Changelog since v2
o Default on
o Logic correction and simplification

Changelog since v1
o Menu section rename
o Make static branch usage similar to init_on_alloc
o Change ordering of menu options

Some hardening options like HARDENED_USERCOPY can be set at boot time
and have negligible cost when disabled. The default for options like
init_on_alloc= can be set at compile time but hardened usercopy is
enabled by default if built in. This incurs overhead when a kernel
wishes to provide optional hardening but the user does not necessarily
care.

Hardening is desirable in some environments but ideally they would be opt-in
by kernel command line as hardening is typically a deliberate decision
whereas the performance overhead is not always obvious to all users.
Patches 1 and 2 move HARDENED_USERCOPY to the Kconfig.hardening and
default it to disabled. Patch 3 moves the static branch check to a fast
path similar to init_on_*. Patch 4 moves FORTIFY_SOURCE to hardening only
because the option is related to hardening and happened to be declared
near HARDENED_USERCOPY.

Building HARDENED_USERCOPY but disabled at runtime has neligible effect
within the noise. Enabling the option by default generally incurs 2-10%
of overhead depending on the workload with some extreme outliers depending
on the exact CPU. While the benchmarks are somewhat synthetic, the overhead
IO-intensive and network-intensive is easily detectable but the root cause
may not be obvious (e.g. 2-14% overhead for netperf TCP_STREAM running
over localhost with different ranges depending on the CPU).

 .../admin-guide/kernel-parameters.txt         |  4 ++-
 include/linux/thread_info.h                   |  8 ++++-
 mm/usercopy.c                                 | 16 +++++----
 security/Kconfig                              | 21 ------------
 security/Kconfig.hardening                    | 33 +++++++++++++++++++
 5 files changed, 52 insertions(+), 30 deletions(-)

-- 
2.43.0

On Thu, 23 Jan 2025 22:11:11 +0000, Mel Gorman wrote:
> Changelog since v2
> o Default on
> o Logic correction and simplification
> 
> Changelog since v1
> o Menu section rename
> o Make static branch usage similar to init_on_alloc
> o Change ordering of menu options
> 
> [...]

Applied to for-next/hardening, thanks!

[1/4] mm: security: Move hardened usercopy under 'Kernel hardening options'
      https://git.kernel.org/kees/c/8907c768bc27
[2/4] mm: security: Allow default HARDENED_USERCOPY to be set at compile time
      https://git.kernel.org/kees/c/caba7c35e832
[3/4] mm: security: Check early if HARDENED_USERCOPY is enabled
      https://git.kernel.org/kees/c/3d2220040476
[4/4] fortify: Move FORTIFY_SOURCE under 'Kernel hardening options'
      https://git.kernel.org/kees/c/b6518de35d6f

Take care,

-- 
Kees Cook

On Thu, Jan 23, 2025 at 10:11:11PM +0000, Mel Gorman wrote:
> Changelog since v2
> o Default on
> o Logic correction and simplification

Thanks, this look good to me! I'll get this added to the hardening tree
once the merge window closes.

-- 
Kees Cook