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 81458C433EF for ; Wed, 18 May 2022 13:33:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237991AbiERNdJ (ORCPT ); Wed, 18 May 2022 09:33:09 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52316 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237999AbiERNdD (ORCPT ); Wed, 18 May 2022 09:33:03 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D40615DD17; Wed, 18 May 2022 06:33:02 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 7D67C6178D; Wed, 18 May 2022 13:33:02 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id C57CDC385A5; Wed, 18 May 2022 13:32:59 +0000 (UTC) Date: Wed, 18 May 2022 14:32:56 +0100 From: Catalin Marinas To: Will Deacon Cc: Francis Laniel , linux-arm-kernel@lists.infradead.org, linux-trace-devel@vger.kernel.org, Mark Brown , Peter Collingbourne , Mark Rutland , Kees Cook , Daniel Kiss , linux-kernel@vger.kernel.org, James Morse Subject: Re: [RFC PATCH v1 1/1] arm64: Forget syscall if different from execve*() Message-ID: References: <20220509151958.441240-1-flaniel@linux.microsoft.com> <20220509151958.441240-2-flaniel@linux.microsoft.com> <20220510105948.GB27557@willie-the-truck> <2826417.e9J7NaK4W3@pwmachine> <20220510140333.GA28104@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20220510140333.GA28104@willie-the-truck> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 10, 2022 at 03:03:33PM +0100, Will Deacon wrote: > On Tue, May 10, 2022 at 03:00:11PM +0100, Francis Laniel wrote: > > Le mardi 10 mai 2022, 11:59:48 BST Will Deacon a �crit : > > > On Mon, May 09, 2022 at 04:19:57PM +0100, Francis Laniel wrote: > > > > diff --git a/arch/arm64/include/asm/processor.h > > > > b/arch/arm64/include/asm/processor.h index 73e38d9a540c..e12ceb363d6a > > > > 100644 > > > > --- a/arch/arm64/include/asm/processor.h > > > > +++ b/arch/arm64/include/asm/processor.h > > > > @@ -34,6 +34,8 @@ > > > > > > > > #include > > > > > > > > +#include > > > > + > > > > > > > > #include > > > > #include > > > > #include > > > > > > > > @@ -250,8 +252,12 @@ void tls_preserve_current_state(void); > > > > > > > > static inline void start_thread_common(struct pt_regs *regs, unsigned > > > > long pc) { > > > > > > > > + s32 previous_syscall = regs->syscallno; > > > > > > > > memset(regs, 0, sizeof(*regs)); > > > > > > > > - forget_syscall(regs); > > > > + if (previous_syscall == __NR_execve || previous_syscall == > > > > __NR_execveat) > > > > + regs->syscallno = previous_syscall; > > > > + else > > > > + forget_syscall(regs); > > > > > > Hmm, this really looks like a bodge and it doesn't handle the compat case > > > either. > > > > > > How do other architectures handle this? > > > > My understanding of others architectures is quite limited, but here are my > > findings and understanding of some of them: > > * arm (32 bits) EABI: start_thread() sets r7 to previous r7 for ELF FDPIC and > > to 0 for other binfmts [1]. > > * arm (32 bits) OABI: syscall number is set to -1 if > > ptrace_report_syscall_entry() failed [2]. > > * mips: start_thread() does not modify current_thread_info->syscall which is > > taken directly from v0 [3, 4]. > > * riscv: start_thread() does not modify a7 [5]. > > * x86_64: start_thread_common() does not touch orig_ax which seems to contain > > the syscall number [6]. > > Hmm, so the million dollar question is why on Earth we have that > forget_syscall() call to start with. Amusingly I've, err, forgotten; > forget_forget_syscall() perhaps? > > Catalin? It's been there since day one afaict. Looking at the old logs, this appeared somewhere between day 0 and 1. We had a memset(regs, 0) with a pt_regs of 36 registers but moved to dedicated names for syscallno etc. and that's where I changed it from 0 to ~0UL. A few weeks ago James Morse (cc'ed) came across this issue and even sent a patch internally to remove forget_syscall() here. But that looks like a bodge and James' suggestion was that maybe the core code can preserve the syscallno and this would fix it for all architectures. -- Catalin