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 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 ++-
 mm/usercopy.c                                 |  3 +-
 security/Kconfig                              | 21 ------------
 security/Kconfig.hardening                    | 33 +++++++++++++++++++
 4 files changed, 38 insertions(+), 23 deletions(-)

-- 
2.43.0

On Fri, Jan 17, 2025 at 01:03:34PM +0000, Mel Gorman wrote:
> 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.

Yeah! I like this. It's been somewhere on my TODO list for a while, so
thank you for doing it!

Nits/ideas in the patch replies...

-- 
Kees Cook