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[67.216.151.25]) by smtp.gmail.com with ESMTPSA id t82sm2316781ywc.26.2019.10.11.08.25.34 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 11 Oct 2019 08:25:35 -0700 (PDT) Subject: Re: [PATCH v2 05/12] arm64: Basic Branch Target Identification support To: Mark Rutland , Dave Martin Cc: linux-kernel@vger.kernel.org, Andrew Jones , Arnd Bergmann , Catalin Marinas , Eugene Syromiatnikov , Florian Weimer , "H.J. Lu" , Jann Horn , Kees Cook , =?UTF-8?Q?Kristina_Mart=c5=a1enko?= , Mark Brown , Paul Elliott , Peter Zijlstra , Sudakshina Das , Szabolcs Nagy , Thomas Gleixner , Will Deacon , Yu-cheng Yu , Amit Kachhap , Vincenzo Frascino , linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org 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> From: Richard Henderson Openpgp: preference=signencrypt Message-ID: <4e09ca54-f353-9448-64ed-4ba1e38c6ebc@linaro.org> Date: Fri, 11 Oct 2019 11:25:33 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20191011151028.GE33537@lakrids.cambridge.arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. r~