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+.. 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 sample interval controls a timer that sets up KFENCE allocations. By
+default, to keep the real sample interval predictable, the normal timer also
+causes CPU wake-ups when the system is completely idle. This may be undesirable
+on power-constrained systems. The boot parameter ``kfence.deferrable=1``
+instead switches to a "deferrable" timer which does not force CPU wake-ups on
+idle systems, at the risk of unpredictable sample intervals. The default is
+configurable via the Kconfig option ``CONFIG_KFENCE_DEFERRABLE``.
+
+.. warning::
+ The KUnit test suite is very likely to fail when using a deferrable timer
+ since it currently causes very unpredictable sample intervals.
+
+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 read in test_out_of_bounds_read+0xa6/0x234
+
+ Out-of-bounds read at 0xffff8c3f2e291fff (1B left of kfence-#72):
+ test_out_of_bounds_read+0xa6/0x234
+ kunit_try_run_case+0x61/0xa0
+ kunit_generic_run_threadfn_adapter+0x16/0x30
+ kthread+0x176/0x1b0
+ ret_from_fork+0x22/0x30
+
+ kfence-#72: 0xffff8c3f2e292000-0xffff8c3f2e29201f, size=32, cache=kmalloc-32
+
+ allocated by task 484 on cpu 0 at 32.919330s:
+ test_alloc+0xfe/0x738
+ test_out_of_bounds_read+0x9b/0x234
+ kunit_try_run_case+0x61/0xa0
+ kunit_generic_run_threadfn_adapter+0x16/0x30
+ kthread+0x176/0x1b0
+ ret_from_fork+0x22/0x30
+
+ CPU: 0 PID: 484 Comm: kunit_try_catch Not tainted 5.13.0-rc3+ #7
+ Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 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 when using the
+kernel command line option ``no_hash_pointers``.
+
+Use-after-free accesses are reported as::
+
+ ==================================================================
+ BUG: KFENCE: use-after-free read in test_use_after_free_read+0xb3/0x143
+
+ Use-after-free read at 0xffff8c3f2e2a0000 (in kfence-#79):
+ test_use_after_free_read+0xb3/0x143
+ kunit_try_run_case+0x61/0xa0
+ kunit_generic_run_threadfn_adapter+0x16/0x30
+ kthread+0x176/0x1b0
+ ret_from_fork+0x22/0x30
+
+ kfence-#79: 0xffff8c3f2e2a0000-0xffff8c3f2e2a001f, size=32, cache=kmalloc-32
+
+ allocated by task 488 on cpu 2 at 33.871326s:
+ test_alloc+0xfe/0x738
+ test_use_after_free_read+0x76/0x143
+ kunit_try_run_case+0x61/0xa0
+ kunit_generic_run_threadfn_adapter+0x16/0x30
+ kthread+0x176/0x1b0
+ ret_from_fork+0x22/0x30
+
+ freed by task 488 on cpu 2 at 33.871358s:
+ test_use_after_free_read+0xa8/0x143
+ kunit_try_run_case+0x61/0xa0
+ kunit_generic_run_threadfn_adapter+0x16/0x30
+ kthread+0x176/0x1b0
+ ret_from_fork+0x22/0x30
+
+ CPU: 2 PID: 488 Comm: kunit_try_catch Tainted: G B 5.13.0-rc3+ #7
+ Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 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 0xffff8c3f2e2a4000 (in kfence-#81):
+ test_double_free+0xdc/0x171
+ kunit_try_run_case+0x61/0xa0
+ kunit_generic_run_threadfn_adapter+0x16/0x30
+ kthread+0x176/0x1b0
+ ret_from_fork+0x22/0x30
+
+ kfence-#81: 0xffff8c3f2e2a4000-0xffff8c3f2e2a401f, size=32, cache=kmalloc-32
+
+ allocated by task 490 on cpu 1 at 34.175321s:
+ test_alloc+0xfe/0x738
+ test_double_free+0x76/0x171
+ kunit_try_run_case+0x61/0xa0
+ kunit_generic_run_threadfn_adapter+0x16/0x30
+ kthread+0x176/0x1b0
+ ret_from_fork+0x22/0x30
+
+ freed by task 490 on cpu 1 at 34.175348s:
+ test_double_free+0xa8/0x171
+ kunit_try_run_case+0x61/0xa0
+ kunit_generic_run_threadfn_adapter+0x16/0x30
+ kthread+0x176/0x1b0
+ ret_from_fork+0x22/0x30
+
+ CPU: 1 PID: 490 Comm: kunit_try_catch Tainted: G B 5.13.0-rc3+ #7
+ Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 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 0xffff8c3f2e33aff9 [ 0xac . . . . . . ] (in kfence-#156):
+ test_kmalloc_aligned_oob_write+0xef/0x184
+ kunit_try_run_case+0x61/0xa0
+ kunit_generic_run_threadfn_adapter+0x16/0x30
+ kthread+0x176/0x1b0
+ ret_from_fork+0x22/0x30
+
+ kfence-#156: 0xffff8c3f2e33afb0-0xffff8c3f2e33aff8, size=73, cache=kmalloc-96
+
+ allocated by task 502 on cpu 7 at 42.159302s:
+ test_alloc+0xfe/0x738
+ test_kmalloc_aligned_oob_write+0x57/0x184
+ kunit_try_run_case+0x61/0xa0
+ kunit_generic_run_threadfn_adapter+0x16/0x30
+ kthread+0x176/0x1b0
+ ret_from_fork+0x22/0x30
+
+ CPU: 7 PID: 502 Comm: kunit_try_catch Tainted: G B 5.13.0-rc3+ #7
+ Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 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 if the kernel was booted with ``no_hash_pointers``; to avoid
+information disclosure otherwise, '!' 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 read in test_invalid_access+0x26/0xe0
+
+ Invalid read 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.
+
+When using ``CONFIG_KFENCE_STATIC_KEYS=y``, KFENCE allocations are "gated"
+through the main allocator's fast-path by relying on static branches via the
+static keys infrastructure. The static branch is toggled to redirect the
+allocation to KFENCE. Depending on sample interval, target workloads, and
+system architecture, this may perform better than the simple dynamic branch.
+Careful benchmarking is recommended.
+
+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.
+
+If pool utilization reaches 75% (default) or above, to reduce the risk of the
+pool eventually being fully occupied by allocated objects yet ensure diverse
+coverage of allocations, KFENCE limits currently covered allocations of the
+same source from further filling up the pool. The "source" of an allocation is
+based on its partial allocation stack trace. A side-effect is that this also
+limits frequent long-lived allocations (e.g. pagecache) of the same source
+filling up the pool permanently, which is the most common risk for the pool
+becoming full and the sampled allocation rate dropping to zero. The threshold
+at which to start limiting currently covered allocations can be configured via
+the boot parameter ``kfence.skip_covered_thresh`` (pool usage%).
+
+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
+<http://llvm.org/docs/GwpAsan.html>`_. 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
+<https://linux.die.net/man/3/efence>`_.
+
+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.