From mboxrd@z Thu Jan 1 00:00:00 1970 From: Kees Cook Subject: Re: [PATCH 07/29] x86: Restore "text" Program Header with dummy section Date: Thu, 10 Oct 2019 09:46:36 -0700 Message-ID: <201910100944.92CFB97157@keescook> References: <20190926175602.33098-1-keescook@chromium.org> <20190926175602.33098-8-keescook@chromium.org> <20191010103305.GD7658@zn.tnic> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20191010103305.GD7658@zn.tnic> Sender: linux-kernel-owner@vger.kernel.org To: Borislav Petkov Cc: Thomas Gleixner , Rick Edgecombe , Ingo Molnar , "H. Peter Anvin" , Dave Hansen , Andy Lutomirski , Arnd Bergmann , linux-arch@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-alpha@vger.kernel.org, linux-ia64@vger.kernel.org, linux-s390@vger.kernel.org, linux-c6x-dev@linux-c6x.org, Yoshinori Sato , Michal Simek , linux-parisc@vger.kernel.org, linux-xtensa@linux-xtensa.org, x86@kernel.org, linux-kernel@vger.kernel.org List-Id: linux-arch.vger.kernel.org On Thu, Oct 10, 2019 at 12:33:05PM +0200, Borislav Petkov wrote: > On Thu, Sep 26, 2019 at 10:55:40AM -0700, Kees Cook wrote: > > Instead of depending on markings in the section following NOTES to > > restore the associated Program Header, use a dummy section, as done > > in other architectures. > > This is very laconic and after some staring at ld.info, I think you mean > this: > > " If you place a section in one or more segments using ':PHDR', then > the linker will place all subsequent allocatable sections which do not > specify ':PHDR' in the same segments." > > but I could be way off. Yes, no? > > IOW, please write in the commit messages first what the problem is > you're addressing. Yeah, that's much improved. I really struggled to describe this, given how weird the linker script logic is here. > > --- a/arch/x86/kernel/vmlinux.lds.S > > +++ b/arch/x86/kernel/vmlinux.lds.S > > @@ -147,8 +147,9 @@ SECTIONS > > } :text = 0x9090 > > > > NOTES :text :note > > + .dummy : { *(.dummy) } :text > > > > - EXCEPTION_TABLE(16) :text = 0x9090 > > + EXCEPTION_TABLE(16) > > This is killing the filler byte but I have a suspicion that'll change > eventually to INT3... :) Yes, though since the exception table isn't executable, filling with 0x90 has no meaning, and I think when I looked at the alignment there were actually no fill bytes from here on. But it doesn't matter; it all goes away in the end. -- Kees Cook From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f194.google.com ([209.85.210.194]:34936 "EHLO mail-pf1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725901AbfJJQqj (ORCPT ); Thu, 10 Oct 2019 12:46:39 -0400 Received: by mail-pf1-f194.google.com with SMTP id 205so4279694pfw.2 for ; Thu, 10 Oct 2019 09:46:38 -0700 (PDT) Date: Thu, 10 Oct 2019 09:46:36 -0700 From: Kees Cook Subject: Re: [PATCH 07/29] x86: Restore "text" Program Header with dummy section Message-ID: <201910100944.92CFB97157@keescook> References: <20190926175602.33098-1-keescook@chromium.org> <20190926175602.33098-8-keescook@chromium.org> <20191010103305.GD7658@zn.tnic> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191010103305.GD7658@zn.tnic> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Borislav Petkov Cc: Thomas Gleixner , Rick Edgecombe , Ingo Molnar , "H. Peter Anvin" , Dave Hansen , Andy Lutomirski , Arnd Bergmann , linux-arch@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-alpha@vger.kernel.org, linux-ia64@vger.kernel.org, linux-s390@vger.kernel.org, linux-c6x-dev@linux-c6x.org, Yoshinori Sato , Michal Simek , linux-parisc@vger.kernel.org, linux-xtensa@linux-xtensa.org, x86@kernel.org, linux-kernel@vger.kernel.org Message-ID: <20191010164636.klRtzk6yt77TU64ZUprkTyDqnhI6exNf0pXv9VHmafI@z> On Thu, Oct 10, 2019 at 12:33:05PM +0200, Borislav Petkov wrote: > On Thu, Sep 26, 2019 at 10:55:40AM -0700, Kees Cook wrote: > > Instead of depending on markings in the section following NOTES to > > restore the associated Program Header, use a dummy section, as done > > in other architectures. > > This is very laconic and after some staring at ld.info, I think you mean > this: > > " If you place a section in one or more segments using ':PHDR', then > the linker will place all subsequent allocatable sections which do not > specify ':PHDR' in the same segments." > > but I could be way off. Yes, no? > > IOW, please write in the commit messages first what the problem is > you're addressing. Yeah, that's much improved. I really struggled to describe this, given how weird the linker script logic is here. > > --- a/arch/x86/kernel/vmlinux.lds.S > > +++ b/arch/x86/kernel/vmlinux.lds.S > > @@ -147,8 +147,9 @@ SECTIONS > > } :text = 0x9090 > > > > NOTES :text :note > > + .dummy : { *(.dummy) } :text > > > > - EXCEPTION_TABLE(16) :text = 0x9090 > > + EXCEPTION_TABLE(16) > > This is killing the filler byte but I have a suspicion that'll change > eventually to INT3... :) Yes, though since the exception table isn't executable, filling with 0x90 has no meaning, and I think when I looked at the alignment there were actually no fill bytes from here on. But it doesn't matter; it all goes away in the end. -- Kees Cook