Messages in this thread |  | | Date | Wed, 6 Jul 2022 17:12:13 -0700 | Subject | Re: [PATCH] random: remove "nordrand" flag in favor of "random.trust_cpu" | From | Randy Dunlap <> |
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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@kernel.org > Cc: Theodore Ts'o <tytso@mit.edu> > Suggested-by: H. Peter Anvin <hpa@zytor.com> > Suggested-by: Borislav Petkov <bp@suse.de> > Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> > --- > This patch builds on top of v4 of "random: remove CONFIG_ARCH_RANDOM": > https://lore.kernel.org/lkml/20220706143521.459565-1-Jason@zx2c4.com/ > > .../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
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