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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:

  c49a0a8013 ("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
This commit is contained in:
Borislav Petkov 2019-08-25 22:50:18 +02:00
parent 54ecb8f702
commit 7879fc4bdc
1 changed files with 21 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

22
arch/x86/kernel/cpu/rdrand.c
View File

@ -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