root/arch/x86/mm/extable.c

DEFINITIONS

1. ex_fixup_addr
2. ex_fixup_handler
3. ex_handler_default
4. ex_handler_fault
5. ex_handler_refcount
6. ex_handler_fprestore
7. ex_handler_uaccess
8. ex_handler_ext
9. ex_handler_rdmsr_unsafe
10. ex_handler_wrmsr_unsafe
11. ex_handler_clear_fs
12. ex_has_fault_handler
13. fixup_exception
14. early_fixup_exception

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 #include <linux/extable.h>
   3 #include <linux/uaccess.h>
   4 #include <linux/sched/debug.h>
   5 #include <xen/xen.h>
   6 
   7 #include <asm/fpu/internal.h>
   8 #include <asm/traps.h>
   9 #include <asm/kdebug.h>
  10 
  11 typedef bool (*ex_handler_t)(const struct exception_table_entry *,
  12                             struct pt_regs *, int, unsigned long,
  13                             unsigned long);
  14 
  15 static inline unsigned long
  16 ex_fixup_addr(const struct exception_table_entry *x)
  17 {
  18         return (unsigned long)&x->fixup + x->fixup;
  19 }
  20 static inline ex_handler_t
  21 ex_fixup_handler(const struct exception_table_entry *x)
  22 {
  23         return (ex_handler_t)((unsigned long)&x->handler + x->handler);
  24 }
  25 
  26 __visible bool ex_handler_default(const struct exception_table_entry *fixup,
  27                                   struct pt_regs *regs, int trapnr,
  28                                   unsigned long error_code,
  29                                   unsigned long fault_addr)
  30 {
  31         regs->ip = ex_fixup_addr(fixup);
  32         return true;
  33 }
  34 EXPORT_SYMBOL(ex_handler_default);
  35 
  36 __visible bool ex_handler_fault(const struct exception_table_entry *fixup,
  37                                 struct pt_regs *regs, int trapnr,
  38                                 unsigned long error_code,
  39                                 unsigned long fault_addr)
  40 {
  41         regs->ip = ex_fixup_addr(fixup);
  42         regs->ax = trapnr;
  43         return true;
  44 }
  45 EXPORT_SYMBOL_GPL(ex_handler_fault);
  46 
  47 /*
  48  * Handler for UD0 exception following a failed test against the
  49  * result of a refcount inc/dec/add/sub.
  50  */
  51 __visible bool ex_handler_refcount(const struct exception_table_entry *fixup,
  52                                    struct pt_regs *regs, int trapnr,
  53                                    unsigned long error_code,
  54                                    unsigned long fault_addr)
  55 {
  56         /* First unconditionally saturate the refcount. */
  57         *(int *)regs->cx = INT_MIN / 2;
  58 
  59         /*
  60          * Strictly speaking, this reports the fixup destination, not
  61          * the fault location, and not the actually overflowing
  62          * instruction, which is the instruction before the "js", but
  63          * since that instruction could be a variety of lengths, just
  64          * report the location after the overflow, which should be close
  65          * enough for finding the overflow, as it's at least back in
  66          * the function, having returned from .text.unlikely.
  67          */
  68         regs->ip = ex_fixup_addr(fixup);
  69 
  70         /*
  71          * This function has been called because either a negative refcount
  72          * value was seen by any of the refcount functions, or a zero
  73          * refcount value was seen by refcount_dec().
  74          *
  75          * If we crossed from INT_MAX to INT_MIN, OF (Overflow Flag: result
  76          * wrapped around) will be set. Additionally, seeing the refcount
  77          * reach 0 will set ZF (Zero Flag: result was zero). In each of
  78          * these cases we want a report, since it's a boundary condition.
  79          * The SF case is not reported since it indicates post-boundary
  80          * manipulations below zero or above INT_MAX. And if none of the
  81          * flags are set, something has gone very wrong, so report it.
  82          */
  83         if (regs->flags & (X86_EFLAGS_OF | X86_EFLAGS_ZF)) {
  84                 bool zero = regs->flags & X86_EFLAGS_ZF;
  85 
  86                 refcount_error_report(regs, zero ? "hit zero" : "overflow");
  87         } else if ((regs->flags & X86_EFLAGS_SF) == 0) {
  88                 /* Report if none of OF, ZF, nor SF are set. */
  89                 refcount_error_report(regs, "unexpected saturation");
  90         }
  91 
  92         return true;
  93 }
  94 EXPORT_SYMBOL(ex_handler_refcount);
  95 
  96 /*
  97  * Handler for when we fail to restore a task's FPU state.  We should never get
  98  * here because the FPU state of a task using the FPU (task->thread.fpu.state)
  99  * should always be valid.  However, past bugs have allowed userspace to set
 100  * reserved bits in the XSAVE area using PTRACE_SETREGSET or sys_rt_sigreturn().
 101  * These caused XRSTOR to fail when switching to the task, leaking the FPU
 102  * registers of the task previously executing on the CPU.  Mitigate this class
 103  * of vulnerability by restoring from the initial state (essentially, zeroing
 104  * out all the FPU registers) if we can't restore from the task's FPU state.
 105  */
 106 __visible bool ex_handler_fprestore(const struct exception_table_entry *fixup,
 107                                     struct pt_regs *regs, int trapnr,
 108                                     unsigned long error_code,
 109                                     unsigned long fault_addr)
 110 {
 111         regs->ip = ex_fixup_addr(fixup);
 112 
 113         WARN_ONCE(1, "Bad FPU state detected at %pB, reinitializing FPU registers.",
 114                   (void *)instruction_pointer(regs));
 115 
 116         __copy_kernel_to_fpregs(&init_fpstate, -1);
 117         return true;
 118 }
 119 EXPORT_SYMBOL_GPL(ex_handler_fprestore);
 120 
 121 __visible bool ex_handler_uaccess(const struct exception_table_entry *fixup,
 122                                   struct pt_regs *regs, int trapnr,
 123                                   unsigned long error_code,
 124                                   unsigned long fault_addr)
 125 {
 126         WARN_ONCE(trapnr == X86_TRAP_GP, "General protection fault in user access. Non-canonical address?");
 127         regs->ip = ex_fixup_addr(fixup);
 128         return true;
 129 }
 130 EXPORT_SYMBOL(ex_handler_uaccess);
 131 
 132 __visible bool ex_handler_ext(const struct exception_table_entry *fixup,
 133                               struct pt_regs *regs, int trapnr,
 134                               unsigned long error_code,
 135                               unsigned long fault_addr)
 136 {
 137         /* Special hack for uaccess_err */
 138         current->thread.uaccess_err = 1;
 139         regs->ip = ex_fixup_addr(fixup);
 140         return true;
 141 }
 142 EXPORT_SYMBOL(ex_handler_ext);
 143 
 144 __visible bool ex_handler_rdmsr_unsafe(const struct exception_table_entry *fixup,
 145                                        struct pt_regs *regs, int trapnr,
 146                                        unsigned long error_code,
 147                                        unsigned long fault_addr)
 148 {
 149         if (pr_warn_once("unchecked MSR access error: RDMSR from 0x%x at rIP: 0x%lx (%pS)\n",
 150                          (unsigned int)regs->cx, regs->ip, (void *)regs->ip))
 151                 show_stack_regs(regs);
 152 
 153         /* Pretend that the read succeeded and returned 0. */
 154         regs->ip = ex_fixup_addr(fixup);
 155         regs->ax = 0;
 156         regs->dx = 0;
 157         return true;
 158 }
 159 EXPORT_SYMBOL(ex_handler_rdmsr_unsafe);
 160 
 161 __visible bool ex_handler_wrmsr_unsafe(const struct exception_table_entry *fixup,
 162                                        struct pt_regs *regs, int trapnr,
 163                                        unsigned long error_code,
 164                                        unsigned long fault_addr)
 165 {
 166         if (pr_warn_once("unchecked MSR access error: WRMSR to 0x%x (tried to write 0x%08x%08x) at rIP: 0x%lx (%pS)\n",
 167                          (unsigned int)regs->cx, (unsigned int)regs->dx,
 168                          (unsigned int)regs->ax,  regs->ip, (void *)regs->ip))
 169                 show_stack_regs(regs);
 170 
 171         /* Pretend that the write succeeded. */
 172         regs->ip = ex_fixup_addr(fixup);
 173         return true;
 174 }
 175 EXPORT_SYMBOL(ex_handler_wrmsr_unsafe);
 176 
 177 __visible bool ex_handler_clear_fs(const struct exception_table_entry *fixup,
 178                                    struct pt_regs *regs, int trapnr,
 179                                    unsigned long error_code,
 180                                    unsigned long fault_addr)
 181 {
 182         if (static_cpu_has(X86_BUG_NULL_SEG))
 183                 asm volatile ("mov %0, %%fs" : : "rm" (__USER_DS));
 184         asm volatile ("mov %0, %%fs" : : "rm" (0));
 185         return ex_handler_default(fixup, regs, trapnr, error_code, fault_addr);
 186 }
 187 EXPORT_SYMBOL(ex_handler_clear_fs);
 188 
 189 __visible bool ex_has_fault_handler(unsigned long ip)
 190 {
 191         const struct exception_table_entry *e;
 192         ex_handler_t handler;
 193 
 194         e = search_exception_tables(ip);
 195         if (!e)
 196                 return false;
 197         handler = ex_fixup_handler(e);
 198 
 199         return handler == ex_handler_fault;
 200 }
 201 
 202 int fixup_exception(struct pt_regs *regs, int trapnr, unsigned long error_code,
 203                     unsigned long fault_addr)
 204 {
 205         const struct exception_table_entry *e;
 206         ex_handler_t handler;
 207 
 208 #ifdef CONFIG_PNPBIOS
 209         if (unlikely(SEGMENT_IS_PNP_CODE(regs->cs))) {
 210                 extern u32 pnp_bios_fault_eip, pnp_bios_fault_esp;
 211                 extern u32 pnp_bios_is_utter_crap;
 212                 pnp_bios_is_utter_crap = 1;
 213                 printk(KERN_CRIT "PNPBIOS fault.. attempting recovery.\n");
 214                 __asm__ volatile(
 215                         "movl %0, %%esp\n\t"
 216                         "jmp *%1\n\t"
 217                         : : "g" (pnp_bios_fault_esp), "g" (pnp_bios_fault_eip));
 218                 panic("do_trap: can't hit this");
 219         }
 220 #endif
 221 
 222         e = search_exception_tables(regs->ip);
 223         if (!e)
 224                 return 0;
 225 
 226         handler = ex_fixup_handler(e);
 227         return handler(e, regs, trapnr, error_code, fault_addr);
 228 }
 229 
 230 extern unsigned int early_recursion_flag;
 231 
 232 /* Restricted version used during very early boot */
 233 void __init early_fixup_exception(struct pt_regs *regs, int trapnr)
 234 {
 235         /* Ignore early NMIs. */
 236         if (trapnr == X86_TRAP_NMI)
 237                 return;
 238 
 239         if (early_recursion_flag > 2)
 240                 goto halt_loop;
 241 
 242         /*
 243          * Old CPUs leave the high bits of CS on the stack
 244          * undefined.  I'm not sure which CPUs do this, but at least
 245          * the 486 DX works this way.
 246          * Xen pv domains are not using the default __KERNEL_CS.
 247          */
 248         if (!xen_pv_domain() && regs->cs != __KERNEL_CS)
 249                 goto fail;
 250 
 251         /*
 252          * The full exception fixup machinery is available as soon as
 253          * the early IDT is loaded.  This means that it is the
 254          * responsibility of extable users to either function correctly
 255          * when handlers are invoked early or to simply avoid causing
 256          * exceptions before they're ready to handle them.
 257          *
 258          * This is better than filtering which handlers can be used,
 259          * because refusing to call a handler here is guaranteed to
 260          * result in a hard-to-debug panic.
 261          *
 262          * Keep in mind that not all vectors actually get here.  Early
 263          * page faults, for example, are special.
 264          */
 265         if (fixup_exception(regs, trapnr, regs->orig_ax, 0))
 266                 return;
 267 
 268         if (fixup_bug(regs, trapnr))
 269                 return;
 270 
 271 fail:
 272         early_printk("PANIC: early exception 0x%02x IP %lx:%lx error %lx cr2 0x%lx\n",
 273                      (unsigned)trapnr, (unsigned long)regs->cs, regs->ip,
 274                      regs->orig_ax, read_cr2());
 275 
 276         show_regs(regs);
 277 
 278 halt_loop:
 279         while (true)
 280                 halt();
 281 }