linux-kernel - Re: [GIT pull] x86/urgent for 5.3-rc5

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Date:   Sun, 25 Aug 2019 22:17:23 +0200
From:   Borislav Petkov <bp@...e.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Pu Wen <puwen@...on.cn>, Thomas Gleixner <tglx@...utronix.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [GIT pull] x86/urgent for 5.3-rc5

On Sun, Aug 25, 2019 at 12:59:25PM -0700, Linus Torvalds wrote:
> I think WARN_ONCE() is good. It's big enough that it will show up in
> dmesg if anybody looks, and if nobody looks I think distros still have
> logging for things like that, don't they?

Probably. Lemme research that.

> Hopefully this never actually triggers in practice, thanks to rdrand
> being turned off on known-bad machines now, and Zen 2 being fixed.

Right. Here's v2, will do a proper patch tomorrow:

---
diff --git a/arch/x86/kernel/cpu/rdrand.c b/arch/x86/kernel/cpu/rdrand.c
index 5c900f9527ff..00a9d13c0a92 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 (!changed)
+		WARN_ONCE(1,
+"RDRAND gives funky smelling output, might consider not using it by booting with \"nordrand\"");
+
 }
 #endif


-- 
Regards/Gruss,
    Boris.

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