From patchwork Fri Feb 26 01:19:26 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Andrew Morton X-Patchwork-Id: 12105467 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 38EAFC433DB for ; Fri, 26 Feb 2021 01:19:31 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id A894164F21 for ; Fri, 26 Feb 2021 01:19:30 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A894164F21 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=linux-foundation.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 424166B00A9; Thu, 25 Feb 2021 20:19:30 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 3ACBF6B00AA; Thu, 25 Feb 2021 20:19:30 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 201868D0001; Thu, 25 Feb 2021 20:19:30 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0047.hostedemail.com [216.40.44.47]) by kanga.kvack.org (Postfix) with ESMTP id F1F2B6B00A9 for ; Thu, 25 Feb 2021 20:19:29 -0500 (EST) Received: from smtpin18.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id BEF6718250DDA for ; Fri, 26 Feb 2021 01:19:29 +0000 (UTC) X-FDA: 77858661258.18.E4F273C Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by imf29.hostedemail.com (Postfix) with ESMTP id 100B7DA for ; Fri, 26 Feb 2021 01:19:28 +0000 (UTC) Received: by mail.kernel.org (Postfix) with ESMTPSA id AFBF564EFA; Fri, 26 Feb 2021 01:19:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1614302368; bh=pwkqpxBQ0dU3SyOgZebHXGc2NEPokQJXy3arlmk7IiM=; h=Date:From:To:Subject:In-Reply-To:From; b=mETOGm66lnPmZwmZ41ES5snKkpB2cXjb/VU5Uvj2nHVuJAQN/FpLdW6by7DREdoKc 1zsyDV+U6TQmaQAmpRb5YikZWDA0uZnSnIfb/KONDWWAvGiOVGNOQwLNTU+Au5DKGZ 6p6D1FrGer0qfBBgsiGBKe8m1yZSHptq/V14NQJc= Date: Thu, 25 Feb 2021 17:19:26 -0800 From: Andrew Morton To: akpm@linux-foundation.org, andreyknvl@google.com, aryabinin@virtuozzo.com, bp@alien8.de, catalin.marinas@arm.com, cl@linux.com, corbet@lwn.net, dave.hansen@linux.intel.com, dvyukov@google.com, edumazet@google.com, elver@google.com, glider@google.com, gregkh@linuxfoundation.org, hdanton@sina.com, hpa@zytor.com, iamjoonsoo.kim@lge.com, jannh@google.com, joern@purestorage.com, keescook@chromium.org, linux-mm@kvack.org, luto@kernel.org, mark.rutland@arm.com, mingo@redhat.com, mm-commits@vger.kernel.org, paulmck@kernel.org, penberg@kernel.org, peterz@infradead.org, rientjes@google.com, sjpark@amazon.de, tglx@linutronix.de, torvalds@linux-foundation.org, vbabka@suse.cz, will@kernel.org Subject: [patch 063/118] kfence, Documentation: add KFENCE documentation Message-ID: <20210226011926.Bo8zOb6qU%akpm@linux-foundation.org> In-Reply-To: <20210225171452.713967e96554bb6a53e44a19@linux-foundation.org> User-Agent: s-nail v14.8.16 X-Stat-Signature: 5kfqwk6xjnd4hnxtxt1qbnphux5wqcmt X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 100B7DA Received-SPF: none (linux-foundation.org>: No applicable sender policy available) receiver=imf29; identity=mailfrom; envelope-from=""; helo=mail.kernel.org; client-ip=198.145.29.99 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1614302368-255890 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: From: Marco Elver Subject: kfence, Documentation: add KFENCE documentation Add KFENCE documentation in dev-tools/kfence.rst, and add to index. [elver@google.com: add missing copyright header to documentation] Link: https://lkml.kernel.org/r/20210118092159.145934-4-elver@google.com Link: https://lkml.kernel.org/r/20201103175841.3495947-8-elver@google.com Signed-off-by: Alexander Potapenko Signed-off-by: Marco Elver Reviewed-by: Dmitry Vyukov Co-developed-by: Alexander Potapenko Reviewed-by: Jann Horn Cc: Andrey Konovalov Cc: Andrey Ryabinin Cc: Andy Lutomirski Cc: Borislav Petkov Cc: Catalin Marinas Cc: Christopher Lameter Cc: Dave Hansen Cc: David Rientjes Cc: Eric Dumazet Cc: Greg Kroah-Hartman Cc: Hillf Danton Cc: "H. Peter Anvin" Cc: Ingo Molnar Cc: Joern Engel Cc: Jonathan Corbet Cc: Joonsoo Kim Cc: Kees Cook Cc: Mark Rutland Cc: Paul E. McKenney Cc: Pekka Enberg Cc: Peter Zijlstra Cc: SeongJae Park Cc: Thomas Gleixner Cc: Vlastimil Babka Cc: Will Deacon Signed-off-by: Andrew Morton --- Documentation/dev-tools/index.rst | 1 Documentation/dev-tools/kfence.rst | 298 +++++++++++++++++++++++++++ lib/Kconfig.kfence | 2 3 files changed, 301 insertions(+) --- a/Documentation/dev-tools/index.rst~kfence-documentation-add-kfence-documentation +++ a/Documentation/dev-tools/index.rst @@ -22,6 +22,7 @@ whole; patches welcome! ubsan kmemleak kcsan + kfence gdb-kernel-debugging kgdb kselftest --- /dev/null +++ a/Documentation/dev-tools/kfence.rst @@ -0,0 +1,298 @@ +.. SPDX-License-Identifier: GPL-2.0 +.. Copyright (C) 2020, Google LLC. + +Kernel Electric-Fence (KFENCE) +============================== + +Kernel Electric-Fence (KFENCE) is a low-overhead sampling-based memory safety +error detector. KFENCE detects heap out-of-bounds access, use-after-free, and +invalid-free errors. + +KFENCE is designed to be enabled in production kernels, and has near zero +performance overhead. Compared to KASAN, KFENCE trades performance for +precision. The main motivation behind KFENCE's design, is that with enough +total uptime KFENCE will detect bugs in code paths not typically exercised by +non-production test workloads. One way to quickly achieve a large enough total +uptime is when the tool is deployed across a large fleet of machines. + +Usage +----- + +To enable KFENCE, configure the kernel with:: + + CONFIG_KFENCE=y + +To build a kernel with KFENCE support, but disabled by default (to enable, set +``kfence.sample_interval`` to non-zero value), configure the kernel with:: + + CONFIG_KFENCE=y + CONFIG_KFENCE_SAMPLE_INTERVAL=0 + +KFENCE provides several other configuration options to customize behaviour (see +the respective help text in ``lib/Kconfig.kfence`` for more info). + +Tuning performance +~~~~~~~~~~~~~~~~~~ + +The most important parameter is KFENCE's sample interval, which can be set via +the kernel boot parameter ``kfence.sample_interval`` in milliseconds. The +sample interval determines the frequency with which heap allocations will be +guarded by KFENCE. The default is configurable via the Kconfig option +``CONFIG_KFENCE_SAMPLE_INTERVAL``. Setting ``kfence.sample_interval=0`` +disables KFENCE. + +The KFENCE memory pool is of fixed size, and if the pool is exhausted, no +further KFENCE allocations occur. With ``CONFIG_KFENCE_NUM_OBJECTS`` (default +255), the number of available guarded objects can be controlled. Each object +requires 2 pages, one for the object itself and the other one used as a guard +page; object pages are interleaved with guard pages, and every object page is +therefore surrounded by two guard pages. + +The total memory dedicated to the KFENCE memory pool can be computed as:: + + ( #objects + 1 ) * 2 * PAGE_SIZE + +Using the default config, and assuming a page size of 4 KiB, results in +dedicating 2 MiB to the KFENCE memory pool. + +Note: On architectures that support huge pages, KFENCE will ensure that the +pool is using pages of size ``PAGE_SIZE``. This will result in additional page +tables being allocated. + +Error reports +~~~~~~~~~~~~~ + +A typical out-of-bounds access looks like this:: + + ================================================================== + BUG: KFENCE: out-of-bounds in test_out_of_bounds_read+0xa3/0x22b + + Out-of-bounds access at 0xffffffffb672efff (1B left of kfence-#17): + test_out_of_bounds_read+0xa3/0x22b + kunit_try_run_case+0x51/0x85 + kunit_generic_run_threadfn_adapter+0x16/0x30 + kthread+0x137/0x160 + ret_from_fork+0x22/0x30 + + kfence-#17 [0xffffffffb672f000-0xffffffffb672f01f, size=32, cache=kmalloc-32] allocated by task 507: + test_alloc+0xf3/0x25b + test_out_of_bounds_read+0x98/0x22b + kunit_try_run_case+0x51/0x85 + kunit_generic_run_threadfn_adapter+0x16/0x30 + kthread+0x137/0x160 + ret_from_fork+0x22/0x30 + + CPU: 4 PID: 107 Comm: kunit_try_catch Not tainted 5.8.0-rc6+ #7 + Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014 + ================================================================== + +The header of the report provides a short summary of the function involved in +the access. It is followed by more detailed information about the access and +its origin. Note that, real kernel addresses are only shown for +``CONFIG_DEBUG_KERNEL=y`` builds. + +Use-after-free accesses are reported as:: + + ================================================================== + BUG: KFENCE: use-after-free in test_use_after_free_read+0xb3/0x143 + + Use-after-free access at 0xffffffffb673dfe0 (in kfence-#24): + test_use_after_free_read+0xb3/0x143 + kunit_try_run_case+0x51/0x85 + kunit_generic_run_threadfn_adapter+0x16/0x30 + kthread+0x137/0x160 + ret_from_fork+0x22/0x30 + + kfence-#24 [0xffffffffb673dfe0-0xffffffffb673dfff, size=32, cache=kmalloc-32] allocated by task 507: + test_alloc+0xf3/0x25b + test_use_after_free_read+0x76/0x143 + kunit_try_run_case+0x51/0x85 + kunit_generic_run_threadfn_adapter+0x16/0x30 + kthread+0x137/0x160 + ret_from_fork+0x22/0x30 + + freed by task 507: + test_use_after_free_read+0xa8/0x143 + kunit_try_run_case+0x51/0x85 + kunit_generic_run_threadfn_adapter+0x16/0x30 + kthread+0x137/0x160 + ret_from_fork+0x22/0x30 + + CPU: 4 PID: 109 Comm: kunit_try_catch Tainted: G W 5.8.0-rc6+ #7 + Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014 + ================================================================== + +KFENCE also reports on invalid frees, such as double-frees:: + + ================================================================== + BUG: KFENCE: invalid free in test_double_free+0xdc/0x171 + + Invalid free of 0xffffffffb6741000: + test_double_free+0xdc/0x171 + kunit_try_run_case+0x51/0x85 + kunit_generic_run_threadfn_adapter+0x16/0x30 + kthread+0x137/0x160 + ret_from_fork+0x22/0x30 + + kfence-#26 [0xffffffffb6741000-0xffffffffb674101f, size=32, cache=kmalloc-32] allocated by task 507: + test_alloc+0xf3/0x25b + test_double_free+0x76/0x171 + kunit_try_run_case+0x51/0x85 + kunit_generic_run_threadfn_adapter+0x16/0x30 + kthread+0x137/0x160 + ret_from_fork+0x22/0x30 + + freed by task 507: + test_double_free+0xa8/0x171 + kunit_try_run_case+0x51/0x85 + kunit_generic_run_threadfn_adapter+0x16/0x30 + kthread+0x137/0x160 + ret_from_fork+0x22/0x30 + + CPU: 4 PID: 111 Comm: kunit_try_catch Tainted: G W 5.8.0-rc6+ #7 + Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014 + ================================================================== + +KFENCE also uses pattern-based redzones on the other side of an object's guard +page, to detect out-of-bounds writes on the unprotected side of the object. +These are reported on frees:: + + ================================================================== + BUG: KFENCE: memory corruption in test_kmalloc_aligned_oob_write+0xef/0x184 + + Corrupted memory at 0xffffffffb6797ff9 [ 0xac . . . . . . ] (in kfence-#69): + test_kmalloc_aligned_oob_write+0xef/0x184 + kunit_try_run_case+0x51/0x85 + kunit_generic_run_threadfn_adapter+0x16/0x30 + kthread+0x137/0x160 + ret_from_fork+0x22/0x30 + + kfence-#69 [0xffffffffb6797fb0-0xffffffffb6797ff8, size=73, cache=kmalloc-96] allocated by task 507: + test_alloc+0xf3/0x25b + test_kmalloc_aligned_oob_write+0x57/0x184 + kunit_try_run_case+0x51/0x85 + kunit_generic_run_threadfn_adapter+0x16/0x30 + kthread+0x137/0x160 + ret_from_fork+0x22/0x30 + + CPU: 4 PID: 120 Comm: kunit_try_catch Tainted: G W 5.8.0-rc6+ #7 + Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014 + ================================================================== + +For such errors, the address where the corruption occurred as well as the +invalidly written bytes (offset from the address) are shown; in this +representation, '.' denote untouched bytes. In the example above ``0xac`` is +the value written to the invalid address at offset 0, and the remaining '.' +denote that no following bytes have been touched. Note that, real values are +only shown for ``CONFIG_DEBUG_KERNEL=y`` builds; to avoid information +disclosure for non-debug builds, '!' is used instead to denote invalidly +written bytes. + +And finally, KFENCE may also report on invalid accesses to any protected page +where it was not possible to determine an associated object, e.g. if adjacent +object pages had not yet been allocated:: + + ================================================================== + BUG: KFENCE: invalid access in test_invalid_access+0x26/0xe0 + + Invalid access at 0xffffffffb670b00a: + test_invalid_access+0x26/0xe0 + kunit_try_run_case+0x51/0x85 + kunit_generic_run_threadfn_adapter+0x16/0x30 + kthread+0x137/0x160 + ret_from_fork+0x22/0x30 + + CPU: 4 PID: 124 Comm: kunit_try_catch Tainted: G W 5.8.0-rc6+ #7 + Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014 + ================================================================== + +DebugFS interface +~~~~~~~~~~~~~~~~~ + +Some debugging information is exposed via debugfs: + +* The file ``/sys/kernel/debug/kfence/stats`` provides runtime statistics. + +* The file ``/sys/kernel/debug/kfence/objects`` provides a list of objects + allocated via KFENCE, including those already freed but protected. + +Implementation Details +---------------------- + +Guarded allocations are set up based on the sample interval. After expiration +of the sample interval, the next allocation through the main allocator (SLAB or +SLUB) returns a guarded allocation from the KFENCE object pool (allocation +sizes up to PAGE_SIZE are supported). At this point, the timer is reset, and +the next allocation is set up after the expiration of the interval. To "gate" a +KFENCE allocation through the main allocator's fast-path without overhead, +KFENCE relies on static branches via the static keys infrastructure. The static +branch is toggled to redirect the allocation to KFENCE. + +KFENCE objects each reside on a dedicated page, at either the left or right +page boundaries selected at random. The pages to the left and right of the +object page are "guard pages", whose attributes are changed to a protected +state, and cause page faults on any attempted access. Such page faults are then +intercepted by KFENCE, which handles the fault gracefully by reporting an +out-of-bounds access, and marking the page as accessible so that the faulting +code can (wrongly) continue executing (set ``panic_on_warn`` to panic instead). + +To detect out-of-bounds writes to memory within the object's page itself, +KFENCE also uses pattern-based redzones. For each object page, a redzone is set +up for all non-object memory. For typical alignments, the redzone is only +required on the unguarded side of an object. Because KFENCE must honor the +cache's requested alignment, special alignments may result in unprotected gaps +on either side of an object, all of which are redzoned. + +The following figure illustrates the page layout:: + + ---+-----------+-----------+-----------+-----------+-----------+--- + | xxxxxxxxx | O : | xxxxxxxxx | : O | xxxxxxxxx | + | xxxxxxxxx | B : | xxxxxxxxx | : B | xxxxxxxxx | + | x GUARD x | J : RED- | x GUARD x | RED- : J | x GUARD x | + | xxxxxxxxx | E : ZONE | xxxxxxxxx | ZONE : E | xxxxxxxxx | + | xxxxxxxxx | C : | xxxxxxxxx | : C | xxxxxxxxx | + | xxxxxxxxx | T : | xxxxxxxxx | : T | xxxxxxxxx | + ---+-----------+-----------+-----------+-----------+-----------+--- + +Upon deallocation of a KFENCE object, the object's page is again protected and +the object is marked as freed. Any further access to the object causes a fault +and KFENCE reports a use-after-free access. Freed objects are inserted at the +tail of KFENCE's freelist, so that the least recently freed objects are reused +first, and the chances of detecting use-after-frees of recently freed objects +is increased. + +Interface +--------- + +The following describes the functions which are used by allocators as well as +page handling code to set up and deal with KFENCE allocations. + +.. kernel-doc:: include/linux/kfence.h + :functions: is_kfence_address + kfence_shutdown_cache + kfence_alloc kfence_free __kfence_free + kfence_ksize kfence_object_start + kfence_handle_page_fault + +Related Tools +------------- + +In userspace, a similar approach is taken by `GWP-ASan +`_. GWP-ASan also relies on guard pages and +a sampling strategy to detect memory unsafety bugs at scale. KFENCE's design is +directly influenced by GWP-ASan, and can be seen as its kernel sibling. Another +similar but non-sampling approach, that also inspired the name "KFENCE", can be +found in the userspace `Electric Fence Malloc Debugger +`_. + +In the kernel, several tools exist to debug memory access errors, and in +particular KASAN can detect all bug classes that KFENCE can detect. While KASAN +is more precise, relying on compiler instrumentation, this comes at a +performance cost. + +It is worth highlighting that KASAN and KFENCE are complementary, with +different target environments. For instance, KASAN is the better debugging-aid, +where test cases or reproducers exists: due to the lower chance to detect the +error, it would require more effort using KFENCE to debug. Deployments at scale +that cannot afford to enable KASAN, however, would benefit from using KFENCE to +discover bugs due to code paths not exercised by test cases or fuzzers. --- a/lib/Kconfig.kfence~kfence-documentation-add-kfence-documentation +++ a/lib/Kconfig.kfence @@ -13,6 +13,8 @@ menuconfig KFENCE to have negligible cost to permit enabling it in production environments. + See for more details. + Note that, KFENCE is not a substitute for explicit testing with tools such as KASAN. KFENCE can detect a subset of bugs that KASAN can detect, albeit at very different performance profiles. If you can