Commit 7879fc4b authored Aug 25, 2019 by Borislav Petkov

x86/rdrand: Sanity-check RDRAND output



It turned out recently that on certain AMD F15h and F16h machines, due
to the BIOS dropping the ball after resume, yet again, RDRAND would not
function anymore:

  c49a0a80 ("x86/CPU/AMD: Clear RDRAND CPUID bit on AMD family 15h/16h")

Add a silly test to the CPU bringup path, to sanity-check the random
data RDRAND returns and scream as loudly as possible if that returned
random data doesn't change.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Pu Wen <puwen@hygon.cn>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/CAHk-=wjWPDauemCmLTKbdMYFB0UveMszZpcrwoUkJRRWKrqaTw@mail.gmail.com

parent 54ecb8f7

arch/x86/kernel/cpu/rdrand.c

+21 −1

Original line number	Diff line number	Diff line
		@@ -29,7 +29,8 @@ __setup("nordrand", x86_rdrand_setup);
		#ifdef CONFIG_ARCH_RANDOM
		void x86_init_rdrand(struct cpuinfo_x86 *c)
		{
		unsigned long tmp;
		unsigned int changed = 0;
		unsigned long tmp, prev;
		int i;

		if (!cpu_has(c, X86_FEATURE_RDRAND))
		@@ -42,5 +43,24 @@ void x86_init_rdrand(struct cpuinfo_x86 *c)
		return;
		}
		}

		/*
		* Stupid sanity-check whether RDRAND does actually generate
		* some at least random-looking data.
		*/
		prev = tmp;
		for (i = 0; i < SANITY_CHECK_LOOPS; i++) {
		if (rdrand_long(&tmp)) {
		if (prev != tmp)
		changed++;

		prev = tmp;
		}
		}

		if (WARN_ON_ONCE(!changed))
		pr_emerg(
		"RDRAND gives funky smelling output, might consider not using it by booting with \"nordrand\"");

		}
		#endif

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