arch/x86/kernel/step.c

   1 /*
   2  * x86 single-step support code, common to 32-bit and 64-bit.
   3  */
   4 #include <linux/sched.h>
   5 #include <linux/sched/task_stack.h>
   6 #include <linux/mm.h>
   7 #include <linux/ptrace.h>
   8 #include <asm/desc.h>
   9 #include <asm/mmu_context.h>
  10
  11 unsigned long convert_ip_to_linear(struct task_struct *child, struct pt_regs *regs)
  12 {
  13         unsigned long addr, seg;
  14
  15         addr = regs->ip;
  16         seg = regs->cs & 0xffff;
  17         if (v8086_mode(regs)) {
  18                 addr = (addr & 0xffff) + (seg << 4);
  19                 return addr;
  20         }
  21
  22 #ifdef CONFIG_MODIFY_LDT_SYSCALL
  23         /*
  24          * We'll assume that the code segments in the GDT
  25          * are all zero-based. That is largely true: the
  26          * TLS segments are used for data, and the PNPBIOS
  27          * and APM bios ones we just ignore here.
  28          */
  29         if ((seg & SEGMENT_TI_MASK) == SEGMENT_LDT) {
  30                 struct desc_struct *desc;
  31                 unsigned long base;
  32
  33                 seg >>= 3;
  34
  35                 mutex_lock(&child->mm->context.lock);
  36                 if (unlikely(!child->mm->context.ldt ||
  37                              seg >= child->mm->context.ldt->size))
  38                         addr = -1L; /* bogus selector, access would fault */
  39                 else {
  40                         desc = &child->mm->context.ldt->entries[seg];
  41                         base = get_desc_base(desc);
  42
  43                         /* 16-bit code segment? */
  44                         if (!desc->d)
  45                                 addr &= 0xffff;
  46                         addr += base;
  47                 }
  48                 mutex_unlock(&child->mm->context.lock);
  49         }
  50 #endif
  51
  52         return addr;
  53 }
  54
  55 static int is_setting_trap_flag(struct task_struct *child, struct pt_regs *regs)
  56 {
  57         int i, copied;
  58         unsigned char opcode[15];
  59         unsigned long addr = convert_ip_to_linear(child, regs);
  60
  61         copied = access_process_vm(child, addr, opcode, sizeof(opcode),
  62                         FOLL_FORCE);
  63         for (i = 0; i < copied; i++) {
  64                 switch (opcode[i]) {
  65                 /* popf and iret */
  66                 case 0x9d: case 0xcf:
  67                         return 1;
  68
  69                         /* CHECKME: 64 65 */
  70
  71                 /* opcode and address size prefixes */
  72                 case 0x66: case 0x67:
  73                         continue;
  74                 /* irrelevant prefixes (segment overrides and repeats) */
  75                 case 0x26: case 0x2e:
  76                 case 0x36: case 0x3e:
  77                 case 0x64: case 0x65:
  78                 case 0xf0: case 0xf2: case 0xf3:
  79                         continue;
  80
  81 #ifdef CONFIG_X86_64
  82                 case 0x40 ... 0x4f:
  83                         if (!user_64bit_mode(regs))
  84                                 /* 32-bit mode: register increment */
  85                                 return 0;
  86                         /* 64-bit mode: REX prefix */
  87                         continue;
  88 #endif
  89
  90                         /* CHECKME: f2, f3 */
  91
  92                 /*
  93                  * pushf: NOTE! We should probably not let
  94                  * the user see the TF bit being set. But
  95                  * it's more pain than it's worth to avoid
  96                  * it, and a debugger could emulate this
  97                  * all in user space if it _really_ cares.
  98                  */
  99                 case 0x9c:
 100                 default:
 101                         return 0;
 102                 }
 103         }
 104         return 0;
 105 }
 106
 107 /*
 108  * Enable single-stepping.  Return nonzero if user mode is not using TF itself.
 109  */
 110 static int enable_single_step(struct task_struct *child)
 111 {
 112         struct pt_regs *regs = task_pt_regs(child);
 113         unsigned long oflags;
 114
 115         /*
 116          * If we stepped into a sysenter/syscall insn, it trapped in
 117          * kernel mode; do_debug() cleared TF and set TIF_SINGLESTEP.
 118          * If user-mode had set TF itself, then it's still clear from
 119          * do_debug() and we need to set it again to restore the user
 120          * state so we don't wrongly set TIF_FORCED_TF below.
 121          * If enable_single_step() was used last and that is what
 122          * set TIF_SINGLESTEP, then both TF and TIF_FORCED_TF are
 123          * already set and our bookkeeping is fine.
 124          */
 125         if (unlikely(test_tsk_thread_flag(child, TIF_SINGLESTEP)))
 126                 regs->flags |= X86_EFLAGS_TF;
 127
 128         /*
 129          * Always set TIF_SINGLESTEP - this guarantees that
 130          * we single-step system calls etc..  This will also
 131          * cause us to set TF when returning to user mode.
 132          */
 133         set_tsk_thread_flag(child, TIF_SINGLESTEP);
 134
 135         oflags = regs->flags;
 136
 137         /* Set TF on the kernel stack.. */
 138         regs->flags |= X86_EFLAGS_TF;
 139
 140         /*
 141          * ..but if TF is changed by the instruction we will trace,
 142          * don't mark it as being "us" that set it, so that we
 143          * won't clear it by hand later.
 144          *
 145          * Note that if we don't actually execute the popf because
 146          * of a signal arriving right now or suchlike, we will lose
 147          * track of the fact that it really was "us" that set it.
 148          */
 149         if (is_setting_trap_flag(child, regs)) {
 150                 clear_tsk_thread_flag(child, TIF_FORCED_TF);
 151                 return 0;
 152         }
 153
 154         /*
 155          * If TF was already set, check whether it was us who set it.
 156          * If not, we should never attempt a block step.
 157          */
 158         if (oflags & X86_EFLAGS_TF)
 159                 return test_tsk_thread_flag(child, TIF_FORCED_TF);
 160
 161         set_tsk_thread_flag(child, TIF_FORCED_TF);
 162
 163         return 1;
 164 }
 165
 166 void set_task_blockstep(struct task_struct *task, bool on)
 167 {
 168         unsigned long debugctl;
 169
 170         /*
 171          * Ensure irq/preemption can't change debugctl in between.
 172          * Note also that both TIF_BLOCKSTEP and debugctl should
 173          * be changed atomically wrt preemption.
 174          *
 175          * NOTE: this means that set/clear TIF_BLOCKSTEP is only safe if
 176          * task is current or it can't be running, otherwise we can race
 177          * with __switch_to_xtra(). We rely on ptrace_freeze_traced() but
 178          * PTRACE_KILL is not safe.
 179          */
 180         local_irq_disable();
 181         debugctl = get_debugctlmsr();
 182         if (on) {
 183                 debugctl |= DEBUGCTLMSR_BTF;
 184                 set_tsk_thread_flag(task, TIF_BLOCKSTEP);
 185         } else {
 186                 debugctl &= ~DEBUGCTLMSR_BTF;
 187                 clear_tsk_thread_flag(task, TIF_BLOCKSTEP);
 188         }
 189         if (task == current)
 190                 update_debugctlmsr(debugctl);
 191         local_irq_enable();
 192 }
 193
 194 /*
 195  * Enable single or block step.
 196  */
 197 static void enable_step(struct task_struct *child, bool block)
 198 {
 199         /*
 200          * Make sure block stepping (BTF) is not enabled unless it should be.
 201          * Note that we don't try to worry about any is_setting_trap_flag()
 202          * instructions after the first when using block stepping.
 203          * So no one should try to use debugger block stepping in a program
 204          * that uses user-mode single stepping itself.
 205          */
 206         if (enable_single_step(child) && block)
 207                 set_task_blockstep(child, true);
 208         else if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
 209                 set_task_blockstep(child, false);
 210 }
 211
 212 void user_enable_single_step(struct task_struct *child)
 213 {
 214         enable_step(child, 0);
 215 }
 216
 217 void user_enable_block_step(struct task_struct *child)
 218 {
 219         enable_step(child, 1);
 220 }
 221
 222 void user_disable_single_step(struct task_struct *child)
 223 {
 224         /*
 225          * Make sure block stepping (BTF) is disabled.
 226          */
 227         if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
 228                 set_task_blockstep(child, false);
 229
 230         /* Always clear TIF_SINGLESTEP... */
 231         clear_tsk_thread_flag(child, TIF_SINGLESTEP);
 232
 233         /* But touch TF only if it was set by us.. */
 234         if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF))
 235                 task_pt_regs(child)->flags &= ~X86_EFLAGS_TF;
 236 }