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Date: Tue, 22 Nov 2022 15:19:14 +0100
From: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: Pedro Falcato <pedro.falcato@gmail.com>
Cc: devel@edk2.groups.io, Michael D Kinney <michael.d.kinney@intel.com>,
	Liming Gao <gaoliming@byosoft.com.cn>,
	Zhiguang Liu <zhiguang.liu@intel.com>
Subject: Re: [PATCH 1/1] MdePkg/BaseRngLib: Add a smoketest for RDRAND and check CPUID
Message-ID: <Y3zaYgO3/JJc/W75@zx2c4.com>
References: <20221122140121.550740-1-pedro.falcato@gmail.com>
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In-Reply-To: <20221122140121.550740-1-pedro.falcato@gmail.com>
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Hi,

On Tue, Nov 22, 2022 at 02:01:21PM +0000, Pedro Falcato wrote:
> RDRAND has notoriously been broken many times over its lifespan.
> Add a smoketest to RDRAND, in order to better sniff out potential
> security concerns.
> 
> Also add a proper CPUID test in order to support older CPUs which may
> not have it; it was previously being tested but then promptly ignored.
> 
> Signed-off-by: Pedro Falcato <pedro.falcato@gmail.com>
> Cc: Michael D Kinney <michael.d.kinney@intel.com>
> Cc: Liming Gao <gaoliming@byosoft.com.cn>
> Cc: Zhiguang Liu <zhiguang.liu@intel.com>

Considering our discussion an hour ago, I would have appreciated you
CC'ing me. I'm not subscribed to this list, and it's not on lore, so
this is a bit of a PITA to subscribe to.

> +STATIC
> +BOOLEAN
> +TestRdRand (
> +  VOID
> +  )
> +{
> +  //
> +  // Test for notoriously broken rdrand implementations that always return the same
> +  // value, like the Zen 3 uarch (all-1s) or other several AMD families on suspend/resume (also all-1s).
> +  // Note that this should be expanded to extensively test for other sorts of
> +  // possible errata. This testing is quite naive.
> +  //

The test that the kernel does is more robust. Maybe try doing that
instead?

  void x86_init_rdrand(struct cpuinfo_x86 *c)
  {
    enum { SAMPLES = 8, MIN_CHANGE = 5 };
    unsigned long sample, prev;
    bool failure = false;
    size_t i, changed;

    if (!cpu_has(c, X86_FEATURE_RDRAND))
      return;

    for (changed = 0, i = 0; i < SAMPLES; ++i) {
      if (!rdrand_long(&sample)) {
        failure = true;
        break;
      }
      changed += i && sample != prev;
      prev = sample;
    }
    if (changed < MIN_CHANGE)
      failure = true;

    if (failure) {
      clear_cpu_cap(c, X86_FEATURE_RDRAND);
      clear_cpu_cap(c, X86_FEATURE_RDSEED);
      pr_emerg("RDRAND is not reliable on this platform; disabling.\n");
    }
  }

Just copy and paste that and convert the Linuxisms to EDK2isms.

Jason