[tip: x86/cpu] x86/rdrand: Sanity-check RDRAND output

[tip-bot2 for Borislav Petkov]

The following commit has been merged into the x86/cpu branch of tip:

Commit-ID:     7879fc4bdc7506d37bd67b6fc29442c53c06dfda
Gitweb:        https://git.kernel.org/tip/7879fc4bdc7506d37bd67b6fc29442c53c06dfda
Author:        Borislav Petkov <bp@xxxxxxx>
AuthorDate:    Sun, 25 Aug 2019 22:50:18 +02:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Tue, 01 Oct 2019 19:55:32 +02:00

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:

  c49a0a80137c ("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@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Cc: Pu Wen <puwen@xxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: x86-ml <x86@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/CAHk-=wjWPDauemCmLTKbdMYFB0UveMszZpcrwoUkJRRWKrqaTw@xxxxxxxxxxxxxx
---
 arch/x86/kernel/cpu/rdrand.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/rdrand.c b/arch/x86/kernel/cpu/rdrand.c
index 5c900f9..c4be620 100644
--- a/arch/x86/kernel/cpu/rdrand.c
+++ b/arch/x86/kernel/cpu/rdrand.c
@@ -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