LXR linux/arch/x86/kernel/step.c

   1// SPDX-License-Identifier: GPL-2.0
   2/*
   3 * x86 single-step support code, common to 32-bit and 64-bit.
   4 */
   5#include <linux/sched.h>
   6#include <linux/sched/task_stack.h>
   7#include <linux/mm.h>
   8#include <linux/ptrace.h>
   9#include <asm/desc.h>
  10#include <asm/mmu_context.h>
  11
  12unsigned long convert_ip_to_linear(struct task_struct *child, struct pt_regs *regs)
  13{
  14        unsigned long addr, seg;
  15
  16        addr = regs->ip;
  17        seg = regs->cs;
  18        if (v8086_mode(regs)) {
  19                addr = (addr & 0xffff) + (seg << 4);
  20                return addr;
  21        }
  22
  23#ifdef CONFIG_MODIFY_LDT_SYSCALL
  24        /*
  25         * We'll assume that the code segments in the GDT
  26         * are all zero-based. That is largely true: the
  27         * TLS segments are used for data, and the PNPBIOS
  28         * and APM bios ones we just ignore here.
  29         */
  30        if ((seg & SEGMENT_TI_MASK) == SEGMENT_LDT) {
  31                struct desc_struct *desc;
  32                unsigned long base;
  33
  34                seg >>= 3;
  35
  36                mutex_lock(&child->mm->context.lock);
  37                if (unlikely(!child->mm->context.ldt ||
  38                             seg >= child->mm->context.ldt->nr_entries))
  39                        addr = -1L; /* bogus selector, access would fault */
  40                else {
  41                        desc = &child->mm->context.ldt->entries[seg];
  42                        base = get_desc_base(desc);
  43
  44                        /* 16-bit code segment? */
  45                        if (!desc->d)
  46                                addr &= 0xffff;
  47                        addr += base;
  48                }
  49                mutex_unlock(&child->mm->context.lock);
  50        }
  51#endif
  52
  53        return addr;
  54}
  55
  56static int is_setting_trap_flag(struct task_struct *child, struct pt_regs *regs)
  57{
  58        int i, copied;
  59        unsigned char opcode[15];
  60        unsigned long addr = convert_ip_to_linear(child, regs);
  61
  62        copied = access_process_vm(child, addr, opcode, sizeof(opcode),
  63                        FOLL_FORCE);
  64        for (i = 0; i < copied; i++) {
  65                switch (opcode[i]) {
  66                /* popf and iret */
  67                case 0x9d: case 0xcf:
  68                        return 1;
  69
  70                        /* CHECKME: 64 65 */
  71
  72                /* opcode and address size prefixes */
  73                case 0x66: case 0x67:
  74                        continue;
  75                /* irrelevant prefixes (segment overrides and repeats) */
  76                case 0x26: case 0x2e:
  77                case 0x36: case 0x3e:
  78                case 0x64: case 0x65:
  79                case 0xf0: case 0xf2: case 0xf3:
  80                        continue;
  81
  82#ifdef CONFIG_X86_64
  83                case 0x40 ... 0x4f:
  84                        if (!user_64bit_mode(regs))
  85                                /* 32-bit mode: register increment */
  86                                return 0;
  87                        /* 64-bit mode: REX prefix */
  88                        continue;
  89#endif
  90
  91                        /* CHECKME: f2, f3 */
  92
  93                /*
  94                 * pushf: NOTE! We should probably not let
  95                 * the user see the TF bit being set. But
  96                 * it's more pain than it's worth to avoid
  97                 * it, and a debugger could emulate this
  98                 * all in user space if it _really_ cares.
  99                 */
 100                case 0x9c:
 101                default:
 102                        return 0;
 103                }
 104        }
 105        return 0;
 106}
 107
 108/*
 109 * Enable single-stepping.  Return nonzero if user mode is not using TF itself.
 110 */
 111static int enable_single_step(struct task_struct *child)
 112{
 113        struct pt_regs *regs = task_pt_regs(child);
 114        unsigned long oflags;
 115
 116        /*
 117         * If we stepped into a sysenter/syscall insn, it trapped in
 118         * kernel mode; do_debug() cleared TF and set TIF_SINGLESTEP.
 119         * If user-mode had set TF itself, then it's still clear from
 120         * do_debug() and we need to set it again to restore the user
 121         * state so we don't wrongly set TIF_FORCED_TF below.
 122         * If enable_single_step() was used last and that is what
 123         * set TIF_SINGLESTEP, then both TF and TIF_FORCED_TF are
 124         * already set and our bookkeeping is fine.
 125         */
 126        if (unlikely(test_tsk_thread_flag(child, TIF_SINGLESTEP)))
 127                regs->flags |= X86_EFLAGS_TF;
 128
 129        /*
 130         * Always set TIF_SINGLESTEP.  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        /*
 136         * Ensure that a trap is triggered once stepping out of a system
 137         * call prior to executing any user instruction.
 138         */
 139        set_task_syscall_work(child, SYSCALL_EXIT_TRAP);
 140
 141        oflags = regs->flags;
 142
 143        /* Set TF on the kernel stack.. */
 144        regs->flags |= X86_EFLAGS_TF;
 145
 146        /*
 147         * ..but if TF is changed by the instruction we will trace,
 148         * don't mark it as being "us" that set it, so that we
 149         * won't clear it by hand later.
 150         *
 151         * Note that if we don't actually execute the popf because
 152         * of a signal arriving right now or suchlike, we will lose
 153         * track of the fact that it really was "us" that set it.
 154         */
 155        if (is_setting_trap_flag(child, regs)) {
 156                clear_tsk_thread_flag(child, TIF_FORCED_TF);
 157                return 0;
 158        }
 159
 160        /*
 161         * If TF was already set, check whether it was us who set it.
 162         * If not, we should never attempt a block step.
 163         */
 164        if (oflags & X86_EFLAGS_TF)
 165                return test_tsk_thread_flag(child, TIF_FORCED_TF);
 166
 167        set_tsk_thread_flag(child, TIF_FORCED_TF);
 168
 169        return 1;
 170}
 171
 172void set_task_blockstep(struct task_struct *task, bool on)
 173{
 174        unsigned long debugctl;
 175
 176        /*
 177         * Ensure irq/preemption can't change debugctl in between.
 178         * Note also that both TIF_BLOCKSTEP and debugctl should
 179         * be changed atomically wrt preemption.
 180         *
 181         * NOTE: this means that set/clear TIF_BLOCKSTEP is only safe if
 182         * task is current or it can't be running, otherwise we can race
 183         * with __switch_to_xtra(). We rely on ptrace_freeze_traced() but
 184         * PTRACE_KILL is not safe.
 185         */
 186        local_irq_disable();
 187        debugctl = get_debugctlmsr();
 188        if (on) {
 189                debugctl |= DEBUGCTLMSR_BTF;
 190                set_tsk_thread_flag(task, TIF_BLOCKSTEP);
 191        } else {
 192                debugctl &= ~DEBUGCTLMSR_BTF;
 193                clear_tsk_thread_flag(task, TIF_BLOCKSTEP);
 194        }
 195        if (task == current)
 196                update_debugctlmsr(debugctl);
 197        local_irq_enable();
 198}
 199
 200/*
 201 * Enable single or block step.
 202 */
 203static void enable_step(struct task_struct *child, bool block)
 204{
 205        /*
 206         * Make sure block stepping (BTF) is not enabled unless it should be.
 207         * Note that we don't try to worry about any is_setting_trap_flag()
 208         * instructions after the first when using block stepping.
 209         * So no one should try to use debugger block stepping in a program
 210         * that uses user-mode single stepping itself.
 211         */
 212        if (enable_single_step(child) && block)
 213                set_task_blockstep(child, true);
 214        else if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
 215                set_task_blockstep(child, false);
 216}
 217
 218void user_enable_single_step(struct task_struct *child)
 219{
 220        enable_step(child, 0);
 221}
 222
 223void user_enable_block_step(struct task_struct *child)
 224{
 225        enable_step(child, 1);
 226}
 227
 228void user_disable_single_step(struct task_struct *child)
 229{
 230        /*
 231         * Make sure block stepping (BTF) is disabled.
 232         */
 233        if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
 234                set_task_blockstep(child, false);
 235
 236        /* Always clear TIF_SINGLESTEP... */
 237        clear_tsk_thread_flag(child, TIF_SINGLESTEP);
 238        clear_task_syscall_work(child, SYSCALL_EXIT_TRAP);
 239
 240        /* But touch TF only if it was set by us.. */
 241        if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF))
 242                task_pt_regs(child)->flags &= ~X86_EFLAGS_TF;
 243}
 244