arch/x86/mm/extable.c

   1 #include <linux/extable.h>
   2 #include <linux/uaccess.h>
   3 #include <linux/sched/debug.h>
   4 #include <xen/xen.h>
   5
   6 #include <asm/fpu/internal.h>
   7 #include <asm/traps.h>
   8 #include <asm/kdebug.h>
   9
  10 typedef bool (*ex_handler_t)(const struct exception_table_entry *,
  11                             struct pt_regs *, int);
  12
  13 static inline unsigned long
  14 ex_fixup_addr(const struct exception_table_entry *x)
  15 {
  16         return (unsigned long)&x->fixup + x->fixup;
  17 }
  18 static inline ex_handler_t
  19 ex_fixup_handler(const struct exception_table_entry *x)
  20 {
  21         return (ex_handler_t)((unsigned long)&x->handler + x->handler);
  22 }
  23
  24 bool ex_handler_default(const struct exception_table_entry *fixup,
  25                        struct pt_regs *regs, int trapnr)
  26 {
  27         regs->ip = ex_fixup_addr(fixup);
  28         return true;
  29 }
  30 EXPORT_SYMBOL(ex_handler_default);
  31
  32 bool ex_handler_fault(const struct exception_table_entry *fixup,
  33                      struct pt_regs *regs, int trapnr)
  34 {
  35         regs->ip = ex_fixup_addr(fixup);
  36         regs->ax = trapnr;
  37         return true;
  38 }
  39 EXPORT_SYMBOL_GPL(ex_handler_fault);
  40
  41 /*
  42  * Handler for UD0 exception following a failed test against the
  43  * result of a refcount inc/dec/add/sub.
  44  */
  45 bool ex_handler_refcount(const struct exception_table_entry *fixup,
  46                          struct pt_regs *regs, int trapnr)
  47 {
  48         /* First unconditionally saturate the refcount. */
  49         *(int *)regs->cx = INT_MIN / 2;
  50
  51         /*
  52          * Strictly speaking, this reports the fixup destination, not
  53          * the fault location, and not the actually overflowing
  54          * instruction, which is the instruction before the "js", but
  55          * since that instruction could be a variety of lengths, just
  56          * report the location after the overflow, which should be close
  57          * enough for finding the overflow, as it's at least back in
  58          * the function, having returned from .text.unlikely.
  59          */
  60         regs->ip = ex_fixup_addr(fixup);
  61
  62         /*
  63          * This function has been called because either a negative refcount
  64          * value was seen by any of the refcount functions, or a zero
  65          * refcount value was seen by refcount_dec().
  66          *
  67          * If we crossed from INT_MAX to INT_MIN, OF (Overflow Flag: result
  68          * wrapped around) will be set. Additionally, seeing the refcount
  69          * reach 0 will set ZF (Zero Flag: result was zero). In each of
  70          * these cases we want a report, since it's a boundary condition.
  71          * The SF case is not reported since it indicates post-boundary
  72          * manipulations below zero or above INT_MAX. And if none of the
  73          * flags are set, something has gone very wrong, so report it.
  74          */
  75         if (regs->flags & (X86_EFLAGS_OF | X86_EFLAGS_ZF)) {
  76                 bool zero = regs->flags & X86_EFLAGS_ZF;
  77
  78                 refcount_error_report(regs, zero ? "hit zero" : "overflow");
  79         } else if ((regs->flags & X86_EFLAGS_SF) == 0) {
  80                 /* Report if none of OF, ZF, nor SF are set. */
  81                 refcount_error_report(regs, "unexpected saturation");
  82         }
  83
  84         return true;
  85 }
  86 EXPORT_SYMBOL(ex_handler_refcount);
  87
  88 /*
  89  * Handler for when we fail to restore a task's FPU state.  We should never get
  90  * here because the FPU state of a task using the FPU (task->thread.fpu.state)
  91  * should always be valid.  However, past bugs have allowed userspace to set
  92  * reserved bits in the XSAVE area using PTRACE_SETREGSET or sys_rt_sigreturn().
  93  * These caused XRSTOR to fail when switching to the task, leaking the FPU
  94  * registers of the task previously executing on the CPU.  Mitigate this class
  95  * of vulnerability by restoring from the initial state (essentially, zeroing
  96  * out all the FPU registers) if we can't restore from the task's FPU state.
  97  */
  98 bool ex_handler_fprestore(const struct exception_table_entry *fixup,
  99                           struct pt_regs *regs, int trapnr)
 100 {
 101         regs->ip = ex_fixup_addr(fixup);
 102
 103         WARN_ONCE(1, "Bad FPU state detected at %pB, reinitializing FPU registers.",
 104                   (void *)instruction_pointer(regs));
 105
 106         __copy_kernel_to_fpregs(&init_fpstate, -1);
 107         return true;
 108 }
 109 EXPORT_SYMBOL_GPL(ex_handler_fprestore);
 110
 111 bool ex_handler_ext(const struct exception_table_entry *fixup,
 112                    struct pt_regs *regs, int trapnr)
 113 {
 114         /* Special hack for uaccess_err */
 115         current->thread.uaccess_err = 1;
 116         regs->ip = ex_fixup_addr(fixup);
 117         return true;
 118 }
 119 EXPORT_SYMBOL(ex_handler_ext);
 120
 121 bool ex_handler_rdmsr_unsafe(const struct exception_table_entry *fixup,
 122                              struct pt_regs *regs, int trapnr)
 123 {
 124         if (pr_warn_once("unchecked MSR access error: RDMSR from 0x%x at rIP: 0x%lx (%pF)\n",
 125                          (unsigned int)regs->cx, regs->ip, (void *)regs->ip))
 126                 show_stack_regs(regs);
 127
 128         /* Pretend that the read succeeded and returned 0. */
 129         regs->ip = ex_fixup_addr(fixup);
 130         regs->ax = 0;
 131         regs->dx = 0;
 132         return true;
 133 }
 134 EXPORT_SYMBOL(ex_handler_rdmsr_unsafe);
 135
 136 bool ex_handler_wrmsr_unsafe(const struct exception_table_entry *fixup,
 137                              struct pt_regs *regs, int trapnr)
 138 {
 139         if (pr_warn_once("unchecked MSR access error: WRMSR to 0x%x (tried to write 0x%08x%08x) at rIP: 0x%lx (%pF)\n",
 140                          (unsigned int)regs->cx, (unsigned int)regs->dx,
 141                          (unsigned int)regs->ax,  regs->ip, (void *)regs->ip))
 142                 show_stack_regs(regs);
 143
 144         /* Pretend that the write succeeded. */
 145         regs->ip = ex_fixup_addr(fixup);
 146         return true;
 147 }
 148 EXPORT_SYMBOL(ex_handler_wrmsr_unsafe);
 149
 150 bool ex_handler_clear_fs(const struct exception_table_entry *fixup,
 151                          struct pt_regs *regs, int trapnr)
 152 {
 153         if (static_cpu_has(X86_BUG_NULL_SEG))
 154                 asm volatile ("mov %0, %%fs" : : "rm" (__USER_DS));
 155         asm volatile ("mov %0, %%fs" : : "rm" (0));
 156         return ex_handler_default(fixup, regs, trapnr);
 157 }
 158 EXPORT_SYMBOL(ex_handler_clear_fs);
 159
 160 bool ex_has_fault_handler(unsigned long ip)
 161 {
 162         const struct exception_table_entry *e;
 163         ex_handler_t handler;
 164
 165         e = search_exception_tables(ip);
 166         if (!e)
 167                 return false;
 168         handler = ex_fixup_handler(e);
 169
 170         return handler == ex_handler_fault;
 171 }
 172
 173 int fixup_exception(struct pt_regs *regs, int trapnr)
 174 {
 175         const struct exception_table_entry *e;
 176         ex_handler_t handler;
 177
 178 #ifdef CONFIG_PNPBIOS
 179         if (unlikely(SEGMENT_IS_PNP_CODE(regs->cs))) {
 180                 extern u32 pnp_bios_fault_eip, pnp_bios_fault_esp;
 181                 extern u32 pnp_bios_is_utter_crap;
 182                 pnp_bios_is_utter_crap = 1;
 183                 printk(KERN_CRIT "PNPBIOS fault.. attempting recovery.\n");
 184                 __asm__ volatile(
 185                         "movl %0, %%esp\n\t"
 186                         "jmp *%1\n\t"
 187                         : : "g" (pnp_bios_fault_esp), "g" (pnp_bios_fault_eip));
 188                 panic("do_trap: can't hit this");
 189         }
 190 #endif
 191
 192         e = search_exception_tables(regs->ip);
 193         if (!e)
 194                 return 0;
 195
 196         handler = ex_fixup_handler(e);
 197         return handler(e, regs, trapnr);
 198 }
 199
 200 extern unsigned int early_recursion_flag;
 201
 202 /* Restricted version used during very early boot */
 203 void __init early_fixup_exception(struct pt_regs *regs, int trapnr)
 204 {
 205         /* Ignore early NMIs. */
 206         if (trapnr == X86_TRAP_NMI)
 207                 return;
 208
 209         if (early_recursion_flag > 2)
 210                 goto halt_loop;
 211
 212         /*
 213          * Old CPUs leave the high bits of CS on the stack
 214          * undefined.  I'm not sure which CPUs do this, but at least
 215          * the 486 DX works this way.
 216          * Xen pv domains are not using the default __KERNEL_CS.
 217          */
 218         if (!xen_pv_domain() && regs->cs != __KERNEL_CS)
 219                 goto fail;
 220
 221         /*
 222          * The full exception fixup machinery is available as soon as
 223          * the early IDT is loaded.  This means that it is the
 224          * responsibility of extable users to either function correctly
 225          * when handlers are invoked early or to simply avoid causing
 226          * exceptions before they're ready to handle them.
 227          *
 228          * This is better than filtering which handlers can be used,
 229          * because refusing to call a handler here is guaranteed to
 230          * result in a hard-to-debug panic.
 231          *
 232          * Keep in mind that not all vectors actually get here.  Early
 233          * fage faults, for example, are special.
 234          */
 235         if (fixup_exception(regs, trapnr))
 236                 return;
 237
 238         if (fixup_bug(regs, trapnr))
 239                 return;
 240
 241 fail:
 242         early_printk("PANIC: early exception 0x%02x IP %lx:%lx error %lx cr2 0x%lx\n",
 243                      (unsigned)trapnr, (unsigned long)regs->cs, regs->ip,
 244                      regs->orig_ax, read_cr2());
 245
 246         show_regs(regs);
 247
 248 halt_loop:
 249         while (true)
 250                 halt();
 251 }