v4.19.13 snapshot.
diff --git a/arch/x86/kernel/doublefault.c b/arch/x86/kernel/doublefault.c
new file mode 100644
index 0000000..0b8cedb
--- /dev/null
+++ b/arch/x86/kernel/doublefault.c
@@ -0,0 +1,83 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/mm.h>
+#include <linux/sched.h>
+#include <linux/sched/debug.h>
+#include <linux/init_task.h>
+#include <linux/fs.h>
+
+#include <linux/uaccess.h>
+#include <asm/pgtable.h>
+#include <asm/processor.h>
+#include <asm/desc.h>
+
+#ifdef CONFIG_X86_32
+
+#define DOUBLEFAULT_STACKSIZE (1024)
+static unsigned long doublefault_stack[DOUBLEFAULT_STACKSIZE];
+#define STACK_START (unsigned long)(doublefault_stack+DOUBLEFAULT_STACKSIZE)
+
+#define ptr_ok(x) ((x) > PAGE_OFFSET && (x) < PAGE_OFFSET + MAXMEM)
+
+static void doublefault_fn(void)
+{
+ struct desc_ptr gdt_desc = {0, 0};
+ unsigned long gdt, tss;
+
+ native_store_gdt(&gdt_desc);
+ gdt = gdt_desc.address;
+
+ printk(KERN_EMERG "PANIC: double fault, gdt at %08lx [%d bytes]\n", gdt, gdt_desc.size);
+
+ if (ptr_ok(gdt)) {
+ gdt += GDT_ENTRY_TSS << 3;
+ tss = get_desc_base((struct desc_struct *)gdt);
+ printk(KERN_EMERG "double fault, tss at %08lx\n", tss);
+
+ if (ptr_ok(tss)) {
+ struct x86_hw_tss *t = (struct x86_hw_tss *)tss;
+
+ printk(KERN_EMERG "eip = %08lx, esp = %08lx\n",
+ t->ip, t->sp);
+
+ printk(KERN_EMERG "eax = %08lx, ebx = %08lx, ecx = %08lx, edx = %08lx\n",
+ t->ax, t->bx, t->cx, t->dx);
+ printk(KERN_EMERG "esi = %08lx, edi = %08lx\n",
+ t->si, t->di);
+ }
+ }
+
+ for (;;)
+ cpu_relax();
+}
+
+struct x86_hw_tss doublefault_tss __cacheline_aligned = {
+ .sp0 = STACK_START,
+ .ss0 = __KERNEL_DS,
+ .ldt = 0,
+ .io_bitmap_base = INVALID_IO_BITMAP_OFFSET,
+
+ .ip = (unsigned long) doublefault_fn,
+ /* 0x2 bit is always set */
+ .flags = X86_EFLAGS_SF | 0x2,
+ .sp = STACK_START,
+ .es = __USER_DS,
+ .cs = __KERNEL_CS,
+ .ss = __KERNEL_DS,
+ .ds = __USER_DS,
+ .fs = __KERNEL_PERCPU,
+
+ .__cr3 = __pa_nodebug(swapper_pg_dir),
+};
+
+/* dummy for do_double_fault() call */
+void df_debug(struct pt_regs *regs, long error_code) {}
+
+#else /* !CONFIG_X86_32 */
+
+void df_debug(struct pt_regs *regs, long error_code)
+{
+ pr_emerg("PANIC: double fault, error_code: 0x%lx\n", error_code);
+ show_regs(regs);
+ panic("Machine halted.");
+}
+#endif