linux/Documentation/dev-tools/kfence.rst

.. 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.