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In a recent discussion[1] with Vitaly Nikolenko and Silvio Cesare, it became clear that moving the freelist pointer away from the edge of allocations would likely improve the overall defensive posture of the inline freelist pointer. My benchmarks show no meaningful change to performance (they seem to show it being faster), so this looks like a reasonable change to make. Instead of having the freelist pointer at the very beginning of an allocation (offset 0) or at the very end of an allocation (effectively offset -sizeof(void *) from the next allocation), move it away from the edges of the allocation and into the middle. This provides some protection against small-sized neighboring overflows (or underflows), for which the freelist pointer is commonly the target. (Large or well controlled overwrites are much more likely to attack live object contents, instead of attempting freelist corruption.) The vaunted kernel build benchmark, across 5 runs. Before: Mean: 250.05 Std Dev: 1.85 and after, which appears mysteriously faster: Mean: 247.13 Std Dev: 0.76 Attempts at running "sysbench --test=memory" show the change to be well in the noise (sysbench seems to be pretty unstable here -- it's not really measuring allocation). Hackbench is more allocation-heavy, and while the std dev is above the difference, it looks like may manifest as an improvement as well: 20 runs of "hackbench -g 20 -l 1000", before: Mean: 36.322 Std Dev: 0.577 and after: Mean: 36.056 Std Dev: 0.598 [1] https://twitter.com/vnik5287/status/1235113523098685440 Signed-off-by: Kees Cook <keescook@chromium.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Acked-by: Christoph Lameter <cl@linux.com> Cc: Vitaly Nikolenko <vnik@duasynt.com> Cc: Silvio Cesare <silvio.cesare@gmail.com> Cc: Christoph Lameter <cl@linux.com>Cc: Pekka Enberg <penberg@kernel.org> Cc: David Rientjes <rientjes@google.com> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com> Link: http://lkml.kernel.org/r/202003051624.AAAC9AECC@keescook Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
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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 Restructured Text 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.