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.
To enable KFENCE, configure the kernel with:
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:
KFENCE provides several other configuration options to customize behaviour (see
the respective help text in
lib/Kconfig.kfence for more info).
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
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
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
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
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.
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
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
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
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 ==================================================================
Some debugging information is exposed via debugfs:
/sys/kernel/debug/kfence/statsprovides runtime statistics.
/sys/kernel/debug/kfence/objectsprovides a list of objects allocated via KFENCE, including those already freed but protected.
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.
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%).
The following describes the functions which are used by allocators as well as page handling code to set up and deal with KFENCE allocations.
is_kfence_address(const void *addr)¶
check if an address belongs to KFENCE pool
const void *addr
address to check
true or false depending on whether the address is within the KFENCE object range.
KFENCE objects live in a separate page range and are not to be intermixed
with regular heap objects (e.g. KFENCE objects must never be added to the
allocator freelists). Failing to do so may and will result in heap
is_kfence_address() must be used to check whether
an object requires specific handling.
This function may be used in fast-paths, and is performance critical. Future changes should take this into account; for instance, we want to avoid introducing another load and therefore need to keep KFENCE_POOL_SIZE a constant (until immediate patching support is added to the kernel).
kfence_shutdown_cache(struct kmem_cache *s)¶
handle shutdown_cache() for KFENCE objects
struct kmem_cache *s
cache being shut down
Before shutting down a cache, one must ensure there are no remaining objects allocated from it. Because KFENCE objects are not referenced from the cache directly, we need to check them here.
Note that shutdown_cache() is internal to SL*B, and kmem_cache_destroy() does not return if allocated objects still exist: it prints an error message and simply aborts destruction of a cache, leaking memory.
If the only such objects are KFENCE objects, we will not leak the entire cache, but instead try to provide more useful debug info by making allocated objects “zombie allocations”. Objects may then still be used or freed (which is handled gracefully), but usage will result in showing KFENCE error reports which include stack traces to the user of the object, the original allocation site, and caller to shutdown_cache().
kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)¶
allocate a KFENCE object with a low probability
struct kmem_cache *s
struct kmem_cache with object requirements
exact size of the object to allocate (can be less than s->size e.g. for kmalloc caches)
NULL - must proceed with allocating as usual,
non-NULL - pointer to a KFENCE object.
kfence_alloc() should be inserted into the heap allocation fast path,
allowing it to transparently return KFENCE-allocated objects with a low
probability using a static branch (the probability is controlled by the
kfence.sample_interval boot parameter).
kfence_ksize(const void *addr)¶
get actual amount of memory allocated for a KFENCE object
const void *addr
pointer to a heap object
0 - not a KFENCE object, must call
non-0 - this many bytes can be accessed without causing a memory error.
kfence_ksize() returns the number of bytes requested for a KFENCE object at
allocation time. This number may be less than the object size of the
corresponding struct kmem_cache.
kfence_object_start(const void *addr)¶
find the beginning of a KFENCE object
const void *addr
address within a KFENCE-allocated object
address of the beginning of the object.
SL[AU]B-allocated objects are laid out within a page one by one, so it is easy to calculate the beginning of an object given a pointer inside it and the object size. The same is not true for KFENCE, which places a single object at either end of the page. This helper function is used to find the beginning of a KFENCE-allocated object.
release a KFENCE heap object to KFENCE pool
object to be freed
Release a KFENCE object and mark it as freed.
try to release an arbitrary heap object to KFENCE pool
object to be freed
false - object doesn’t belong to KFENCE pool and was ignored,
true - object was released to KFENCE pool.
Release a KFENCE object and mark it as freed. May be called on any object, even non-KFENCE objects, to simplify integration of the hooks into the allocator’s free codepath. The allocator must check the return value to determine if it was a KFENCE object or not.
kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs)¶
perform page fault handling for KFENCE pages
unsigned long addr
is access a write
struct pt_regs *regs
current struct pt_regs (can be NULL, but shows full stack trace)
false - address outside KFENCE pool,
true - page fault handled by KFENCE, no additional handling required.
A page fault inside KFENCE pool indicates a memory error, such as an out-of-bounds access, a use-after-free or an invalid memory access. In these cases KFENCE prints an error message and marks the offending page as present, so that the kernel can proceed.