arch/x86/kernel/doublefault_32.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 #include <linux/mm.h>
   3 #include <linux/sched.h>
   4 #include <linux/sched/debug.h>
   5 #include <linux/init_task.h>
   6 #include <linux/fs.h>
   7
   8 #include <linux/uaccess.h>
   9 #include <asm/processor.h>
  10 #include <asm/desc.h>
  11 #include <asm/traps.h>
  12
  13 #define ptr_ok(x) ((x) > PAGE_OFFSET && (x) < PAGE_OFFSET + MAXMEM)
  14
  15 #define TSS(x) this_cpu_read(cpu_tss_rw.x86_tss.x)
  16
  17 static void set_df_gdt_entry(unsigned int cpu);
  18
  19 /*
  20  * Called by double_fault with CR0.TS and EFLAGS.NT cleared.  The CPU thinks
  21  * we're running the doublefault task.  Cannot return.
  22  */
  23 asmlinkage noinstr void __noreturn doublefault_shim(void)
  24 {
  25         unsigned long cr2;
  26         struct pt_regs regs;
  27
  28         BUILD_BUG_ON(sizeof(struct doublefault_stack) != PAGE_SIZE);
  29
  30         cr2 = native_read_cr2();
  31
  32         /* Reset back to the normal kernel task. */
  33         force_reload_TR();
  34         set_df_gdt_entry(smp_processor_id());
  35
  36         trace_hardirqs_off();
  37
  38         /*
  39          * Fill in pt_regs.  A downside of doing this in C is that the unwinder
  40          * won't see it (no ENCODE_FRAME_POINTER), so a nested stack dump
  41          * won't successfully unwind to the source of the double fault.
  42          * The main dump from exc_double_fault() is fine, though, since it
  43          * uses these regs directly.
  44          *
  45          * If anyone ever cares, this could be moved to asm.
  46          */
  47         regs.ss         = TSS(ss);
  48         regs.__ssh      = 0;
  49         regs.sp         = TSS(sp);
  50         regs.flags      = TSS(flags);
  51         regs.cs         = TSS(cs);
  52         /* We won't go through the entry asm, so we can leave __csh as 0. */
  53         regs.__csh      = 0;
  54         regs.ip         = TSS(ip);
  55         regs.orig_ax    = 0;
  56         regs.gs         = TSS(gs);
  57         regs.__gsh      = 0;
  58         regs.fs         = TSS(fs);
  59         regs.__fsh      = 0;
  60         regs.es         = TSS(es);
  61         regs.__esh      = 0;
  62         regs.ds         = TSS(ds);
  63         regs.__dsh      = 0;
  64         regs.ax         = TSS(ax);
  65         regs.bp         = TSS(bp);
  66         regs.di         = TSS(di);
  67         regs.si         = TSS(si);
  68         regs.dx         = TSS(dx);
  69         regs.cx         = TSS(cx);
  70         regs.bx         = TSS(bx);
  71
  72         exc_double_fault(&regs, 0, cr2);
  73
  74         /*
  75          * x86_32 does not save the original CR3 anywhere on a task switch.
  76          * This means that, even if we wanted to return, we would need to find
  77          * some way to reconstruct CR3.  We could make a credible guess based
  78          * on cpu_tlbstate, but that would be racy and would not account for
  79          * PTI.
  80          *
  81          * Instead, don't bother.  We can return through
  82          * rewind_stack_do_exit() instead.
  83          */
  84         panic("cannot return from double fault\n");
  85 }
  86
  87 DEFINE_PER_CPU_PAGE_ALIGNED(struct doublefault_stack, doublefault_stack) = {
  88         .tss = {
  89                 /*
  90                  * No sp0 or ss0 -- we never run CPL != 0 with this TSS
  91                  * active.  sp is filled in later.
  92                  */
  93                 .ldt            = 0,
  94         .io_bitmap_base = IO_BITMAP_OFFSET_INVALID,
  95
  96                 .ip             = (unsigned long) asm_exc_double_fault,
  97                 .flags          = X86_EFLAGS_FIXED,
  98                 .es             = __USER_DS,
  99                 .cs             = __KERNEL_CS,
 100                 .ss             = __KERNEL_DS,
 101                 .ds             = __USER_DS,
 102                 .fs             = __KERNEL_PERCPU,
 103 #ifndef CONFIG_X86_32_LAZY_GS
 104                 .gs             = __KERNEL_STACK_CANARY,
 105 #endif
 106
 107                 .__cr3          = __pa_nodebug(swapper_pg_dir),
 108         },
 109 };
 110
 111 static void set_df_gdt_entry(unsigned int cpu)
 112 {
 113         /* Set up doublefault TSS pointer in the GDT */
 114         __set_tss_desc(cpu, GDT_ENTRY_DOUBLEFAULT_TSS,
 115                        &get_cpu_entry_area(cpu)->doublefault_stack.tss);
 116
 117 }
 118
 119 void doublefault_init_cpu_tss(void)
 120 {
 121         unsigned int cpu = smp_processor_id();
 122         struct cpu_entry_area *cea = get_cpu_entry_area(cpu);
 123
 124         /*
 125          * The linker isn't smart enough to initialize percpu variables that
 126          * point to other places in percpu space.
 127          */
 128         this_cpu_write(doublefault_stack.tss.sp,
 129                        (unsigned long)&cea->doublefault_stack.stack +
 130                        sizeof(doublefault_stack.stack));
 131
 132         set_df_gdt_entry(cpu);
 133 }