x86/CPU: Fix FPDSS on Zen1

bp3tk0v · opsiff · commit 7d459b4712c6 · 2026-04-21T14:59:55.000+08:00
commit e55d98e7756135f32150b9b8f75d580d0d4b2dd3 upstream.

Zen1's hardware divider can leave, under certain circumstances, partial
results from previous operations.  Those results can be leaked by
another, attacker thread.

Fix that with a chicken bit.

Signed-off-by: Borislav Petkov (AMD) &lt;bp@alien8.de&gt;
Signed-off-by: Linus Torvalds &lt;torvalds@linux-foundation.org&gt;
Signed-off-by: Greg Kroah-Hartman &lt;gregkh@linuxfoundation.org&gt;
(cherry picked from commit ad17f07e95e6e8505e2153e5b391f0d27eacce25)
Signed-off-by: Wentao Guan &lt;guanwentao@uniontech.com&gt;
diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
@@ -645,6 +645,9 @@
 #define MSR_AMD64_DC_CFG		0xc0011022
 #define MSR_AMD64_TW_CFG		0xc0011023
 
+#define MSR_AMD64_FP_CFG		0xc0011028
+#define MSR_AMD64_FP_CFG_ZEN1_DENORM_FIX_BIT	9
+
 #define MSR_AMD64_DE_CFG		0xc0011029
 #define MSR_AMD64_DE_CFG_LFENCE_SERIALIZE_BIT	 1
 #define MSR_AMD64_DE_CFG_LFENCE_SERIALIZE	BIT_ULL(MSR_AMD64_DE_CFG_LFENCE_SERIALIZE_BIT)
diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
@@ -950,6 +950,9 @@ static void init_amd_zen1(struct cpuinfo_x86 *c)
 		msr_clear_bit(MSR_K7_HWCR, MSR_K7_HWCR_IRPERF_EN_BIT);
 		clear_cpu_cap(c, X86_FEATURE_IRPERF);
 	}
+
+	pr_notice_once("AMD Zen1 FPDSS bug detected, enabling mitigation.\n");
+	msr_set_bit(MSR_AMD64_FP_CFG, MSR_AMD64_FP_CFG_ZEN1_DENORM_FIX_BIT);
 }
 
 static const struct x86_cpu_id amd_zenbleed_microcode[] = {

Original file line number	Diff line number	Diff line change
`@@ -950,6 +950,9 @@ static void init_amd_zen1(struct cpuinfo_x86 *c)`
`950`	`950`	`msr_clear_bit(MSR_K7_HWCR, MSR_K7_HWCR_IRPERF_EN_BIT);`
`951`	`951`	`clear_cpu_cap(c, X86_FEATURE_IRPERF);`
`952`	`952`	`}`
	`953`	`+`
	`954`	`+ pr_notice_once("AMD Zen1 FPDSS bug detected, enabling mitigation.\n");`
	`955`	`+ msr_set_bit(MSR_AMD64_FP_CFG, MSR_AMD64_FP_CFG_ZEN1_DENORM_FIX_BIT);`
`953`	`956`	`}`
`954`	`957`
`955`	`958`	`static const struct x86_cpu_id amd_zenbleed_microcode[] = {`