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=-8.4 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_IN_DEF_DKIM_WL 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 68ABFC55185 for ; Wed, 22 Apr 2020 23:52:02 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id BEA652076E for ; Wed, 22 Apr 2020 23:52:01 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="b1xFmE2s" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BEA652076E Authentication-Results: mail.kernel.org; dmarc=fail (p=reject dis=none) header.from=google.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-18612-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 20185 invoked by uid 550); 22 Apr 2020 23:51:54 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 20162 invoked from network); 22 Apr 2020 23:51:54 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=QgMVZz3XwLkExErntao4WLiI0VS/DsZOFmHjZC/Mbow=; b=b1xFmE2sV5VW4aIlU/whxS/awWX9vekQ0Ico8eQgciMKy7zuNVk5XtOpg39bOTWcKs gmE9IbR9W35bFsypZV4vL3HSs5YmoSf5fN8616fkMWbslDLfiUYdepcmXnGAGCT9UBow tcegupNB12V2vId+LwrLQi0D9fZMOT/3eaM73mrtZdPpGBHCbmDq0L+M54abVOxMotXA LxFAdJGBFIgJQJDG8i+fcobu58I88gq4sWg+B8YpHYN1E99k4GEnsYVLt9Kz7c/v8YvO 1Ag5ItZ7vliNh16MSWpcpImUWRG2bw/nM3eiMhbI7Z3GgjX0L1CaNHTEcHJUd8nG/PfW SEpw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=QgMVZz3XwLkExErntao4WLiI0VS/DsZOFmHjZC/Mbow=; b=chRcROO0iy5frlFnSz2mcdOl6oYOzGNyPsI4YpPOyH4NT2PoZaGHvVhI61gzd+NHvP RTsSBVvtP/Am4METjmLqLkurJgexY9pLZbem3Yg+M/tDW/GKBJzyhr+pA0oCcE8Xcxee LHCGWvj2+gAzIQDeXyNz3BpHupwtVxU4mwO507jdFTtnlXCTU4qVG9pAbZHyr7jcoy+s IEpzCdhf2Gh0BtmuAbldg7Lw9jnWg6Cv8FO9gXN84dNidQ/2asMhfF2fhunfB0k3xQ7d 8peOoj+N3nrLHDKnBvwoamCC8ZFv68RKf2ujNbKZX1I/VW0v4joBVH30envQA0vIgUUG Uq/g== X-Gm-Message-State: AGi0PuYtkPf+0wuoaocvHrbavo1Dbf5hO3oC0XLsQEOSyM4misNluOT1 Ar7xliIYBYUHf8477PkPJOQIVQ== X-Google-Smtp-Source: APiQypKd8hMC33L8Wj5RKdsZ70cGbYI15nshdaNSvjex+B8eCij/WOOT64Lb2ePxq5TYoHX5bEGXjw== X-Received: by 2002:a17:902:b097:: with SMTP id p23mr1161570plr.195.1587599501845; Wed, 22 Apr 2020 16:51:41 -0700 (PDT) Date: Wed, 22 Apr 2020 16:51:34 -0700 From: Sami Tolvanen To: Will Deacon Cc: Catalin Marinas , James Morse , Steven Rostedt , Ard Biesheuvel , Mark Rutland , Masahiro Yamada , Michal Marek , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dave Martin , Kees Cook , Laura Abbott , Marc Zyngier , Masami Hiramatsu , Nick Desaulniers , Jann Horn , Miguel Ojeda , clang-built-linux@googlegroups.com, kernel-hardening@lists.openwall.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v11 01/12] add support for Clang's Shadow Call Stack (SCS) Message-ID: <20200422235134.GA211149@google.com> References: <20191018161033.261971-1-samitolvanen@google.com> <20200416161245.148813-1-samitolvanen@google.com> <20200416161245.148813-2-samitolvanen@google.com> <20200420171727.GB24386@willie-the-truck> <20200420211830.GA5081@google.com> <20200422173938.GA3069@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200422173938.GA3069@willie-the-truck> On Wed, Apr 22, 2020 at 06:39:47PM +0100, Will Deacon wrote: > On Mon, Apr 20, 2020 at 02:18:30PM -0700, Sami Tolvanen wrote: > > On Mon, Apr 20, 2020 at 06:17:28PM +0100, Will Deacon wrote: > > > > + * The shadow call stack is aligned to SCS_SIZE, and grows > > > > + * upwards, so we can mask out the low bits to extract the base > > > > + * when the task is not running. > > > > + */ > > > > + return (void *)((unsigned long)task_scs(tsk) & ~(SCS_SIZE - 1)); > > > > > > Could we avoid forcing this alignment it we stored the SCS pointer as a > > > (base,offset) pair instead? That might be friendlier on the allocations > > > later on. > > > > The idea is to avoid storing the current task's shadow stack address in > > memory, which is why I would rather not store the base address either. > > What I mean is that, instead of storing the current shadow stack pointer, > we instead store a base and an offset. We can still clear the base, as you > do with the pointer today, and I don't see that the offset is useful to > an attacker on its own. I see what you mean. However, even if we store the base address + the offset, we still need aligned allocation if we want to clear the address. This would basically just move __scs_base() logic to cpu_switch_to() / scs_save(). > But more generally, is it really worthwhile to do this clearing at all? Can > you (or Kees?) provide some justification for it, please? We don't do it > for anything else, e.g. the pointer authentication keys, so something > feels amiss here. Like Kees pointed out, this makes it slightly harder to locate the current task's shadow stack pointer. I realize there are other useful targets in thread_info, but we would rather not make this any easier than necessary. Is your primary concern here the cost of doing this, or just that it doesn't sufficiently improve security? Sami