From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Martin Subject: Re: [PATCH v2 05/12] arm64: Basic Branch Target Identification support Date: Fri, 11 Oct 2019 16:32:26 +0100 Message-ID: <20191011153225.GL27757@arm.com> References: <1570733080-21015-1-git-send-email-Dave.Martin@arm.com> <1570733080-21015-6-git-send-email-Dave.Martin@arm.com> <20191011151028.GE33537@lakrids.cambridge.arm.com> <4e09ca54-f353-9448-64ed-4ba1e38c6ebc@linaro.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <4e09ca54-f353-9448-64ed-4ba1e38c6ebc@linaro.org> Sender: linux-kernel-owner@vger.kernel.org To: Richard Henderson Cc: Mark Rutland , Paul Elliott , Peter Zijlstra , Catalin Marinas , Will Deacon , Yu-cheng Yu , Amit Kachhap , Vincenzo Frascino , linux-arch@vger.kernel.org, Eugene Syromiatnikov , Szabolcs Nagy , "H.J. Lu" , Andrew Jones , Kees Cook , Arnd Bergmann , Jann Horn , Kristina =?utf-8?Q?Mart=C5=A1enko?= , Mark Brown , Thomas Gleixner , linux-arm-kernel@lists.infradead.org, Fl List-Id: linux-arch.vger.kernel.org On Fri, Oct 11, 2019 at 11:25:33AM -0400, Richard Henderson wrote: > On 10/11/19 11:10 AM, Mark Rutland wrote: > > On Thu, Oct 10, 2019 at 07:44:33PM +0100, Dave Martin wrote: > >> @@ -730,6 +730,11 @@ static void setup_return > >> regs->regs[29] = (unsigned long)&user->next_frame->fp; > >> regs->pc = (unsigned long)ka->sa.sa_handler; > >> > >> + if (system_supports_bti()) { > >> + regs->pstate &= ~PSR_BTYPE_MASK; > >> + regs->pstate |= PSR_BTYPE_CALL; > >> + } > >> + > > > > I think we might need a comment as to what we're trying to ensure here. > > > > I was under the (perhaps mistaken) impression that we'd generate a > > pristine pstate for a signal handler, and it's not clear to me that we > > must ensure the first instruction is a target instruction. > > I think it makes sense to treat entry into a signal handler as a call. Code > that has been compiled for BTI, and whose page has been marked with PROT_BTI, > will already have the pauth/bti markup at the beginning of the signal handler > function; we might as well verify that. > > Otherwise sigaction becomes a hole by which an attacker can force execution to > start at any arbitrary address. Ack, that's the intended rationale -- I also outlined this in the commit message. Does this sound reasonable? Either way, I feel we should do this: any function in a PROT_BTI page should have a suitable landing pad. There's no reason I can see why a protection given to any other callback function should be omitted for a signal handler. Note, if the signal handler isn't in a PROT_BTI page then overriding BTYPE here will not trigger a Branch Target exception. I'm happy to drop a brief comment into the code also, once we're agreed on what the code should be doing. Cheers ---Dave From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from foss.arm.com ([217.140.110.172]:35922 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726328AbfJKPcb (ORCPT ); Fri, 11 Oct 2019 11:32:31 -0400 Date: Fri, 11 Oct 2019 16:32:26 +0100 From: Dave Martin Subject: Re: [PATCH v2 05/12] arm64: Basic Branch Target Identification support Message-ID: <20191011153225.GL27757@arm.com> References: <1570733080-21015-1-git-send-email-Dave.Martin@arm.com> <1570733080-21015-6-git-send-email-Dave.Martin@arm.com> <20191011151028.GE33537@lakrids.cambridge.arm.com> <4e09ca54-f353-9448-64ed-4ba1e38c6ebc@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4e09ca54-f353-9448-64ed-4ba1e38c6ebc@linaro.org> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Richard Henderson Cc: Mark Rutland , Paul Elliott , Peter Zijlstra , Catalin Marinas , Will Deacon , Yu-cheng Yu , Amit Kachhap , Vincenzo Frascino , linux-arch@vger.kernel.org, Eugene Syromiatnikov , Szabolcs Nagy , "H.J. Lu" , Andrew Jones , Kees Cook , Arnd Bergmann , Jann Horn , Kristina =?utf-8?Q?Mart=C5=A1enko?= , Mark Brown , Thomas Gleixner , linux-arm-kernel@lists.infradead.org, Florian Weimer , linux-kernel@vger.kernel.org, Sudakshina Das Message-ID: <20191011153226.e-vKWZ-XwubPI5-JX_ZU8rFVAmyfZ2RF9KxQ8d6sTIE@z> On Fri, Oct 11, 2019 at 11:25:33AM -0400, Richard Henderson wrote: > On 10/11/19 11:10 AM, Mark Rutland wrote: > > On Thu, Oct 10, 2019 at 07:44:33PM +0100, Dave Martin wrote: > >> @@ -730,6 +730,11 @@ static void setup_return > >> regs->regs[29] = (unsigned long)&user->next_frame->fp; > >> regs->pc = (unsigned long)ka->sa.sa_handler; > >> > >> + if (system_supports_bti()) { > >> + regs->pstate &= ~PSR_BTYPE_MASK; > >> + regs->pstate |= PSR_BTYPE_CALL; > >> + } > >> + > > > > I think we might need a comment as to what we're trying to ensure here. > > > > I was under the (perhaps mistaken) impression that we'd generate a > > pristine pstate for a signal handler, and it's not clear to me that we > > must ensure the first instruction is a target instruction. > > I think it makes sense to treat entry into a signal handler as a call. Code > that has been compiled for BTI, and whose page has been marked with PROT_BTI, > will already have the pauth/bti markup at the beginning of the signal handler > function; we might as well verify that. > > Otherwise sigaction becomes a hole by which an attacker can force execution to > start at any arbitrary address. Ack, that's the intended rationale -- I also outlined this in the commit message. Does this sound reasonable? Either way, I feel we should do this: any function in a PROT_BTI page should have a suitable landing pad. There's no reason I can see why a protection given to any other callback function should be omitted for a signal handler. Note, if the signal handler isn't in a PROT_BTI page then overriding BTYPE here will not trigger a Branch Target exception. I'm happy to drop a brief comment into the code also, once we're agreed on what the code should be doing. Cheers ---Dave