From patchwork Thu Jun 13 15:51:36 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Vincenzo Frascino X-Patchwork-Id: 10992551 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id DCAC218A6 for ; Thu, 13 Jun 2019 15:51:59 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id CB1DE212DA for ; Thu, 13 Jun 2019 15:51:59 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id C93412228E; Thu, 13 Jun 2019 15:51:59 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.9 required=2.0 tests=BAYES_00,MAILING_LIST_MULTI, RCVD_IN_DNSWL_HI autolearn=ham version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 2709E22B1F for ; Thu, 13 Jun 2019 15:51:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389899AbfFMPv5 (ORCPT ); Thu, 13 Jun 2019 11:51:57 -0400 Received: from foss.arm.com ([217.140.110.172]:43860 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2389743AbfFMPv4 (ORCPT ); Thu, 13 Jun 2019 11:51:56 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id D6CF03EF; Thu, 13 Jun 2019 08:51:55 -0700 (PDT) Received: from e119884-lin.cambridge.arm.com (e119884-lin.cambridge.arm.com [10.1.196.72]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 827973F246; Thu, 13 Jun 2019 08:51:54 -0700 (PDT) From: Vincenzo Frascino To: linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org Cc: Catalin Marinas , Will Deacon , Andrey Konovalov , Alexander Viro Subject: [PATCH v5 1/2] arm64: Define Documentation/arm64/tagged-address-abi.txt Date: Thu, 13 Jun 2019 16:51:36 +0100 Message-Id: <20190613155137.47675-2-vincenzo.frascino@arm.com> X-Mailer: git-send-email 2.21.0 In-Reply-To: <20190613155137.47675-1-vincenzo.frascino@arm.com> References: <20190613155137.47675-1-vincenzo.frascino@arm.com> MIME-Version: 1.0 Sender: linux-kselftest-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kselftest@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP On arm64 the TCR_EL1.TBI0 bit has been always enabled hence the userspace (EL0) is allowed to set a non-zero value in the top byte but the resulting pointers are not allowed at the user-kernel syscall ABI boundary. With the relaxed ABI proposed through this document, it is now possible to pass tagged pointers to the syscalls, when these pointers are in memory ranges obtained by an anonymous (MAP_ANONYMOUS) mmap(). This change in the ABI requires a mechanism to requires the userspace to opt-in to such an option. Specify and document the way in which sysctl and prctl() can be used in combination to allow the userspace to opt-in this feature. Cc: Catalin Marinas Cc: Will Deacon CC: Andrey Konovalov Signed-off-by: Vincenzo Frascino Acked-by: Szabolcs Nagy --- Documentation/arm64/tagged-address-abi.txt | 134 +++++++++++++++++++++ 1 file changed, 134 insertions(+) create mode 100644 Documentation/arm64/tagged-address-abi.txt diff --git a/Documentation/arm64/tagged-address-abi.txt b/Documentation/arm64/tagged-address-abi.txt new file mode 100644 index 000000000000..0ae900d4bb2d --- /dev/null +++ b/Documentation/arm64/tagged-address-abi.txt @@ -0,0 +1,134 @@ +ARM64 TAGGED ADDRESS ABI +======================== + +This document describes the usage and semantics of the Tagged Address +ABI on arm64. + +1. Introduction +--------------- + +On arm64 the TCR_EL1.TBI0 bit has been always enabled on the kernel, hence +the userspace (EL0) is entitled to perform a user memory access through a +64-bit pointer with a non-zero top byte but the resulting pointers are not +allowed at the user-kernel syscall ABI boundary. + +This document describes a relaxation of the ABI that makes it possible to +to pass tagged pointers to the syscalls, when these pointers are in memory +ranges obtained as described in section 2. + +Since it is not desirable to relax the ABI to allow tagged user addresses +into the kernel indiscriminately, arm64 provides a new sysctl interface +(/proc/sys/abi/tagged_addr) that is used to prevent the applications from +enabling the relaxed ABI and a new prctl() interface that can be used to +enable or disable the relaxed ABI. +A detailed description of the newly introduced mechanisms will be provided +in section 2. + +2. ARM64 Tagged Address ABI +--------------------------- + +From the kernel syscall interface perspective, we define, for the purposes +of this document, a "valid tagged pointer" as a pointer that either has a +zero value set in the top byte or has a non-zero value, it is in memory +ranges privately owned by a userspace process and it is obtained in one of +the following ways: + - mmap() done by the process itself, where either: + * flags have MAP_PRIVATE and MAP_ANONYMOUS + * flags have MAP_PRIVATE and the file descriptor refers to a regular + file or "/dev/zero" + - brk() system call done by the process itself (i.e. the heap area between + the initial location of the program break at process creation and its + current location). + - any memory mapped by the kernel in the process's address space during + creation and following the restrictions presented above (i.e. data, bss, + stack). + +The ARM64 Tagged Address ABI is an opt-in feature, and an application can +control it using the following: + - /proc/sys/abi/tagged_addr: a new sysctl interface that can be used to + prevent the applications from enabling the relaxed ABI. + The sysctl is meant also for testing purposes in order to provide a + simple way for the userspace to verify the return error checking of + the prctl() commands without having to reconfigure the kernel. + The sysctl supports the following configuration options: + - 0: Disable ARM64 Tagged Address ABI for all the applications. + - 1 (Default): Enable ARM64 Tagged Address ABI for all the + applications. + If the ARM64 Tagged Address ABI is disabled at a certain point in + time, all the applications that were using tagging before this event + occurs, will continue to use tagging. + + - prctl()s: + - PR_SET_TAGGED_ADDR_CTRL: can be used to enable or disable the Tagged + Address ABI. + The (unsigned int) arg2 argument is a bit mask describing the + control mode used: + - PR_TAGGED_ADDR_ENABLE: Enable ARM64 Tagged Address ABI. + The arguments arg3, arg4, and arg5 are ignored. + + - PR_GET_TAGGED_ADDR_CTRL: can be used to check the status of the Tagged + Address ABI. + The arguments arg2, arg3, arg4, and arg5 are ignored. + +The ABI properties set by the mechanisms described above are inherited by threads +of the same application and fork()'ed children but cleared by execve(). + +As a consequence of invoking PR_SET_TAGGED_ADDR_CTRL prctl() by an applications, +the ABI guarantees the following behaviours: + + - Every current or newly introduced syscall can accept any valid tagged + pointers. + + - If a non valid tagged pointer is passed to a syscall then the behaviour + is undefined. + + - Every valid tagged pointer is expected to work as an untagged one. + + - The kernel preserves any valid tagged pointers and returns them to the + userspace unchanged (i.e. on syscall return) in all the cases except the + ones documented in the "Preserving tags" section of tagged-pointers.txt. + +A definition of the meaning of tagged pointers on arm64 can be found in: +Documentation/arm64/tagged-pointers.txt. + +3. ARM64 Tagged Address ABI Exceptions +-------------------------------------- + +The behaviours described in section 2, with particular reference to the +acceptance by the syscalls of any valid tagged pointer are not applicable +to the following cases: + - mmap() addr parameter. + - mremap() new_address parameter. + - prctl_set_mm() struct prctl_map fields. + - prctl_set_mm_map() struct prctl_map fields. + +Any attempt to use non-zero tagged pointers will lead to undefined behaviour. + +4. Example of correct usage +--------------------------- + +void main(void) +{ + static int tbi_enabled = 0; + unsigned long tag = 0; + + char *ptr = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE, + MAP_ANONYMOUS, -1, 0); + + if (prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE, + 0, 0, 0) == 0) + tbi_enabled = 1; + + if (ptr == (void *)-1) /* MAP_FAILED */ + return -1; + + if (tbi_enabled) + tag = rand() & 0xff; + + ptr = (char *)((unsigned long)ptr | (tag << TAG_SHIFT)); + + *ptr = 'a'; + + ... +} + From patchwork Thu Jun 13 15:51:37 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Vincenzo Frascino X-Patchwork-Id: 10992559 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id DFCF514BB for ; Thu, 13 Jun 2019 15:52:08 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id D157922376 for ; Thu, 13 Jun 2019 15:52:08 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id CFAB222BF1; Thu, 13 Jun 2019 15:52:08 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.9 required=2.0 tests=BAYES_00,MAILING_LIST_MULTI, RCVD_IN_DNSWL_HI autolearn=unavailable version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 5DAF822376 for ; Thu, 13 Jun 2019 15:52:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389915AbfFMPv7 (ORCPT ); Thu, 13 Jun 2019 11:51:59 -0400 Received: from foss.arm.com ([217.140.110.172]:43874 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2389906AbfFMPv6 (ORCPT ); Thu, 13 Jun 2019 11:51:58 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 6B8E0A78; Thu, 13 Jun 2019 08:51:57 -0700 (PDT) Received: from e119884-lin.cambridge.arm.com (e119884-lin.cambridge.arm.com [10.1.196.72]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 1748E3F246; Thu, 13 Jun 2019 08:51:55 -0700 (PDT) From: Vincenzo Frascino To: linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org Cc: Catalin Marinas , Will Deacon , Andrey Konovalov , Alexander Viro Subject: [PATCH v5 2/2] arm64: Relax Documentation/arm64/tagged-pointers.txt Date: Thu, 13 Jun 2019 16:51:37 +0100 Message-Id: <20190613155137.47675-3-vincenzo.frascino@arm.com> X-Mailer: git-send-email 2.21.0 In-Reply-To: <20190613155137.47675-1-vincenzo.frascino@arm.com> References: <20190613155137.47675-1-vincenzo.frascino@arm.com> MIME-Version: 1.0 Sender: linux-kselftest-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kselftest@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP On arm64 the TCR_EL1.TBI0 bit has been always enabled hence the userspace (EL0) is allowed to set a non-zero value in the top byte but the resulting pointers are not allowed at the user-kernel syscall ABI boundary. With the relaxed ABI proposed in this set, it is now possible to pass tagged pointers to the syscalls, when these pointers are in memory ranges obtained by an anonymous (MAP_ANONYMOUS) mmap(). Relax the requirements described in tagged-pointers.txt to be compliant with the behaviours guaranteed by the ARM64 Tagged Address ABI. Cc: Catalin Marinas Cc: Will Deacon CC: Andrey Konovalov Signed-off-by: Vincenzo Frascino --- Documentation/arm64/tagged-pointers.txt | 23 ++++++++++++++++------- 1 file changed, 16 insertions(+), 7 deletions(-) diff --git a/Documentation/arm64/tagged-pointers.txt b/Documentation/arm64/tagged-pointers.txt index a25a99e82bb1..e33af14478e3 100644 --- a/Documentation/arm64/tagged-pointers.txt +++ b/Documentation/arm64/tagged-pointers.txt @@ -18,7 +18,8 @@ Passing tagged addresses to the kernel -------------------------------------- All interpretation of userspace memory addresses by the kernel assumes -an address tag of 0x00. +an address tag of 0x00, unless the userspace opts-in the ARM64 Tagged +Address ABI via the PR_SET_TAGGED_ADDR_CTRL prctl(). This includes, but is not limited to, addresses found in: @@ -31,18 +32,23 @@ This includes, but is not limited to, addresses found in: - the frame pointer (x29) and frame records, e.g. when interpreting them to generate a backtrace or call graph. -Using non-zero address tags in any of these locations may result in an -error code being returned, a (fatal) signal being raised, or other modes -of failure. +Using non-zero address tags in any of these locations when the +userspace application did not opt-in to the ARM64 Tagged Address ABI +may result in an error code being returned, a (fatal) signal being raised, +or other modes of failure. -For these reasons, passing non-zero address tags to the kernel via -system calls is forbidden, and using a non-zero address tag for sp is -strongly discouraged. +For these reasons, when the userspace application did not opt-in, passing +non-zero address tags to the kernel via system calls is forbidden, and using +a non-zero address tag for sp is strongly discouraged. Programs maintaining a frame pointer and frame records that use non-zero address tags may suffer impaired or inaccurate debug and profiling visibility. +A definition of the meaning of ARM64 Tagged Address ABI and of the +guarantees that the ABI provides when the userspace opts-in via prctl() +can be found in: Documentation/arm64/tagged-address-abi.txt. + Preserving tags --------------- @@ -57,6 +63,9 @@ be preserved. The architecture prevents the use of a tagged PC, so the upper byte will be set to a sign-extension of bit 55 on exception return. +These behaviours are preserved even when the userspace opts-in to the ARM64 +Tagged Address ABI via the PR_SET_TAGGED_ADDR_CTRL prctl(). + Other considerations --------------------