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SubjectRe: [PATCH] random: add a config option to trust the CPU's hwrng
On 07/17/2018 06:43 PM, Theodore Ts'o wrote:
> This gives the user building their own kernel (or a Linux
> distribution) the option of deciding whether or not to trust the CPU's
> hardware random number generator (e.g., RDRAND for x86 CPU's) as being
> correctly implemented and not having a back door introduced (perhaps
> courtesy of a Nation State's law enforcement or intelligence
> agencies).
> This will prevent getrandom(2) from blocking, if there is a
> willingness to trust the CPU manufacturer.
> Signed-off-by: Theodore Ts'o <>
> ---
> I'm not sure Linux distro's will thank us for this. The problem is
> trusting the CPU manfuacturer can be an emotional / political issue.
> For example, assume that China has decided that as a result of the
> "death sentence" that the US government threatened to impose on ZTE
> after they were caught introducing privacy violating malware on US
> comsumers, that they needed to be self-sufficient in their technology
> sector, and so they decided the needed to produce their own CPU.
> Even if I were convinced that Intel hadn't backdoored RDRAND (or an
> NSA agent backdoored RDRAND for them) such that the NSA had a NOBUS
> (nobody but us) capability to crack RDRAND generated numbers, if we
> made a change to unconditionally trust RDRAND, then I didn't want the
> upstream kernel developers to have to answer the question, "why are
> you willing to trust Intel, but you aren't willing to trust a company
> owned and controlled by a PLA general?" (Or a company owned and
> controlled by one of Putin's Oligarchs, if that makes you feel
> better.)
> With this patch, we don't put ourselves in this position --- but we
> do put the Linux distro's in this position intead. The upside is it
> gives the choice to each person building their own Linux kernel to
> decide whether trusting RDRAND is worth it to avoid hangs due to
> userspace trying to get cryptographic-grade entropy early in the boot
> process. (Note: I trust RDRAND more than I do Jitter Entropy.)
> drivers/char/Kconfig | 14 ++++++++++++++
> drivers/char/random.c | 11 ++++++++++-
> 2 files changed, 24 insertions(+), 1 deletion(-)
> diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
> index 212f447938ae..fe2930c4ecf0 100644
> --- a/drivers/char/Kconfig
> +++ b/drivers/char/Kconfig
> @@ -554,3 +554,17 @@ config ADI
> endmenu

Hi Ted,

In case you go further with this:

> + bool "Trust the CPU manufacturer to initialize Linux's CRNG"
> + depends on (X86 || X86_64 || X86_32 || S390 || PPC)

depends on X86 || S390 || PPC
should be sufficient.

> + default n
> + help

and all 4 lines above should be indented with one tab instead of spaces.

> + Assume that CPU manufacurer (e.g., Intel or AMD for RDSEED or


> + RDRAND, IBM for the S390 and Power PC architectures) is trustworthy
> + for the purposes of initializing Linux's CRNG. Since this is not
> + something that can be indepedently audited, this amounts to trusting


> + that CPU manufacturer (perhaps with the insistance or requirement


> + of a Nation State's intelligence or law enforcement agencies)
> + has not installed a hidden back door to compromise the CPU's
> + random number generation facilities.


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