against reg array size Message-ID: <202205201115.5E830F0@keescook> References: <20220520165705.2140042-1-keescook@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 20, 2022 at 05:32:04PM +0000, Sean Christopherson wrote: > On Fri, May 20, 2022, Kees Cook wrote: > > GCC 12 sees that it might be possible for "nr" to be outside the _regs > > array. Add explicit bounds checking. > > I think GCC 12 is wrong. I think it's more like GCC is extremely conservative about these things, and assumes the worst when, for whatever reason, it can't track something. > There are four uses of reg_rmw() that don't use hardcoded registers: > > $ git grep reg_rmw | grep -v VCPU_REGS_ > emulate.c:static ulong *reg_rmw(struct x86_emulate_ctxt *ctxt, unsigned nr) > 1 emulate.c: ulong *preg = reg_rmw(ctxt, reg); > 2 emulate.c: p = (unsigned char *)reg_rmw(ctxt, modrm_reg & 3) + 1; > 3 emulate.c: p = reg_rmw(ctxt, modrm_reg); > 4 emulate.c: assign_register(reg_rmw(ctxt, reg), val, ctxt->op_bytes); > > #1 has three users, but two of those use hardcoded registers. > > $ git grep register_address_increment | grep -v VCPU_REGS_ > emulate.c:register_address_increment(struct x86_emulate_ctxt *ctxt, int reg, int inc) > emulate.c: register_address_increment(ctxt, reg, df * op->bytes); > > and that last one is string_addr_inc(), which is only called with RDI or RSI. > > #2 can't overflow as the register can only be 0-3 (yay AH/BH/CH/DH operands). > > #3 is the !highbyte path of decode_register(), and is a bit messy, but modrm_reg > is always sanitized. > > $ git grep -E "decode_register\(" > emulate.c:static void *decode_register(struct x86_emulate_ctxt *ctxt, u8 modrm_reg, > a emulate.c: op->addr.reg = decode_register(ctxt, reg, ctxt->d & ByteOp); > b emulate.c: op->addr.reg = decode_register(ctxt, ctxt->modrm_rm, > c emulate.c: ctxt->memop.addr.reg = decode_register(ctxt, > ctxt->modrm_rm, true); > > For (b) and (c), modrm_reg == ctxt->modrm_rm, which is computed in one place and > is bounded to 0-15: > > base_reg = (ctxt->rex_prefix << 3) & 8; /* REX.B */ > ctxt->modrm_rm = base_reg | (ctxt->modrm & 0x07); > > For (a), "reg" is either modrm_reg or a register that is encoded in the opcode, > both of which are again bounded to 0-15: > > unsigned reg = ctxt->modrm_reg; > > if (!(ctxt->d & ModRM)) > reg = (ctxt->b & 7) | ((ctxt->rex_prefix & 1) << 3); > > and > > ctxt->modrm_reg = ((ctxt->rex_prefix << 1) & 8); /* REX.R */ > ctxt->modrm_reg |= (ctxt->modrm & 0x38) >> 3; > > #4 is em_popa() and is just funky hardcoding of popping RAX-RDI, minus RSP. > > I did the same exercise for reg_reg() and write_reg(), and the handful of > non-hardcoded use are all bounded in similar ways. Thanks for digging into this. I tried to do the same and started to lose track of things. > > > In function 'reg_read', > > inlined from 'reg_rmw' at ../arch/x86/kvm/emulate.c:266:2: > > Is there more of the "stack" available? I don't mind the WARN too much, but if > there is a bug lurking I would much rather fix the bug. Agreed, but I haven't found a way to get more context here. I think I found a separate place where GCC really does look to have a bug, as it complains about array usage that is explicitly bounded. :P -- Kees Cook