linux-kernel - [PATCH] x86/microcode: Fix crashes on early 486 CPUs due to usage of 'cpuid'.

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Message-ID: <CANpbe9Wm3z8fy9HbgS8cuhoj0TREYEEkBipDuhgkWFvqX0UoVQ@mail.gmail.com>
Date: Sat, 19 Oct 2024 08:29:04 +0200
From: Oerg866 <oerg866@...glemail.com>
To: linux-kernel@...r.kernel.org
Cc: Borislav Petkov <bp@...en8.de>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH] x86/microcode: Fix crashes on early 486 CPUs due to usage of 'cpuid'.

Starting with v6.7-rc1, the kernel was no longer able to boot on early
i486-class CPUs.

To clarify, this was caused by the comprehensive microcode rework, from

0a23fb262d17f("Merge tag 'x86_microcode_for_v6.7_rc1'
of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip").

The breaking changes were introduced with these specific commits:

4c585af7180c1("x86/boot/32: Temporarily map initrd for microcode loading")

...causes immediate reboot.

a7939f0167203("x86/microcode/amd: Cache builtin/initrd microcode early")

...causes kernel panic early on.

They assume the host CPU supports the CPUID instruction, which
the pre-"enhanced" 486 type ones do not.

These changes make every kernel version from 6.7 to 6.11.4 bootable on
them.
---
 arch/x86/kernel/cpu/microcode/amd.c | 8 ++++++--
 arch/x86/kernel/head32.c            | 2 +-
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/cpu/microcode/amd.c
b/arch/x86/kernel/cpu/microcode/amd.c
index c0d56c02b8da..71fa388573ac 100644
--- a/arch/x86/kernel/cpu/microcode/amd.c
+++ b/arch/x86/kernel/cpu/microcode/amd.c
@@ -516,14 +516,18 @@ static enum ucode_state load_microcode_amd(u8
family, const u8 *data, size_t siz

 static int __init save_microcode_in_initrd(void)
 {
- unsigned int cpuid_1_eax = native_cpuid_eax(1);
+ unsigned int cpuid_1_eax;
  struct cpuinfo_x86 *c = &boot_cpu_data;
  struct cont_desc desc = { 0 };
  enum ucode_state ret;
  struct cpio_data cp;

- if (dis_ucode_ldr || c->x86_vendor != X86_VENDOR_AMD || c->x86 < 0x10)
+ if (!have_cpuid_p() || dis_ucode_ldr ||
+        c->x86_vendor != X86_VENDOR_AMD || c->x86 < 0x10) {
         return 0;
+ }
+
+ cpuid_1_eax = native_cpuid_eax(1);

  find_blobs_in_containers(cpuid_1_eax, &cp);
  if (!(cp.data && cp.size))
diff --git a/arch/x86/kernel/head32.c b/arch/x86/kernel/head32.c
index de001b2146ab..79fd9f11dbcb 100644
--- a/arch/x86/kernel/head32.c
+++ b/arch/x86/kernel/head32.c
@@ -146,7 +146,7 @@ void __init __no_stack_protector mk_early_pgtbl_32(void)

 #ifdef CONFIG_MICROCODE_INITRD32
  /* Running on a hypervisor? */
- if (native_cpuid_ecx(1) & BIT(31))
+ if (have_cpuid_p() && (native_cpuid_ecx(1) & BIT(31)))
         return;

  params = (struct boot_params *)__pa_nodebug(&boot_params);
--
2.34.1