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Introduce burst mode, which can be configured with kfence.burst=$count, where the burst count denotes the additional successive slab allocations to be allocated through KFENCE for each sample interval. The idea is that this can give developers an additional knob to make KFENCE more aggressive when debugging specific issues of systems where either rebooting or recompiling the kernel with KASAN is not possible. Experiment: To assess the effectiveness of the new option, we randomly picked a recent out-of-bounds [1] and use-after-free bug [2], each with a reproducer provided by syzbot, that initially detected these bugs with KASAN. We then tried to reproduce the bugs with KFENCE below. [1] Fixed by:7c55b78818("jfs: xattr: fix buffer overflow for invalid xattr") https://syzkaller.appspot.com/bug?id=9d1b59d4718239da6f6069d3891863c25f9f24a2 [2] Fixed by:f8ad00f3fb("l2tp: fix possible UAF when cleaning up tunnels") https://syzkaller.appspot.com/bug?id=4f34adc84f4a3b080187c390eeef60611fd450e1 The following KFENCE configs were compared. A pool size of 1023 objects was used for all configurations. Baseline kfence.sample_interval=100 kfence.skip_covered_thresh=75 kfence.burst=0 Aggressive kfence.sample_interval=1 kfence.skip_covered_thresh=10 kfence.burst=0 AggressiveBurst kfence.sample_interval=1 kfence.skip_covered_thresh=10 kfence.burst=1000 Each reproducer was run 10 times (after a fresh reboot), with the following detection counts for each KFENCE config: | Detection Count out of 10 | | OOB [1] | UAF [2] | ------------------+-------------+-------------+ Default | 0/10 | 0/10 | Aggressive | 0/10 | 0/10 | AggressiveBurst | 8/10 | 8/10 | With the Default and even the Aggressive configs the results are unsurprising, given KFENCE has not been designed for deterministic bug detection of small test cases. However, when enabling burst mode with relatively large burst count, KFENCE can start to detect heap memory-safety bugs even in simpler test cases with high probability (in the above cases with ~80% probability). Link: https://lkml.kernel.org/r/20240805124203.2692278-1-elver@google.com Signed-off-by: Marco Elver <elver@google.com> Reviewed-by: Alexander Potapenko <glider@google.com> Cc: Andrey Konovalov <andreyknvl@gmail.com> Cc: Dmitry Vyukov <dvyukov@google.com> Cc: Jann Horn <jannh@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.