From patchwork Wed Dec 8 04:48:08 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Peter Collingbourne X-Patchwork-Id: 12695282 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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 4C78DC433F5 for ; Wed, 8 Dec 2021 04:52:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:Cc:To:From:Subject:References: Mime-Version:Message-Id:In-Reply-To:Date:Reply-To:Content-ID: Content-Description:Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc :Resent-Message-ID:List-Owner; bh=bfSTcTA3Q0izeT7Kl8PakYkyadecqVuaHtMnc0DA2CQ=; b=Q7COisRVhekz+FvM6eIdp4Hg0R 2adZ8wOA3vQj6pqfVCWj9ud1jY03quaUQNGWScZmSyrdb+8yu2yYF+nVrXEmFboDO5JBY1UHShLzP r5feQd0/ClDe+Vqmo0QTFukjewkAtIpb30o3baJZVNfgGSIzs2FDQNcKo4UBfK7228+Wpr+ZbSiYj JDX7iLm5qhw/pW/wMGjT/JZZZeQ0ZaVs21XVLFGnIH4jWBsDw7jMIal3iuze8klvIaLxnASkrAD0N TM9zp87kOhspnzGyYA0jOlVSFxcQ6OjU8z8Aku4d3ptmh69XucNNR+DLx0deK2eaaCkisxwkqdeG/ blwkJjyw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1muouQ-00BFHw-KK; Wed, 08 Dec 2021 04:50:28 +0000 Received: from mail-yb1-xb4a.google.com ([2607:f8b0:4864:20::b4a]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1muosb-00BERK-Le for linux-arm-kernel@lists.infradead.org; Wed, 08 Dec 2021 04:48:35 +0000 Received: by mail-yb1-xb4a.google.com with SMTP id b15-20020a25ae8f000000b005c20f367790so2589815ybj.2 for ; Tue, 07 Dec 2021 20:48:32 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=date:in-reply-to:message-id:mime-version:references:subject:from:to :cc; bh=a1e01/uQIMruIVMxVHsJI+lKzq1arm+BxjGSeQVMHMA=; b=X5XGUkPM9DeYjKTNyLx2ogk3uEMOMqK3PIrwP0pN9HrJlyX1XaukdQVdrACI73UoI3 C73Tp66+6etXU4aZ1PWgFJ4WBFWZtFEroPEq02Gkr6xcejitZp76DXS8oTs3s5VOjqNP Eyg7FdUgoYYDf886Tg9frjeUBQTjA5v56tFO2t8rXOkyLvD+UDdxYtuf7+mZtcYhMF5S b3Up/nJeyx6W7qbr365Vs8T1S46gGaXdJ0rB1ND35ibD7sdIEKCvszFhOUOJO+BqxfKl YWTcweeQkvDzzswEj6LzSd2IGtcsswqvdESk1anfzaMnXZSgwyB3WViy7sqFYkQNCL7y sCug== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:in-reply-to:message-id:mime-version :references:subject:from:to:cc; bh=a1e01/uQIMruIVMxVHsJI+lKzq1arm+BxjGSeQVMHMA=; b=RwRVD399T112uEyJVJIln3INL2BjU5ZfR9/J9FXdXfBZQW0pN7IBqynS4m9BbJUKlg 4uScHqze+iosZvpqife0eY0+H+ROFXS723QRKgP2Nla3qJpc0SAiBkHrh/2bvBreVy1S ipNmHCvixPhVZ4q5a/BFr5YEFQJm7yGLBgY9jW8CpzlU/OjyCa+BqX+WXJ3ISB1csQVI hwZZ+dAZJ0y31n+j7bOfXtYi/vUusyKCHV2aGNg5CDapuIkyRFm3PAmSV/H2OWJhQMbr GlFqm+lBJMwBnGMzZKz5EyG9HeFOx+IjtxEtkIwU1A+jrMIMJdjbMQZJCPVHNm4+krnU lB5A== X-Gm-Message-State: AOAM530bo6IfxBLFyzzshI9FOHDJPcflxhBeQNUD7pfzbMP3+xg4GQub puRznoIJmUFQBKXItcUgsyLel4A= X-Google-Smtp-Source: ABdhPJygyqIWeu10SeLNwpOjQa4crQYQeJJQ5eWQYbtj8qvL/UTnq8W35uhTakuFmHjpK4j8LLrTDGc= X-Received: from pcc-desktop.svl.corp.google.com ([2620:15c:2ce:200:be2d:924d:844b:d2fa]) (user=pcc job=sendgmr) by 2002:a25:b0a8:: with SMTP id f40mr54053425ybj.125.1638938912219; Tue, 07 Dec 2021 20:48:32 -0800 (PST) Date: Tue, 7 Dec 2021 20:48:08 -0800 In-Reply-To: <20211208044808.872554-1-pcc@google.com> Message-Id: <20211208044808.872554-7-pcc@google.com> Mime-Version: 1.0 References: <20211208044808.872554-1-pcc@google.com> X-Mailer: git-send-email 2.34.1.173.g76aa8bc2d0-goog Subject: [PATCH v3 6/6] Documentation: document uaccess logging From: Peter Collingbourne To: Catalin Marinas , Will Deacon , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Thomas Gleixner , Andy Lutomirski , Kees Cook , Andrew Morton , Masahiro Yamada , Sami Tolvanen , YiFei Zhu , Mark Rutland , Frederic Weisbecker , Viresh Kumar , Andrey Konovalov , Peter Collingbourne , Gabriel Krisman Bertazi , Chris Hyser , Daniel Vetter , Chris Wilson , Arnd Bergmann , Dmitry Vyukov , Christian Brauner , "Eric W. Biederman" , Alexey Gladkov , Ran Xiaokai , David Hildenbrand , Xiaofeng Cao , Cyrill Gorcunov , Thomas Cedeno , Marco Elver , Alexander Potapenko Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Evgenii Stepanov X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20211207_204833_765803_AD1FD861 X-CRM114-Status: GOOD ( 31.51 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Add documentation for the uaccess logging feature. Link: https://linux-review.googlesource.com/id/Ia626c0ca91bc0a3d8067d7f28406aa40693b65a2 Signed-off-by: Peter Collingbourne --- v3: - document what happens if passing NULL to prctl - be explicit about meaning of addr and size Documentation/admin-guide/index.rst | 1 + Documentation/admin-guide/uaccess-logging.rst | 151 ++++++++++++++++++ 2 files changed, 152 insertions(+) create mode 100644 Documentation/admin-guide/uaccess-logging.rst diff --git a/Documentation/admin-guide/index.rst b/Documentation/admin-guide/index.rst index 1bedab498104..4f6ee447ab2f 100644 --- a/Documentation/admin-guide/index.rst +++ b/Documentation/admin-guide/index.rst @@ -54,6 +54,7 @@ ABI will be found here. :maxdepth: 1 sysfs-rules + uaccess-logging The rest of this manual consists of various unordered guides on how to configure specific aspects of kernel behavior to your liking. diff --git a/Documentation/admin-guide/uaccess-logging.rst b/Documentation/admin-guide/uaccess-logging.rst new file mode 100644 index 000000000000..24def38bbdf8 --- /dev/null +++ b/Documentation/admin-guide/uaccess-logging.rst @@ -0,0 +1,151 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=============== +Uaccess Logging +=============== + +Background +---------- + +Userspace tools such as sanitizers (ASan, MSan, HWASan) and tools +making use of the ARM Memory Tagging Extension (MTE) need to +monitor all memory accesses in a program so that they can detect +memory errors. Furthermore, fuzzing tools such as syzkaller need to +monitor all memory accesses so that they know which parts of memory +to fuzz. For accesses made purely in userspace, this is achieved +via compiler instrumentation, or for MTE, via direct hardware +support. However, accesses made by the kernel on behalf of the user +program via syscalls (i.e. uaccesses) are normally invisible to +these tools. + +Traditionally, the sanitizers have handled this by interposing the libc +syscall stubs with a wrapper that checks the memory based on what we +believe the uaccesses will be. However, this creates a maintenance +burden: each syscall must be annotated with its uaccesses in order +to be recognized by the sanitizer, and these annotations must be +continuously updated as the kernel changes. + +The kernel's uaccess logging feature provides userspace tools with +the address and size of each userspace access, thereby allowing these +tools to report memory errors involving these accesses without needing +annotations for every syscall. + +By relying on the kernel's actual uaccesses, rather than a +reimplementation of them, the userspace memory safety tools may +play a dual role of verifying the validity of kernel accesses. Even +a sanitizer whose syscall wrappers have complete knowledge of the +kernel's intended API may vary from the kernel's actual uaccesses due +to kernel bugs. A sanitizer with knowledge of the kernel's actual +uaccesses may produce more accurate error reports that reveal such +bugs. For example, a kernel that accesses more memory than expected +by the userspace program could indicate that either userspace or the +kernel has the wrong idea about which kernel functionality is being +requested -- either way, there is a bug. + +Interface +--------- + +The feature may be used via the following prctl: + +.. code-block:: c + + uint64_t addr = 0; /* Generally will be a TLS slot or equivalent */ + prctl(PR_SET_UACCESS_DESCRIPTOR_ADDR_ADDR, &addr, 0, 0, 0); + +Supplying a non-zero address as the second argument to ``prctl`` +will cause the kernel to read an address (referred to as the *uaccess +descriptor address*) from that address on each kernel entry. Specifying +an address of NULL as the second argument will restore the kernel's +default behavior, i.e. no uaccess descriptor address is read. + +When entering the kernel with a non-zero uaccess descriptor address +to handle a syscall, the kernel will read a data structure of type +``struct uaccess_descriptor`` from the uaccess descriptor address, +which is defined as follows: + +.. code-block:: c + + struct uaccess_descriptor { + uint64_t addr, size; + }; + +This data structure contains the address and size (in array elements) +of a *uaccess buffer*, which is an array of data structures of type +``struct uaccess_buffer_entry``. Before returning to userspace, the +kernel will log information about uaccesses to sequential entries +in the uaccess buffer. It will also store ``NULL`` to the uaccess +descriptor address, and store the address and size of the unused +portion of the uaccess buffer to the uaccess descriptor. + +The format of a uaccess buffer entry is defined as follows: + +.. code-block:: c + + struct uaccess_buffer_entry { + uint64_t addr, size, flags; + }; + +``addr`` and ``size`` contain the address and size of the user memory +access. On arm64, tag bits are preserved in the ``addr`` field. There +is currently one flag bit assignment for the ``flags`` field: + +.. code-block:: c + + #define UACCESS_BUFFER_FLAG_WRITE 1 + +This flag is set if the access was a write, or clear if it was a +read. The meaning of all other flag bits is reserved. + +When entering the kernel with a non-zero uaccess descriptor +address for a reason other than a syscall (for example, when +IPI'd due to an incoming asynchronous signal), any signals other +than ``SIGKILL`` and ``SIGSTOP`` are masked as if by calling +``sigprocmask(SIG_SETMASK, set, NULL)`` where ``set`` has been +initialized with ``sigfillset(set)``. This is to prevent incoming +signals from interfering with uaccess logging. + +Example +------- + +Here is an example of a code snippet that will enumerate the accesses +performed by a ``uname(2)`` syscall: + +.. code-block:: c + + struct uaccess_buffer_entry entries[64]; + struct uaccess_descriptor desc; + uint64_t desc_addr = 0; + prctl(PR_SET_UACCESS_DESCRIPTOR_ADDR_ADDR, &desc_addr, 0, 0, 0); + + desc.addr = (uint64_t)&entries; + desc.size = 64; + desc_addr = (uint64_t)&desc; + + struct utsname un; + uname(&un); + + struct uaccess_buffer_entry* entries_end = (struct uaccess_buffer_entry*)desc.addr; + for (struct uaccess_buffer_entry* entry = entries; entry != entries_end; ++entry) { + printf("%s at 0x%lx size 0x%lx\n", entry->flags & UACCESS_BUFFER_FLAG_WRITE ? "WRITE" : "READ", + (unsigned long)entry->addr, (unsigned long)entry->size); + } + +Limitations +----------- + +This feature is currently only supported on the arm64, s390 and x86 +architectures. + +Uaccess buffers are a "best-effort" mechanism for logging uaccesses. Of +course, not all of the accesses may fit in the buffer, but aside from +that, not all internal kernel APIs that access userspace memory are +covered. Therefore, userspace programs should tolerate unreported +accesses. + +On the other hand, the kernel guarantees that it will not +(intentionally) report accessing more data than it is specified +to read. For example, if the kernel implements a syscall that is +specified to read a data structure of size ``N`` bytes by first +reading a page's worth of data and then only using the first ``N`` +bytes from it, the kernel will either report reading ``N`` bytes or +not report the access at all.