[patch 1/2] x86/microcode/32: Move early loading after paging enable

From: Thomas Gleixner
Date: Tue Aug 22 2023 - 08:20:23 EST

32-bit loads microcode before paging is enabled. The commit which
introduced that has zero justification in the changelog. The cover letter
has slightly more content, but it does not give any technical justification
either:

"The problem in current microcode loading method is that we load a
microcode way, way too late; ideally we should load it before turning
paging on. This may only be practical on 32 bits since we can't get to
64-bit mode without paging on, but we should still do it as early as at
all possible."

Handwaving word salad with zero technical content.

Someone claimed in an offlist conversation that this is required for curing
the ATOM erratum AAE44/AAF40/AAG38/AAH41. That erratum requires an
microcode update in order to make the usage of PSE safe. But during early
boot PSE is completely irrelevant and it is evaluated way later.

Neither is it relevant for the AP on single core HT enabled CPUs as the
microcode loading on the AP is not doing anything.

On dual core CPUs there is a theoretical problem if a split of an
executable large page between enabling paging including PSE and loading the
microcode happens. But that's only theoretical, it's practically irrelevant
because the affected dual core CPUs are 64bit enabled and therefore have
paging and PSE enabled before loading the microcode on the second core. So
why would it work on 64-bit but not on 32-bit?

The erratum:

"AAG38 Code Fetch May Occur to Incorrect Address After a Large Page is
Split Into 4-Kbyte Pages

Problem: If software clears the PS (page size) bit in a present PDE
(page directory entry), that will cause linear addresses mapped through
this PDE to use 4-KByte pages instead of using a large page after old
TLB entries are invalidated. Due to this erratum, if a code fetch uses
this PDE before the TLB entry for the large page is invalidated then it
may fetch from a different physical address than specified by either the
old large page translation or the new 4-KByte page translation. This
erratum may also cause speculative code fetches from incorrect addresses."

The practical relevance for this is exactly zero because there is no
splitting of large text pages during early boot-time, i.e. between paging
enable and microcode loading, and neither during CPU hotplug.

IOW, this load microcode before paging enable is yet another voodoo
programming solution in search of a problem. What's worse is that it causes
at least two serious problems:

1) When stackprotector is enabled then the microcode loader code has the
stackprotector mechanics enabled. The read from the per CPU variable
__stack_chk_guard is always accessing the virtual address either
directly on UP or via FS on SMP. In physical address mode this results
in an access to memory above 3GB. So this works by chance as the
hardware returns the same value when there is no RAM at this physical
address. When there is RAM populated above 3G then the read is by
chance the same as nothing changes that memory during the very early
boot stage. That's not necessarily true during runtime CPU hotplug.

2) When function tracing is enabled, then the relevant microcode loader
functions and the functions invoked from there will call into the
tracing code and evaluate global and per CPU variables in physical
address mode. What could potentially go wrong?

Cure this and move the microcode loading after the early paging enable and
remove the gunk in the microcode loader which is required to handle
physical address mode.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Cc: Peter Anvin <hpa@xxxxxxxxx>
Link: https://lore.kernel.org/lkml/1356075872-3054-1-git-send-email-fenghua.yu@xxxxxxxxx
---
arch/x86/kernel/cpu/microcode/amd.c | 31 +-------
arch/x86/kernel/cpu/microcode/core.c | 40 ++---------
arch/x86/kernel/cpu/microcode/intel.c | 108 +++----------------------------
arch/x86/kernel/cpu/microcode/internal.h | 2
arch/x86/kernel/head32.c | 3
arch/x86/kernel/head_32.S | 10 --
arch/x86/kernel/smpboot.c | 12 +--
7 files changed, 35 insertions(+), 171 deletions(-)

--- a/arch/x86/kernel/cpu/microcode/amd.c
+++ b/arch/x86/kernel/cpu/microcode/amd.c
@@ -456,14 +456,8 @@ static bool early_apply_microcode(u32 cp
{
struct cont_desc desc = { 0 };
struct microcode_amd *mc;
- u32 rev, dummy, *new_rev;
bool ret = false;
-
-#ifdef CONFIG_X86_32
- new_rev = (u32 *)__pa_nodebug(&ucode_new_rev);
-#else
- new_rev = &ucode_new_rev;
-#endif
+ u32 rev, dummy;

desc.cpuid_1_eax = cpuid_1_eax;

@@ -484,8 +478,8 @@ static bool early_apply_microcode(u32 cp
return ret;

if (!__apply_microcode_amd(mc)) {
- *new_rev = mc->hdr.patch_id;
- ret = true;
+ ucode_new_rev = mc->hdr.patch_id;
+ ret = true;
}

return ret;
@@ -514,26 +508,13 @@ static bool get_builtin_microcode(struct

static void find_blobs_in_containers(unsigned int cpuid_1_eax, struct cpio_data *ret)
{
- struct ucode_cpu_info *uci;
struct cpio_data cp;
- const char *path;
- bool use_pa;
-
- if (IS_ENABLED(CONFIG_X86_32)) {
- uci = (struct ucode_cpu_info *)__pa_nodebug(ucode_cpu_info);
- path = (const char *)__pa_nodebug(ucode_path);
- use_pa = true;
- } else {
- uci = ucode_cpu_info;
- path = ucode_path;
- use_pa = false;
- }

if (!get_builtin_microcode(&cp, x86_family(cpuid_1_eax)))
- cp = find_microcode_in_initrd(path, use_pa);
+ cp = find_microcode_in_initrd(ucode_path);

/* Needed in load_microcode_amd() */
- uci->cpu_sig.sig = cpuid_1_eax;
+ ucode_cpu_info->cpu_sig.sig = cpuid_1_eax;

*ret = cp;
}
@@ -562,7 +543,7 @@ int __init save_microcode_in_initrd_amd(
enum ucode_state ret;
struct cpio_data cp;

- cp = find_microcode_in_initrd(ucode_path, false);
+ cp = find_microcode_in_initrd(ucode_path);
if (!(cp.data && cp.size))
return -EINVAL;

--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -226,40 +226,31 @@ static int __init save_microcode_in_init
return ret;
}

-struct cpio_data find_microcode_in_initrd(const char *path, bool use_pa)
+struct cpio_data find_microcode_in_initrd(const char *path)
{
#ifdef CONFIG_BLK_DEV_INITRD
unsigned long start = 0;
size_t size;

#ifdef CONFIG_X86_32
- struct boot_params *params;
-
- if (use_pa)
- params = (struct boot_params *)__pa_nodebug(&boot_params);
- else
- params = &boot_params;
-
- size = params->hdr.ramdisk_size;
-
+ size = boot_params.hdr.ramdisk_size;
/*
* Set start only if we have an initrd image. We cannot use initrd_start
* because it is not set that early yet.
*/
if (size)
- start = params->hdr.ramdisk_image;
+ start = boot_params.hdr.ramdisk_image;

-# else /* CONFIG_X86_64 */
+#else /* CONFIG_X86_64 */
size = (unsigned long)boot_params.ext_ramdisk_size << 32;
size |= boot_params.hdr.ramdisk_size;

if (size) {
start = (unsigned long)boot_params.ext_ramdisk_image << 32;
start |= boot_params.hdr.ramdisk_image;
-
start += PAGE_OFFSET;
}
-# endif
+#endif

/*
* Fixup the start address: after reserve_initrd() runs, initrd_start
@@ -270,23 +261,10 @@ struct cpio_data find_microcode_in_initr
* initrd_gone is for the hotplug case where we've thrown out initrd
* already.
*/
- if (!use_pa) {
- if (initrd_gone)
- return (struct cpio_data){ NULL, 0, "" };
- if (initrd_start)
- start = initrd_start;
- } else {
- /*
- * The picture with physical addresses is a bit different: we
- * need to get the *physical* address to which the ramdisk was
- * relocated, i.e., relocated_ramdisk (not initrd_start) and
- * since we're running from physical addresses, we need to access
- * relocated_ramdisk through its *physical* address too.
- */
- u64 *rr = (u64 *)__pa_nodebug(&relocated_ramdisk);
- if (*rr)
- start = *rr;
- }
+ if (initrd_gone)
+ return (struct cpio_data){ NULL, 0, "" };
+ if (initrd_start)
+ start = initrd_start;

return find_cpio_data(path, (void *)start, size, NULL);
#else /* !CONFIG_BLK_DEV_INITRD */
--- a/arch/x86/kernel/cpu/microcode/intel.c
+++ b/arch/x86/kernel/cpu/microcode/intel.c
@@ -319,15 +319,8 @@ static void save_microcode_patch(struct
if (!intel_find_matching_signature(p->data, uci->cpu_sig.sig, uci->cpu_sig.pf))
return;

- /*
- * Save for early loading. On 32-bit, that needs to be a physical
- * address as the APs are running from physical addresses, before
- * paging has been enabled.
- */
- if (IS_ENABLED(CONFIG_X86_32))
- intel_ucode_patch = (struct microcode_intel *)__pa_nodebug(p->data);
- else
- intel_ucode_patch = p->data;
+ /* Save for early loading */
+ intel_ucode_patch = p->data;
}

/*
@@ -420,66 +413,10 @@ static bool load_builtin_intel_microcode
return false;
}

-static void print_ucode_info(int old_rev, int new_rev, unsigned int date)
-{
- pr_info_once("updated early: 0x%x -> 0x%x, date = %04x-%02x-%02x\n",
- old_rev,
- new_rev,
- date & 0xffff,
- date >> 24,
- (date >> 16) & 0xff);
-}
-
-#ifdef CONFIG_X86_32
-
-static int delay_ucode_info;
-static int current_mc_date;
-static int early_old_rev;
-
-/*
- * Print early updated ucode info after printk works. This is delayed info dump.
- */
-void show_ucode_info_early(void)
-{
- struct ucode_cpu_info uci;
-
- if (delay_ucode_info) {
- intel_cpu_collect_info(&uci);
- print_ucode_info(early_old_rev, uci.cpu_sig.rev, current_mc_date);
- delay_ucode_info = 0;
- }
-}
-
-/*
- * At this point, we can not call printk() yet. Delay printing microcode info in
- * show_ucode_info_early() until printk() works.
- */
-static void print_ucode(int old_rev, int new_rev, int date)
-{
- int *delay_ucode_info_p;
- int *current_mc_date_p;
- int *early_old_rev_p;
-
- delay_ucode_info_p = (int *)__pa_nodebug(&delay_ucode_info);
- current_mc_date_p = (int *)__pa_nodebug(&current_mc_date);
- early_old_rev_p = (int *)__pa_nodebug(&early_old_rev);
-
- *delay_ucode_info_p = 1;
- *current_mc_date_p = date;
- *early_old_rev_p = old_rev;
-}
-#else
-
-static inline void print_ucode(int old_rev, int new_rev, int date)
-{
- print_ucode_info(old_rev, new_rev, date);
-}
-#endif
-
static int apply_microcode_early(struct ucode_cpu_info *uci, bool early)
{
struct microcode_intel *mc;
- u32 rev, old_rev;
+ u32 rev, old_rev, date;

mc = uci->mc;
if (!mc)
@@ -513,11 +450,9 @@ static int apply_microcode_early(struct

uci->cpu_sig.rev = rev;

- if (early)
- print_ucode(old_rev, uci->cpu_sig.rev, mc->hdr.date);
- else
- print_ucode_info(old_rev, uci->cpu_sig.rev, mc->hdr.date);
-
+ date = mc->hdr.date;
+ pr_info_once("updated early: 0x%x -> 0x%x, date = %04x-%02x-%02x\n",
+ old_rev, rev, date & 0xffff, date >> 24, (date >> 16) & 0xff);
return 0;
}

@@ -535,7 +470,7 @@ int __init save_microcode_in_initrd_inte
intel_ucode_patch = NULL;

if (!load_builtin_intel_microcode(&cp))
- cp = find_microcode_in_initrd(ucode_path, false);
+ cp = find_microcode_in_initrd(ucode_path);

if (!(cp.data && cp.size))
return 0;
@@ -551,21 +486,11 @@ int __init save_microcode_in_initrd_inte
*/
static struct microcode_intel *__load_ucode_intel(struct ucode_cpu_info *uci)
{
- static const char *path;
struct cpio_data cp;
- bool use_pa;
-
- if (IS_ENABLED(CONFIG_X86_32)) {
- path = (const char *)__pa_nodebug(ucode_path);
- use_pa = true;
- } else {
- path = ucode_path;
- use_pa = false;
- }

/* try built-in microcode first */
if (!load_builtin_intel_microcode(&cp))
- cp = find_microcode_in_initrd(path, use_pa);
+ cp = find_microcode_in_initrd(ucode_path);

if (!(cp.data && cp.size))
return NULL;
@@ -591,24 +516,15 @@ void __init load_ucode_intel_bsp(void)

void load_ucode_intel_ap(void)
{
- struct microcode_intel *patch, **iup;
struct ucode_cpu_info uci;

- if (IS_ENABLED(CONFIG_X86_32))
- iup = (struct microcode_intel **) __pa_nodebug(&intel_ucode_patch);
- else
- iup = &intel_ucode_patch;
-
- if (!*iup) {
- patch = __load_ucode_intel(&uci);
- if (!patch)
+ if (!intel_ucode_patch) {
+ intel_ucode_patch = __load_ucode_intel(&uci);
+ if (!intel_ucode_patch)
return;
-
- *iup = patch;
}

- uci.mc = *iup;
-
+ uci.mc = intel_ucode_patch;
apply_microcode_early(&uci, true);
}

--- a/arch/x86/kernel/cpu/microcode/internal.h
+++ b/arch/x86/kernel/cpu/microcode/internal.h
@@ -44,7 +44,7 @@ struct microcode_ops {
};

extern struct ucode_cpu_info ucode_cpu_info[];
-struct cpio_data find_microcode_in_initrd(const char *path, bool use_pa);
+struct cpio_data find_microcode_in_initrd(const char *path);

#define MAX_UCODE_COUNT 128

--- a/arch/x86/kernel/head32.c
+++ b/arch/x86/kernel/head32.c
@@ -19,6 +19,7 @@
#include <asm/apic.h>
#include <asm/io_apic.h>
#include <asm/bios_ebda.h>
+#include <asm/microcode.h>
#include <asm/tlbflush.h>
#include <asm/bootparam_utils.h>

@@ -34,6 +35,8 @@ asmlinkage __visible void __init __noret
/* Make sure IDT is set up before any exception happens */
idt_setup_early_handler();

+ load_ucode_bsp();
+
cr4_init_shadow();

sanitize_boot_params(&boot_params);
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -118,11 +118,6 @@ SYM_CODE_START(startup_32)
movl %eax, pa(olpc_ofw_pgd)
#endif

-#ifdef CONFIG_MICROCODE
- /* Early load ucode on BSP. */
- call load_ucode_bsp
-#endif
-
/* Create early pagetables. */
call mk_early_pgtbl_32

@@ -157,11 +152,6 @@ SYM_FUNC_START(startup_32_smp)
movl %eax,%ss
leal -__PAGE_OFFSET(%ecx),%esp

-#ifdef CONFIG_MICROCODE
- /* Early load ucode on AP. */
- call load_ucode_ap
-#endif
-
.Ldefault_entry:
movl $(CR0_STATE & ~X86_CR0_PG),%eax
movl %eax,%cr0
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -258,12 +258,9 @@ static void notrace start_secondary(void
cpu_init_exception_handling();

/*
- * 32-bit systems load the microcode from the ASM startup code for
- * historical reasons.
- *
- * On 64-bit systems load it before reaching the AP alive
- * synchronization point below so it is not part of the full per
- * CPU serialized bringup part when "parallel" bringup is enabled.
+ * Load the microcode before reaching the AP alive synchronization
+ * point below so it is not part of the full per CPU serialized
+ * bringup part when "parallel" bringup is enabled.
*
* That's even safe when hyperthreading is enabled in the CPU as
* the core code starts the primary threads first and leaves the
@@ -276,8 +273,7 @@ static void notrace start_secondary(void
* CPUID, MSRs etc. must be strictly serialized to maintain
* software state correctness.
*/
- if (IS_ENABLED(CONFIG_X86_64))
- load_ucode_ap();
+ load_ucode_ap();

/*
* Synchronization point with the hotplug core. Sets this CPUs