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, unsigned long,
  12                             unsigned long);
  13
  14 static inline unsigned long
  15 ex_fixup_addr(const struct exception_table_entry *x)
  16 {
  17         return (unsigned long)&x->fixup + x->fixup;
  18 }
  19 static inline ex_handler_t
  20 ex_fixup_handler(const struct exception_table_entry *x)
  21 {
  22         return (ex_handler_t)((unsigned long)&x->handler + x->handler);
  23 }
  24
  25 __visible bool ex_handler_default(const struct exception_table_entry *fixup,
  26                                   struct pt_regs *regs, int trapnr,
  27                                   unsigned long error_code,
  28                                   unsigned long fault_addr)
  29 {
  30         regs->ip = ex_fixup_addr(fixup);
  31         return true;
  32 }
  33 EXPORT_SYMBOL(ex_handler_default);
  34
  35 __visible bool ex_handler_fault(const struct exception_table_entry *fixup,
  36                                 struct pt_regs *regs, int trapnr,
  37                                 unsigned long error_code,
  38                                 unsigned long fault_addr)
  39 {
  40         regs->ip = ex_fixup_addr(fixup);
  41         regs->ax = trapnr;
  42         return true;
  43 }
  44 EXPORT_SYMBOL_GPL(ex_handler_fault);
  45
  46 /*
  47  * Handler for UD0 exception following a failed test against the
  48  * result of a refcount inc/dec/add/sub.
  49  */
  50 __visible bool ex_handler_refcount(const struct exception_table_entry *fixup,
  51                                    struct pt_regs *regs, int trapnr,
  52                                    unsigned long error_code,
  53                                    unsigned long fault_addr)
  54 {
  55         /* First unconditionally saturate the refcount. */
  56         *(int *)regs->cx = INT_MIN / 2;
  57
  58         /*
  59          * Strictly speaking, this reports the fixup destination, not
  60          * the fault location, and not the actually overflowing
  61          * instruction, which is the instruction before the "js", but
  62          * since that instruction could be a variety of lengths, just
  63          * report the location after the overflow, which should be close
  64          * enough for finding the overflow, as it's at least back in
  65          * the function, having returned from .text.unlikely.
  66          */
  67         regs->ip = ex_fixup_addr(fixup);
  68
  69         /*
  70          * This function has been called because either a negative refcount
  71          * value was seen by any of the refcount functions, or a zero
  72          * refcount value was seen by refcount_dec().
  73          *
  74          * If we crossed from INT_MAX to INT_MIN, OF (Overflow Flag: result
  75          * wrapped around) will be set. Additionally, seeing the refcount
  76          * reach 0 will set ZF (Zero Flag: result was zero). In each of
  77          * these cases we want a report, since it's a boundary condition.
  78          * The SF case is not reported since it indicates post-boundary
  79          * manipulations below zero or above INT_MAX. And if none of the
  80          * flags are set, something has gone very wrong, so report it.
  81          */
  82         if (regs->flags & (X86_EFLAGS_OF | X86_EFLAGS_ZF)) {
  83                 bool zero = regs->flags & X86_EFLAGS_ZF;
  84
  85                 refcount_error_report(regs, zero ? "hit zero" : "overflow");
  86         } else if ((regs->flags & X86_EFLAGS_SF) == 0) {
  87                 /* Report if none of OF, ZF, nor SF are set. */
  88                 refcount_error_report(regs, "unexpected saturation");
  89         }
  90
  91         return true;
  92 }
  93 EXPORT_SYMBOL(ex_handler_refcount);
  94
  95 /*
  96  * Handler for when we fail to restore a task's FPU state.  We should never get
  97  * here because the FPU state of a task using the FPU (task->thread.fpu.state)
  98  * should always be valid.  However, past bugs have allowed userspace to set
  99  * reserved bits in the XSAVE area using PTRACE_SETREGSET or sys_rt_sigreturn().
 100  * These caused XRSTOR to fail when switching to the task, leaking the FPU
 101  * registers of the task previously executing on the CPU.  Mitigate this class
 102  * of vulnerability by restoring from the initial state (essentially, zeroing
 103  * out all the FPU registers) if we can't restore from the task's FPU state.
 104  */
 105 __visible bool ex_handler_fprestore(const struct exception_table_entry *fixup,
 106                                     struct pt_regs *regs, int trapnr,
 107                                     unsigned long error_code,
 108                                     unsigned long fault_addr)
 109 {
 110         regs->ip = ex_fixup_addr(fixup);
 111
 112         WARN_ONCE(1, "Bad FPU state detected at %pB, reinitializing FPU registers.",
 113                   (void *)instruction_pointer(regs));
 114
 115         __copy_kernel_to_fpregs(&init_fpstate, -1);
 116         return true;
 117 }
 118 EXPORT_SYMBOL_GPL(ex_handler_fprestore);
 119
 120 /* Helper to check whether a uaccess fault indicates a kernel bug. */
 121 static bool bogus_uaccess(struct pt_regs *regs, int trapnr,
 122                           unsigned long fault_addr)
 123 {
 124         /* This is the normal case: #PF with a fault address in userspace. */
 125         if (trapnr == X86_TRAP_PF && fault_addr < TASK_SIZE_MAX)
 126                 return false;
 127
 128         /*
 129          * This code can be reached for machine checks, but only if the #MC
 130          * handler has already decided that it looks like a candidate for fixup.
 131          * This e.g. happens when attempting to access userspace memory which
 132          * the CPU can't access because of uncorrectable bad memory.
 133          */
 134         if (trapnr == X86_TRAP_MC)
 135                 return false;
 136
 137         /*
 138          * There are two remaining exception types we might encounter here:
 139          *  - #PF for faulting accesses to kernel addresses
 140          *  - #GP for faulting accesses to noncanonical addresses
 141          * Complain about anything else.
 142          */
 143         if (trapnr != X86_TRAP_PF && trapnr != X86_TRAP_GP) {
 144                 WARN(1, "unexpected trap %d in uaccess\n", trapnr);
 145                 return false;
 146         }
 147
 148         /*
 149          * This is a faulting memory access in kernel space, on a kernel
 150          * address, in a usercopy function. This can e.g. be caused by improper
 151          * use of helpers like __put_user and by improper attempts to access
 152          * userspace addresses in KERNEL_DS regions.
 153          * The one (semi-)legitimate exception are probe_kernel_{read,write}(),
 154          * which can be invoked from places like kgdb, /dev/mem (for reading)
 155          * and privileged BPF code (for reading).
 156          * The probe_kernel_*() functions set the kernel_uaccess_faults_ok flag
 157          * to tell us that faulting on kernel addresses, and even noncanonical
 158          * addresses, in a userspace accessor does not necessarily imply a
 159          * kernel bug, root might just be doing weird stuff.
 160          */
 161         if (current->kernel_uaccess_faults_ok)
 162                 return false;
 163
 164         /* This is bad. Refuse the fixup so that we go into die(). */
 165         if (trapnr == X86_TRAP_PF) {
 166                 pr_emerg("BUG: pagefault on kernel address 0x%lx in non-whitelisted uaccess\n",
 167                          fault_addr);
 168         } else {
 169                 pr_emerg("BUG: GPF in non-whitelisted uaccess (non-canonical address?)\n");
 170         }
 171         return true;
 172 }
 173
 174 __visible bool ex_handler_uaccess(const struct exception_table_entry *fixup,
 175                                   struct pt_regs *regs, int trapnr,
 176                                   unsigned long error_code,
 177                                   unsigned long fault_addr)
 178 {
 179         if (bogus_uaccess(regs, trapnr, fault_addr))
 180                 return false;
 181         regs->ip = ex_fixup_addr(fixup);
 182         return true;
 183 }
 184 EXPORT_SYMBOL(ex_handler_uaccess);
 185
 186 __visible bool ex_handler_ext(const struct exception_table_entry *fixup,
 187                               struct pt_regs *regs, int trapnr,
 188                               unsigned long error_code,
 189                               unsigned long fault_addr)
 190 {
 191         if (bogus_uaccess(regs, trapnr, fault_addr))
 192                 return false;
 193         /* Special hack for uaccess_err */
 194         current->thread.uaccess_err = 1;
 195         regs->ip = ex_fixup_addr(fixup);
 196         return true;
 197 }
 198 EXPORT_SYMBOL(ex_handler_ext);
 199
 200 __visible bool ex_handler_rdmsr_unsafe(const struct exception_table_entry *fixup,
 201                                        struct pt_regs *regs, int trapnr,
 202                                        unsigned long error_code,
 203                                        unsigned long fault_addr)
 204 {
 205         if (pr_warn_once("unchecked MSR access error: RDMSR from 0x%x at rIP: 0x%lx (%pF)\n",
 206                          (unsigned int)regs->cx, regs->ip, (void *)regs->ip))
 207                 show_stack_regs(regs);
 208
 209         /* Pretend that the read succeeded and returned 0. */
 210         regs->ip = ex_fixup_addr(fixup);
 211         regs->ax = 0;
 212         regs->dx = 0;
 213         return true;
 214 }
 215 EXPORT_SYMBOL(ex_handler_rdmsr_unsafe);
 216
 217 __visible bool ex_handler_wrmsr_unsafe(const struct exception_table_entry *fixup,
 218                                        struct pt_regs *regs, int trapnr,
 219                                        unsigned long error_code,
 220                                        unsigned long fault_addr)
 221 {
 222         if (pr_warn_once("unchecked MSR access error: WRMSR to 0x%x (tried to write 0x%08x%08x) at rIP: 0x%lx (%pF)\n",
 223                          (unsigned int)regs->cx, (unsigned int)regs->dx,
 224                          (unsigned int)regs->ax,  regs->ip, (void *)regs->ip))
 225                 show_stack_regs(regs);
 226
 227         /* Pretend that the write succeeded. */
 228         regs->ip = ex_fixup_addr(fixup);
 229         return true;
 230 }
 231 EXPORT_SYMBOL(ex_handler_wrmsr_unsafe);
 232
 233 __visible bool ex_handler_clear_fs(const struct exception_table_entry *fixup,
 234                                    struct pt_regs *regs, int trapnr,
 235                                    unsigned long error_code,
 236                                    unsigned long fault_addr)
 237 {
 238         if (static_cpu_has(X86_BUG_NULL_SEG))
 239                 asm volatile ("mov %0, %%fs" : : "rm" (__USER_DS));
 240         asm volatile ("mov %0, %%fs" : : "rm" (0));
 241         return ex_handler_default(fixup, regs, trapnr, error_code, fault_addr);
 242 }
 243 EXPORT_SYMBOL(ex_handler_clear_fs);
 244
 245 __visible bool ex_has_fault_handler(unsigned long ip)
 246 {
 247         const struct exception_table_entry *e;
 248         ex_handler_t handler;
 249
 250         e = search_exception_tables(ip);
 251         if (!e)
 252                 return false;
 253         handler = ex_fixup_handler(e);
 254
 255         return handler == ex_handler_fault;
 256 }
 257
 258 int fixup_exception(struct pt_regs *regs, int trapnr, unsigned long error_code,
 259                     unsigned long fault_addr)
 260 {
 261         const struct exception_table_entry *e;
 262         ex_handler_t handler;
 263
 264 #ifdef CONFIG_PNPBIOS
 265         if (unlikely(SEGMENT_IS_PNP_CODE(regs->cs))) {
 266                 extern u32 pnp_bios_fault_eip, pnp_bios_fault_esp;
 267                 extern u32 pnp_bios_is_utter_crap;
 268                 pnp_bios_is_utter_crap = 1;
 269                 printk(KERN_CRIT "PNPBIOS fault.. attempting recovery.\n");
 270                 __asm__ volatile(
 271                         "movl %0, %%esp\n\t"
 272                         "jmp *%1\n\t"
 273                         : : "g" (pnp_bios_fault_esp), "g" (pnp_bios_fault_eip));
 274                 panic("do_trap: can't hit this");
 275         }
 276 #endif
 277
 278         e = search_exception_tables(regs->ip);
 279         if (!e)
 280                 return 0;
 281
 282         handler = ex_fixup_handler(e);
 283         return handler(e, regs, trapnr, error_code, fault_addr);
 284 }
 285
 286 extern unsigned int early_recursion_flag;
 287
 288 /* Restricted version used during very early boot */
 289 void __init early_fixup_exception(struct pt_regs *regs, int trapnr)
 290 {
 291         /* Ignore early NMIs. */
 292         if (trapnr == X86_TRAP_NMI)
 293                 return;
 294
 295         if (early_recursion_flag > 2)
 296                 goto halt_loop;
 297
 298         /*
 299          * Old CPUs leave the high bits of CS on the stack
 300          * undefined.  I'm not sure which CPUs do this, but at least
 301          * the 486 DX works this way.
 302          * Xen pv domains are not using the default __KERNEL_CS.
 303          */
 304         if (!xen_pv_domain() && regs->cs != __KERNEL_CS)
 305                 goto fail;
 306
 307         /*
 308          * The full exception fixup machinery is available as soon as
 309          * the early IDT is loaded.  This means that it is the
 310          * responsibility of extable users to either function correctly
 311          * when handlers are invoked early or to simply avoid causing
 312          * exceptions before they're ready to handle them.
 313          *
 314          * This is better than filtering which handlers can be used,
 315          * because refusing to call a handler here is guaranteed to
 316          * result in a hard-to-debug panic.
 317          *
 318          * Keep in mind that not all vectors actually get here.  Early
 319          * page faults, for example, are special.
 320          */
 321         if (fixup_exception(regs, trapnr, regs->orig_ax, 0))
 322                 return;
 323
 324         if (fixup_bug(regs, trapnr))
 325                 return;
 326
 327 fail:
 328         early_printk("PANIC: early exception 0x%02x IP %lx:%lx error %lx cr2 0x%lx\n",
 329                      (unsigned)trapnr, (unsigned long)regs->cs, regs->ip,
 330                      regs->orig_ax, read_cr2());
 331
 332         show_regs(regs);
 333
 334 halt_loop:
 335         while (true)
 336                 halt();
 337 }