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.
Error
kernel-doc missing