Re: [PATCH] random: remove "nordrand" flag in favor of "random.trust_cpu"

From: Randy Dunlap
Date: Wed Jul 06 2022 - 20:12:24 EST


Hi Jason,

On 7/6/22 17:00, Jason A. Donenfeld wrote:
> The decision of whether or not to trust RDRAND is controlled by the
> "random.trust_cpu" boot time parameter or the CONFIG_RANDOM_TRUST_CPU
> compile time default. The "nordrand" flag was added during the early
> days of RDRAND, when there were worries that merely using its values
> could compromise the RNG. However, these days, RDRAND values are not
> used directly but always go through the RNG's hash function, making
> "nordrand" no longer useful.
>
> Rather, the correct switch is "random.trust_cpu", which not only handles
> the relevant trust issue directly, but also is general to multiple CPU
> types, not just x86.
>
> However, x86 RDRAND does have a history of being occassionally

occasionally

> problematic. Prior, when the kernel would notice something strange, it'd
> warn in dmesg and suggest enabling "nordrand". We can improve on that by
> making the test a little bit better and then taking the step of
> automatically disabling "nordrand" if we detect it's problematic.

enabling
? I.e., disabling RDRAND and RDSEED.


>
> Also extend the basic sanity test to RDSEED in addition to RDRAND, and
> disable both if either one fails.
>
> Cc: x86@xxxxxxxxxx
> Cc: Theodore Ts'o <tytso@xxxxxxx>
> Suggested-by: H. Peter Anvin <hpa@xxxxxxxxx>
> Suggested-by: Borislav Petkov <bp@xxxxxxx>
> Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
> ---
> This patch builds on top of v4 of "random: remove CONFIG_ARCH_RANDOM":
> https://lore.kernel.org/lkml/20220706143521.459565-1-Jason@xxxxxxxxx/
>
> .../admin-guide/kernel-parameters.txt | 5 --
> arch/x86/kernel/cpu/amd.c | 2 +-
> arch/x86/kernel/cpu/rdrand.c | 73 +++++++++----------
> 3 files changed, 36 insertions(+), 44 deletions(-)

> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index 0c0b09796ced..216fc2f53cbe 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -808,7 +808,7 @@ static void clear_rdrand_cpuid_bit(struct cpuinfo_x86 *c)
> return;
>
> /*
> - * The nordrand option can clear X86_FEATURE_RDRAND, so check for
> + * The self test can clear X86_FEATURE_RDRAND, so check for

Preferably: self-test

> * RDRAND support using the CPUID function directly.
> */
> if (!(cpuid_ecx(1) & BIT(30)) || rdrand_force)


--
~Randy