From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on archive.lwn.net X-Spam-Level: X-Spam-Status: No, score=-5.7 required=5.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI,SPF_HELO_NONE,SPF_NONE autolearn=unavailable autolearn_force=no version=3.4.2 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by archive.lwn.net (Postfix) with ESMTP id 15F377DBEB for ; Thu, 15 Aug 2019 15:44:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730860AbfHOPoQ (ORCPT ); Thu, 15 Aug 2019 11:44:16 -0400 Received: from foss.arm.com ([217.140.110.172]:45766 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729975AbfHOPoQ (ORCPT ); Thu, 15 Aug 2019 11:44:16 -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 7DC811597; Thu, 15 Aug 2019 08:44:15 -0700 (PDT) Received: from arrakis.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id CC42A3F738; Thu, 15 Aug 2019 08:44:13 -0700 (PDT) From: Catalin Marinas To: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org Cc: Andrew Morton , Vincenzo Frascino , Will Deacon , Andrey Konovalov , Szabolcs Nagy , Kevin Brodsky , Dave P Martin , Dave Hansen , linux-doc@vger.kernel.org, linux-arch@vger.kernel.org Subject: [PATCH v8 4/5] arm64: Define Documentation/arm64/tagged-address-abi.rst Date: Thu, 15 Aug 2019 16:44:02 +0100 Message-Id: <20190815154403.16473-5-catalin.marinas@arm.com> X-Mailer: git-send-email 2.23.0.rc0 In-Reply-To: <20190815154403.16473-1-catalin.marinas@arm.com> References: <20190815154403.16473-1-catalin.marinas@arm.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org From: Vincenzo Frascino On AArch64 the TCR_EL1.TBI0 bit is set by default, allowing userspace (EL0) to perform memory accesses through 64-bit pointers with a non-zero top byte. Introduce the document describing the relaxation of the syscall ABI that allows userspace to pass certain tagged pointers to kernel syscalls. Cc: Will Deacon Cc: Andrey Konovalov Cc: Szabolcs Nagy Cc: Kevin Brodsky Signed-off-by: Vincenzo Frascino Co-developed-by: Catalin Marinas Signed-off-by: Catalin Marinas --- Documentation/arm64/tagged-address-abi.rst | 155 +++++++++++++++++++++ 1 file changed, 155 insertions(+) create mode 100644 Documentation/arm64/tagged-address-abi.rst diff --git a/Documentation/arm64/tagged-address-abi.rst b/Documentation/arm64/tagged-address-abi.rst new file mode 100644 index 000000000000..8808337775d6 --- /dev/null +++ b/Documentation/arm64/tagged-address-abi.rst @@ -0,0 +1,155 @@ +========================== +AArch64 TAGGED ADDRESS ABI +========================== + +Authors: Vincenzo Frascino + Catalin Marinas + +Date: 15 August 2019 + +This document describes the usage and semantics of the Tagged Address +ABI on AArch64 Linux. + +1. Introduction +--------------- + +On AArch64 the TCR_EL1.TBI0 bit is set by default, allowing userspace +(EL0) to perform memory accesses through 64-bit pointers with a non-zero +top byte. This document describes the relaxation of the syscall ABI that +allows userspace to pass certain tagged pointers to kernel syscalls. + +2. AArch64 Tagged Address ABI +----------------------------- + +From the kernel syscall interface perspective and for the purposes of +this document, a "valid tagged pointer" is a pointer with a potentially +non-zero top-byte that references an address in the user process address +space obtained in one of the following ways: + +- mmap() done by the process itself (or its parent), where either: + + - flags have the **MAP_ANONYMOUS** bit set + - the file descriptor refers to a regular file (including those + returned by memfd_create()) 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 address space of the process + during creation and with the same restrictions as for mmap() above + (e.g. data, bss, stack). + +The AArch64 Tagged Address ABI has two stages of relaxation depending +how the user addresses are used by the kernel: + +1. User addresses not accessed by the kernel but used for address space + management (e.g. mmap(), mprotect(), madvise()). The use of valid + tagged pointers in this context is always allowed. + +2. User addresses accessed by the kernel (e.g. write()). This ABI + relaxation is disabled by default and the application thread needs to + explicitly enable it via **prctl()** as follows: + + - **PR_SET_TAGGED_ADDR_CTRL**: enable or disable the AArch64 Tagged + Address ABI for the calling thread. + + The (unsigned int) arg2 argument is a bit mask describing the + control mode used: + + - **PR_TAGGED_ADDR_ENABLE**: enable AArch64 Tagged Address ABI. + Default status is disabled. + + Arguments arg3, arg4, and arg5 must be 0. + + - **PR_GET_TAGGED_ADDR_CTRL**: get the status of the AArch64 Tagged + Address ABI for the calling thread. + + Arguments arg2, arg3, arg4, and arg5 must be 0. + + The ABI properties described above are thread-scoped, inherited on + clone() and fork() and cleared on exec(). + + Calling prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE, 0, 0, 0) + returns -EINVAL if the AArch64 Tagged Address ABI is globally disabled + by sysctl abi.tagged_addr_disabled=1. The default sysctl + abi.tagged_addr_disabled configuration is 0. + +When the AArch64 Tagged Address ABI is enabled for a thread, the +following behaviours are guaranteed: + +- All syscalls except the cases mentioned in section 3 can accept any + valid tagged pointer. + +- The syscall behaviour is undefined for invalid tagged pointers: it may + result in an error code being returned, a (fatal) signal being raised, + or other modes of failure. + +- A valid tagged pointer has the same semantics as the corresponding + untagged pointer. + +A definition of the meaning of tagged pointers on AArch64 can be found +in Documentation/arm64/tagged-pointers.rst. + +3. AArch64 Tagged Address ABI Exceptions +----------------------------------------- + +The following system call parameters must be untagged regardless of the +ABI relaxation: + +- prctl() other than arguments pointing to user structures to be + accessed by the kernel. + +- ioctl() other than arguments pointing to user structures to be + accessed by the kernel. + +- shmat() and shmdt(). + +Any attempt to use non-zero tagged pointers may result in an error code +being returned, a (fatal) signal being raised, or other modes of +failure. + +4. Example of correct usage +--------------------------- +.. code-block:: c + + #include + #include + #include + #include + #include + + #define PR_SET_TAGGED_ADDR_CTRL 55 + #define PR_TAGGED_ADDR_ENABLE (1UL << 0) + + #define TAG_SHIFT 56 + + int main(void) + { + int tbi_enabled = 0; + unsigned long tag = 0; + char *ptr; + + /* check/enable the tagged address ABI */ + if (!prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE, 0, 0, 0)) + tbi_enabled = 1; + + /* memory allocation */ + ptr = mmap(NULL, sysconf(_SC_PAGE_SIZE), PROT_READ | PROT_WRITE, + MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); + if (ptr == MAP_FAILED) + return 1; + + /* set a non-zero tag if the ABI is available */ + if (tbi_enabled) + tag = rand() & 0xff; + ptr = (char *)((unsigned long)ptr | (tag << TAG_SHIFT)); + + /* memory access to a tagged address */ + strcpy(ptr, "tagged pointer\n"); + + /* syscall with a tagged pointer */ + write(1, ptr, strlen(ptr)); + + return 0; + }