On Fri, 11 Sep 2020 at 09:14, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote: > > On Mon, Sep 7, 2020 at 3:41 PM Marco Elver <elver@xxxxxxxxxx> wrote: > > > > Add KFENCE documentation in dev-tools/kfence.rst, and add to index. > > > > Co-developed-by: Alexander Potapenko <glider@xxxxxxxxxx> > > Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx> > > Signed-off-by: Marco Elver <elver@xxxxxxxxxx> > > --- > > Documentation/dev-tools/index.rst | 1 + > > Documentation/dev-tools/kfence.rst | 285 +++++++++++++++++++++++++++++ > > 2 files changed, 286 insertions(+) > > create mode 100644 Documentation/dev-tools/kfence.rst > > > > diff --git a/Documentation/dev-tools/index.rst b/Documentation/dev-tools/index.rst > > index f7809c7b1ba9..1b1cf4f5c9d9 100644 > > --- a/Documentation/dev-tools/index.rst > > +++ b/Documentation/dev-tools/index.rst > > @@ -22,6 +22,7 @@ whole; patches welcome! > > ubsan > > kmemleak > > kcsan > > + kfence > > gdb-kernel-debugging > > kgdb > > kselftest > > diff --git a/Documentation/dev-tools/kfence.rst b/Documentation/dev-tools/kfence.rst > > new file mode 100644 > > index 000000000000..254f4f089104 > > --- /dev/null > > +++ b/Documentation/dev-tools/kfence.rst > > @@ -0,0 +1,285 @@ > > +.. SPDX-License-Identifier: GPL-2.0 > > + > > +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 > > + > > +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. > > + > > +With the Kconfig option ``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. > > + > > +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 (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 in: > > + __kfence_alloc+0x42d/0x4c0 > > + __kmalloc+0x133/0x200 > > + 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. > > + > > +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: > > + 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 in: > > + __kfence_alloc+0x277/0x4c0 > > + __kmalloc+0x133/0x200 > > + 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 > > Empty line between stacks for consistency and readability. Done for v2. > > + freed in: > > + kfence_guarded_free+0x158/0x380 > > + __kfence_free+0x38/0xc0 > > + 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 in: > > + __kfence_alloc+0x42d/0x4c0 > > + __kmalloc+0x133/0x200 > > + 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 in: > > + kfence_guarded_free+0x158/0x380 > > + __kfence_free+0x38/0xc0 > > + 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 > > + > > + Detected corrupted memory at 0xffffffffb6797ff9 [ 0xac . . . . . . ]: > > + 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 in: > > + __kfence_alloc+0x277/0x4c0 > > + __kmalloc+0x133/0x200 > > + 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 as well as the corrupt bytes > > +are shown. > > + > > +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, a guarded allocation from the KFENCE object pool is > > +returned to the main allocator (SLAB or SLUB). 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 > > Do we mention anywhere explicitly that KFENCE currently only supports > allocations <=page_size? > May be worth mentioning. It kinda follows from implementation but > quite implicitly. One may also be confused assuming KFENCE handles > larger allocations, but then not being able to figure out. We can add a note that "allocation sizes up to PAGE_SIZE are supported". Thanks, -- Marco
