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