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=-6.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, 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 DE14AC433DB for ; Mon, 29 Mar 2021 18:41:42 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id 364FC6195D for ; Mon, 29 Mar 2021 18:41:42 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 364FC6195D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=chromium.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-21076-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 17577 invoked by uid 550); 29 Mar 2021 18:41:34 -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 17557 invoked from network); 29 Mar 2021 18:41:33 -0000 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=aYL/dbQ14QTXXMSkWSQ/z+qzj6yL6Ve3lzeO159+/Vc=; b=ILRNFBaUrvYS27aWFLaYXO8LPj6pKTA2SBD59ksr9Fpfa7kG44gHIoQIIeiA0uW0DT INDEJxHCxLo9mNI1mXPwmIRMdAheSsDUl9E7dL0goqI8aLORifPfviS8NWFxCGPsRDpU h2HExNb2DqeqeKKnXrybXZvXygAADq2smXaJA= 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=aYL/dbQ14QTXXMSkWSQ/z+qzj6yL6Ve3lzeO159+/Vc=; b=TznanbPacdS+xSKYVhb9pKWOxrV/PGv/rpvYK/XevtdY9c0SyZl3jQC/n++qOKKaQq 7o4EKvZ2aWUr5E+9plmkGNUhUFTbq4kJy4n6z+m0QKOgpxjRrh1ZULNsBhhgvPCNTo/7 cfUdPPiTIyQmlNBM+RA2japnFYETyaYtNusccFJkT9j8NSalrYhpbSh8zBDYFQcyiSkW 226DDxyMjwwIYlSZgrfxgK10ofJtwrwTmuOE7AZee83pT0ACFiTw0Gv6aHyvNyiXotjZ 8X0yFio9OsKcLYnjGRQ9K5SHyXaakaZI5CcIQMMMoId9KDhWm3Q2opkkkVLx/QCZA2uf OrjQ== X-Gm-Message-State: AOAM530yIRp3/gsWU/NIFYItyOdx2/s/908fkaLd0fAz4UGXjWj6xZin OfRzPjXxG67zFKphDVpJkoR0Jw== X-Google-Smtp-Source: ABdhPJxjjtBRwQY4aFK1R1QLvwpTRJ9D0T4m5/43mE5p+szS6JXZVD5wBPAm/iQ4MzX4uEt8W4cEIA== X-Received: by 2002:aa7:8a19:0:b029:1f6:6839:7211 with SMTP id m25-20020aa78a190000b02901f668397211mr25877536pfa.40.1617043281318; Mon, 29 Mar 2021 11:41:21 -0700 (PDT) Date: Mon, 29 Mar 2021 11:41:18 -0700 From: Kees Cook To: Thomas Gleixner Cc: Elena Reshetova , x86@kernel.org, Andy Lutomirski , Peter Zijlstra , Catalin Marinas , Will Deacon , Mark Rutland , Alexander Potapenko , Alexander Popov , Ard Biesheuvel , Jann Horn , Vlastimil Babka , David Hildenbrand , Mike Rapoport , Andrew Morton , Jonathan Corbet , Randy Dunlap , kernel-hardening@lists.openwall.com, linux-hardening@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v7 3/6] stack: Optionally randomize kernel stack offset each syscall Message-ID: <202103291139.54AA7CDE@keescook> References: <20210319212835.3928492-1-keescook@chromium.org> <20210319212835.3928492-4-keescook@chromium.org> <87eefzcpc4.ffs@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87eefzcpc4.ffs@nanos.tec.linutronix.de> On Sun, Mar 28, 2021 at 04:42:03PM +0200, Thomas Gleixner wrote: > On Fri, Mar 19 2021 at 14:28, Kees Cook wrote: > > +/* > > + * Do not use this anywhere else in the kernel. This is used here because > > + * it provides an arch-agnostic way to grow the stack with correct > > + * alignment. Also, since this use is being explicitly masked to a max of > > + * 10 bits, stack-clash style attacks are unlikely. For more details see > > + * "VLAs" in Documentation/process/deprecated.rst > > VLAs are bad, VLAs to the rescue! :) I'm aware of the irony, but luto's idea really makes things easy. As documented there, though, this has a hard-coded (low) upper bound, so it's not like "regular" VLA use. > > > + * The asm statement is designed to convince the compiler to keep the > > + * allocation around even after "ptr" goes out of scope. > > + */ > > +void *__builtin_alloca(size_t size); > > + > > +#define add_random_kstack_offset() do { \ > > + if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ > > + &randomize_kstack_offset)) { \ > > + u32 offset = this_cpu_read(kstack_offset); \ > > Not that it matters on x86, but as this has to be called in the > interrupt disabled region of the syscall entry, shouldn't this be a > raw_cpu_read(). The asm-generic version has a preempt_disable/enable > pair around the raw read for native wordsize reads, otherwise a > irqsave/restore pair. > > __this_cpu_read() is fine as well, but that has an sanity check before > the raw read when CONFIG_DEBUG_PREEMPT is on, which is harmless but > also pointless in this case. > > Probably the same for the counterpart this_cpu_write(). Oh! Excellent point. I think this will make a big difference on arm64. I will adjust and test. -- Kees Cook