linux-kernel - [PATCH stable] x86/microcode: do not cache microcode if it will not be used

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Date: Mon, 15 Jan 2024 11:22:02 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: stable@...r.kernel.org,
	x86@...nel.org,
	Borislav Petkov <bp@...e.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>
Subject: [PATCH stable] x86/microcode: do not cache microcode if it will not be used

[ Upstream commit a7939f01672034a58ad3fdbce69bb6c665ce0024 ]

Builtin/initrd microcode will not be used the ucode loader is disabled.
But currently, save_microcode_in_initrd is always performed and it
accesses MSR_IA32_UCODE_REV even if dis_ucode_ldr is true, and in
particular even if X86_FEATURE_HYPERVISOR is set; the TDX module does not
implement the MSR and the result is a call trace at boot for TDX guests.

Mainline Linux fixed this as part of a more complex rework of microcode
caching that went into 6.7 (see in particular commits dd5e3e3ca6,
"x86/microcode/intel: Simplify early loading"; and a7939f0167203,
"x86/microcode/amd: Cache builtin/initrd microcode early").  Do the bare
minimum in stable kernels, setting initrd_gone just like mainline Linux
does in mark_initrd_gone().

Note that save_microcode_in_initrd() is not in the microcode application
path, which runs with paging disabled on 32-bit systems, so it can (and
has to) use dis_ucode_ldr instead of check_loader_disabled_ap().

Cc: stable@...r.kernel.org # v6.6+
Cc: x86@...nel.org # v6.6+
Cc: Borislav Petkov <bp@...e.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kernel/cpu/microcode/core.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index 35d39a13dc90..503b5da56685 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -214,6 +214,11 @@ static int __init save_microcode_in_initrd(void)
 	struct cpuinfo_x86 *c = &boot_cpu_data;
 	int ret = -EINVAL;

+	if (dis_ucode_ldr) {
+		ret = 0;
+		goto out;
+	}
+
 	switch (c->x86_vendor) {
 	case X86_VENDOR_INTEL:
 		if (c->x86 >= 6)
@@ -227,6 +230,7 @@ static int __init save_microcode_in_initrd(void)
 		break;
 	}

+out:
 	initrd_gone = true;

 	return ret;
-- 
2.43.0