linux-kernel - [PATCH v2] x86/amd: Disable RDSEED on AMD Zen5 because of an error.

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Message-ID: <20251018024010.4112396-1-gourry@gourry.net>
Date: Fri, 17 Oct 2025 22:40:10 -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@...r.kernel.org
Subject: [PATCH v2] x86/amd: Disable RDSEED on AMD Zen5 because of an error.

Under unknown conditions, Zen5 chips running rdseed can produce
(val=0,CF=1) 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 a bug which causes rdseed to silently fail.

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.

This was observed on more than 1 Zen5 model, so it should be disabled
for all of Zen5 until/unless a comprehensive blacklist can be built.

Cc: stable@...r.kernel.org
Signed-off-by: Gregory Price <gourry@...rry.net>
---
 arch/x86/kernel/cpu/amd.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 5398db4dedb4..1af30518d3e7 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -1037,6 +1037,10 @@ 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. */
+	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