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x86/CPU: Fix FPDSS on Zen1

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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) <bp@alien8.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Borislav Petkov (AMD)
2026-04-07 11:40:03 +02:00
committed by Linus Torvalds
parent 43cfbdda5a
commit e55d98e775
2 changed files with 6 additions and 0 deletions
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3
arch/x86/include/asm/msr-index.h
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@@ -674,6 +674,9 @@
#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)

3
arch/x86/kernel/cpu/amd.c
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@@ -944,6 +944,9 @@ 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[] = {