On Wed, Jul 06, 2022 at 02:32:25AM +0200, Jason A. Donenfeld wrote:

When RDRAND was introduced, there was much discussion on whether it
should be trusted and how the kernel should handle that. Initially, two
mechanisms cropped up, CONFIG_ARCH_RANDOM, a compile time switch, and
"nordrand", a boot-time switch.

Later the thinking evolved. With a properly designed RNG, using RDRAND
values alone won't harm anything, even if the outputs are malicious.
Rather, the issue is whether those values are being *trusted* to be good
or not. And so a new set of options were introduced as the real
ones that people use -- CONFIG_RANDOM_TRUST_CPU and "random.trust_cpu".
With these options, RDRAND is used, but it's not always credited. So in
the worst case, it does nothing, and in the best case, maybe it helps.

Along the way, CONFIG_ARCH_RANDOM's meaning got sort of pulled into the
center and became something certain platforms force-select.

The old options don't really help with much, and it's a bit odd to have
special handling for these instructions when the kernel can deal fine
with the existence or untrusted existence or broken existence or
non-existence of that CPU capability.

So this commit simplifies things down to the two options that are
actually used, and removes the confusing old ones that aren't used or
useful. It leaves "nordrand" for now, as the removal of that will take a
different route.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Thomas Gleixner <redacted>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 arch/arm64/Kconfig                                |  8 --------
 arch/arm64/include/asm/archrandom.h               | 10 ----------
 arch/arm64/kernel/cpufeature.c                    |  2 --
 arch/powerpc/Kconfig                              |  3 ---
 arch/powerpc/include/asm/archrandom.h             |  3 ---
 arch/powerpc/include/asm/machdep.h                |  2 --
 arch/powerpc/platforms/microwatt/Kconfig          |  1 -
 arch/powerpc/platforms/powernv/Kconfig            |  1 -
 arch/powerpc/platforms/pseries/Kconfig            |  1 -
 arch/s390/Kconfig                                 | 15 ---------------
 arch/s390/configs/zfcpdump_defconfig              |  1 -
 arch/s390/crypto/Makefile                         |  2 +-
 arch/s390/include/asm/archrandom.h                |  3 ---
 arch/x86/Kconfig                                  |  9 ---------
 arch/x86/include/asm/archrandom.h                 | 10 +---------
 arch/x86/kernel/cpu/rdrand.c                      |  2 --
 drivers/char/Kconfig                              |  1 -
 drivers/char/hw_random/s390-trng.c                |  9 ---------
 include/linux/random.h                            |  9 +--------
 .../selftests/wireguard/qemu/kernel.config        |  1 -
 20 files changed, 3 insertions(+), 90 deletions(-)

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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