linux-kernel - [PATCH] x86/amd: Disable RDSEED on AMD Zen5 Turin because of an error.

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Message-ID: <20251016182107.3496116-1-gourry@gourry.net>
Date: Thu, 16 Oct 2025 14:21:07 -0400
From: Gregory Price <gourry@...rry.net>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
	tglx@...utronix.de,
	mingo@...hat.com,
	bp@...en8.de,
	dave.hansen@...ux.intel.com,
	hpa@...or.com,
	peterz@...radead.org,
	mario.limonciello@....com,
	riel@...riel.com,
	yazen.ghannam@....com,
	me@...aill.net,
	kai.huang@...el.com,
	sandipan.das@....com,
	darwi@...utronix.de,
	stable@...nel.org
Subject: [PATCH] x86/amd: Disable RDSEED on AMD Zen5 Turin because of an error.

Under unknown architectural conditions, Zen5 chips running rdseed
can produce (val=0,CF=1) as a "random" result over 10% of the time
(when rdseed is successful).  CF=1 indicates success, while val=0
is typically only produced when rdseed fails (CF=0).

This suggests there is an architectural issue which causes rdseed
to misclassify a failure as a success under unknown conditions.

This was reproduced reliably by launching 2-threads per available
core, 1-thread per for hamming on RDSEED, and 1-thread per core
collectively eating and hammering on ~90% of memory.

Fix was modeled after a different RDSEED issue in Zen2 Cyan Skillfish.

Link: https://lore.kernel.org/all/20250715130819.461718765@linuxfoundation.org/
Cc: <stable@...nel.org>
Signed-off-by: Gregory Price <gourry@...rry.net>
---
 arch/x86/kernel/cpu/amd.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 5398db4dedb4..9c3b2f010f8c 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -1037,6 +1037,12 @@ static void init_amd_zen4(struct cpuinfo_x86 *c)

 static void init_amd_zen5(struct cpuinfo_x86 *c)
 {
+	/* Disable RDSEED on AMD Turin because of an error. */
+	if (c->x86_model == 0x11 && c->x86_stepping == 0x0) {
+		clear_cpu_cap(c, X86_FEATURE_RDSEED);
+		msr_clear_bit(MSR_AMD64_CPUID_FN_7, 18);
+		pr_emerg("RDSEED is not reliable on this platform; disabling.\n");
+	}
 }

 static void init_amd(struct cpuinfo_x86 *c)
-- 
2.51.0