[PATCH v2 00/10] KFENCE: A low-overhead sampling-based memory safety error detector

Qian Cai cai at redhat.com
Fri Sep 18 07:17:06 EDT 2020

On Tue, 2020-09-15 at 15:20 +0200, Marco Elver wrote:
> This adds the Kernel Electric-Fence (KFENCE) infrastructure. KFENCE is a
> low-overhead sampling-based memory safety error detector of heap
> use-after-free, invalid-free, and out-of-bounds access errors.  This
> series enables KFENCE for the x86 and arm64 architectures, and adds
> KFENCE hooks to the SLAB and SLUB allocators.
> 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.
> KFENCE objects each reside on a dedicated page, at either the left or
> right page boundaries. 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 to them. Such page
> faults are then intercepted by KFENCE, which handles the fault
> gracefully by reporting a memory access error.
> Guarded allocations are set up based on a sample interval (can be set
> via kfence.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. At this point, the timer
> is reset, and the next allocation is set up after the expiration of the
> interval.
> To enable/disable 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.
> The KFENCE memory pool is of fixed size, and if the pool is exhausted no
> further KFENCE allocations occur. The default config is conservative
> with only 255 objects, resulting in a pool size of 2 MiB (with 4 KiB
> pages).
> We have verified by running synthetic benchmarks (sysbench I/O,
> hackbench) that a kernel with KFENCE is performance-neutral compared to
> a non-KFENCE baseline kernel.
> KFENCE is inspired by GWP-ASan [1], a userspace tool with similar
> properties. The name "KFENCE" is a homage to the Electric Fence Malloc
> Debugger [2].
> For more details, see Documentation/dev-tools/kfence.rst added in the
> series -- also viewable here:

Does anybody else grow tried of all those different *imperfect* versions of in-
kernel memory safety error detectors? KASAN-generic, KFENCE, KASAN-tag-based
etc. Then, we have old things like page_poison, SLUB debugging, debug_pagealloc
etc which are pretty much inefficient to detect bugs those days compared to
KASAN. Can't we work towards having a single implementation and clean up all
those mess?

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