text: // 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/processor.h>
  #include <asm/desc.h>
  #include <asm/traps.h>
  
  #define ptr_ok(x) ((x) > PAGE_OFFSET && (x) < PAGE_OFFSET + MAXMEM)
  
  #define TSS(x) this_cpu_read(cpu_tss_rw.x86_tss.x)
  
  static void set_df_gdt_entry(unsigned int cpu);
  
  /*
   * Called by double_fault with CR0.TS and EFLAGS.NT cleared.  The CPU thinks
   * we're running the doublefault task.  Cannot return.
   */
  asmlinkage noinstr void __noreturn doublefault_shim(void)
  {
  	unsigned long cr2;
  	struct pt_regs regs;
  
  	BUILD_BUG_ON(sizeof(struct doublefault_stack) != PAGE_SIZE);
  
  	cr2 = native_read_cr2();
  
  	/* Reset back to the normal kernel task. */
  	force_reload_TR();
  	set_df_gdt_entry(smp_processor_id());
  
  	trace_hardirqs_off();
  
  	/*
  	 * Fill in pt_regs.  A downside of doing this in C is that the unwinder
  	 * won't see it (no ENCODE_FRAME_POINTER), so a nested stack dump
  	 * won't successfully unwind to the source of the double fault.
  	 * The main dump from exc_double_fault() is fine, though, since it
  	 * uses these regs directly.
  	 *
  	 * If anyone ever cares, this could be moved to asm.
  	 */
  	regs.ss		= TSS(ss);
  	regs.__ssh	= 0;
  	regs.sp		= TSS(sp);
  	regs.flags	= TSS(flags);
  	regs.cs		= TSS(cs);
  	/* We won't go through the entry asm, so we can leave __csh as 0. */
  	regs.__csh	= 0;
  	regs.ip		= TSS(ip);
  	regs.orig_ax	= 0;
  	regs.gs		= TSS(gs);
  	regs.__gsh	= 0;
  	regs.fs		= TSS(fs);
  	regs.__fsh	= 0;
  	regs.es		= TSS(es);
  	regs.__esh	= 0;
  	regs.ds		= TSS(ds);
  	regs.__dsh	= 0;
  	regs.ax		= TSS(ax);
  	regs.bp		= TSS(bp);
  	regs.di		= TSS(di);
  	regs.si		= TSS(si);
  	regs.dx		= TSS(dx);
  	regs.cx		= TSS(cx);
  	regs.bx		= TSS(bx);
  
  	exc_double_fault(&regs, 0, cr2);
  
  	/*
  	 * x86_32 does not save the original CR3 anywhere on a task switch.
  	 * This means that, even if we wanted to return, we would need to find
  	 * some way to reconstruct CR3.  We could make a credible guess based
  	 * on cpu_tlbstate, but that would be racy and would not account for
  	 * PTI.
  	 *
  	 * Instead, don't bother.  We can return through
  	 * rewind_stack_do_exit() instead.
  	 */
  	panic("cannot return from double fault\n");
  }
  
  DEFINE_PER_CPU_PAGE_ALIGNED(struct doublefault_stack, doublefault_stack) = {
  	.tss = {
                  /*
                   * No sp0 or ss0 -- we never run CPL != 0 with this TSS
                   * active.  sp is filled in later.
                   */
  		.ldt		= 0,
  	.io_bitmap_base	= IO_BITMAP_OFFSET_INVALID,
  
  		.ip		= (unsigned long) asm_exc_double_fault,
  		.flags		= X86_EFLAGS_FIXED,
  		.es		= __USER_DS,
  		.cs		= __KERNEL_CS,
  		.ss		= __KERNEL_DS,
  		.ds		= __USER_DS,
  		.fs		= __KERNEL_PERCPU,
  		.gs		= 0,
  
  		.__cr3		= __pa_nodebug(swapper_pg_dir),
  	},
  };
  
  static void set_df_gdt_entry(unsigned int cpu)
  {
  	/* Set up doublefault TSS pointer in the GDT */
  	__set_tss_desc(cpu, GDT_ENTRY_DOUBLEFAULT_TSS,
  		       &get_cpu_entry_area(cpu)->doublefault_stack.tss);
  
  }
  
  void doublefault_init_cpu_tss(void)
  {
  	unsigned int cpu = smp_processor_id();
  	struct cpu_entry_area *cea = get_cpu_entry_area(cpu);
  
  	/*
  	 * The linker isn't smart enough to initialize percpu variables that
  	 * point to other places in percpu space.
  	 */
          this_cpu_write(doublefault_stack.tss.sp,
                         (unsigned long)&cea->doublefault_stack.stack +
                         sizeof(doublefault_stack.stack));
  
  	set_df_gdt_entry(cpu);
  }
  