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From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org,
	linux-kernel@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	"Borislav Petkov (AMD)" <bp@alien8.de>,
	x86@kernel.org
Subject: [PATCH 6.6 129/150] x86/microcode: do not cache microcode if it will not be used
Date: Thu, 18 Jan 2024 11:49:11 +0100
Message-ID: <20240118104326.023345747@linuxfoundation.org>
In-Reply-To: <20240118104320.029537060@linuxfoundation.org>
References: <20240118104320.029537060@linuxfoundation.org>
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6.6-stable review patch.  If anyone has any objections, please let me know.

------------------

No relevant upstream kernel due to refactoring in 6.7

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@vger.kernel.org # v6.6+
Cc: x86@kernel.org # v6.6+
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 arch/x86/kernel/cpu/microcode/core.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -208,6 +208,11 @@ static int __init save_microcode_in_init
 	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)
@@ -221,6 +226,7 @@ static int __init save_microcode_in_init
 		break;
 	}
 
+out:
 	initrd_gone = true;
 
 	return ret;