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 10 #include <asm/desc.h> 11 #include <asm/debugreg.h> 12 #include <asm/mmu_context.h> 13 14 unsigned long convert_ip_to_linear(struct task_struct *child, struct pt_regs *regs) 15 { 16 unsigned long addr, seg; 17 18 addr = regs->ip; 19 seg = regs->cs; 20 if (v8086_mode(regs)) { 21 addr = (addr & 0xffff) + (seg << 4); 22 return addr; 23 } 24 25 #ifdef CONFIG_MODIFY_LDT_SYSCALL 26 /* 27 * We'll assume that the code segments in the GDT 28 * are all zero-based. That is largely true: the 29 * TLS segments are used for data, and the PNPBIOS 30 * and APM bios ones we just ignore here. 31 */ 32 if ((seg & SEGMENT_TI_MASK) == SEGMENT_LDT) { 33 struct desc_struct *desc; 34 unsigned long base; 35 36 seg >>= 3; 37 38 mutex_lock(&child->mm->context.lock); 39 if (unlikely(!child->mm->context.ldt || 40 seg >= child->mm->context.ldt->nr_entries)) 41 addr = -1L; /* bogus selector, access would fault */ 42 else { 43 desc = &child->mm->context.ldt->entries[seg]; 44 base = get_desc_base(desc); 45 46 /* 16-bit code segment? */ 47 if (!desc->d) 48 addr &= 0xffff; 49 addr += base; 50 } 51 mutex_unlock(&child->mm->context.lock); 52 } 53 #endif 54 55 return addr; 56 } 57 58 static int is_setting_trap_flag(struct task_struct *child, struct pt_regs *regs) 59 { 60 int i, copied; 61 unsigned char opcode[15]; 62 unsigned long addr = convert_ip_to_linear(child, regs); 63 64 copied = access_process_vm(child, addr, opcode, sizeof(opcode), 65 FOLL_FORCE); 66 for (i = 0; i < copied; i++) { 67 switch (opcode[i]) { 68 /* popf and iret */ 69 case 0x9d: case 0xcf: 70 return 1; 71 72 /* CHECKME: 64 65 */ 73 74 /* opcode and address size prefixes */ 75 case 0x66: case 0x67: 76 continue; 77 /* irrelevant prefixes (segment overrides and repeats) */ 78 case 0x26: case 0x2e: 79 case 0x36: case 0x3e: 80 case 0x64: case 0x65: 81 case 0xf0: case 0xf2: case 0xf3: 82 continue; 83 84 #ifdef CONFIG_X86_64 85 case 0x40 ... 0x4f: 86 if (!user_64bit_mode(regs)) 87 /* 32-bit mode: register increment */ 88 return 0; 89 /* 64-bit mode: REX prefix */ 90 continue; 91 #endif 92 93 /* CHECKME: f2, f3 */ 94 95 /* 96 * pushf: NOTE! We should probably not let 97 * the user see the TF bit being set. But 98 * it's more pain than it's worth to avoid 99 * it, and a debugger could emulate this 100 * all in user space if it _really_ cares. 101 */ 102 case 0x9c: 103 default: 104 return 0; 105 } 106 } 107 return 0; 108 } 109 110 /* 111 * Enable single-stepping. Return nonzero if user mode is not using TF itself. 112 */ 113 static int enable_single_step(struct task_struct *child) 114 { 115 struct pt_regs *regs = task_pt_regs(child); 116 unsigned long oflags; 117 118 /* 119 * If we stepped into a sysenter/syscall insn, it trapped in 120 * kernel mode; do_debug() cleared TF and set TIF_SINGLESTEP. 121 * If user-mode had set TF itself, then it's still clear from 122 * do_debug() and we need to set it again to restore the user 123 * state so we don't wrongly set TIF_FORCED_TF below. 124 * If enable_single_step() was used last and that is what 125 * set TIF_SINGLESTEP, then both TF and TIF_FORCED_TF are 126 * already set and our bookkeeping is fine. 127 */ 128 if (unlikely(test_tsk_thread_flag(child, TIF_SINGLESTEP))) 129 regs->flags |= X86_EFLAGS_TF; 130 131 /* 132 * Always set TIF_SINGLESTEP. This will also 133 * cause us to set TF when returning to user mode. 134 */ 135 set_tsk_thread_flag(child, TIF_SINGLESTEP); 136 137 /* 138 * Ensure that a trap is triggered once stepping out of a system 139 * call prior to executing any user instruction. 140 */ 141 set_task_syscall_work(child, SYSCALL_EXIT_TRAP); 142 143 oflags = regs->flags; 144 145 /* Set TF on the kernel stack.. */ 146 regs->flags |= X86_EFLAGS_TF; 147 148 /* 149 * ..but if TF is changed by the instruction we will trace, 150 * don't mark it as being "us" that set it, so that we 151 * won't clear it by hand later. 152 * 153 * Note that if we don't actually execute the popf because 154 * of a signal arriving right now or suchlike, we will lose 155 * track of the fact that it really was "us" that set it. 156 */ 157 if (is_setting_trap_flag(child, regs)) { 158 clear_tsk_thread_flag(child, TIF_FORCED_TF); 159 return 0; 160 } 161 162 /* 163 * If TF was already set, check whether it was us who set it. 164 * If not, we should never attempt a block step. 165 */ 166 if (oflags & X86_EFLAGS_TF) 167 return test_tsk_thread_flag(child, TIF_FORCED_TF); 168 169 set_tsk_thread_flag(child, TIF_FORCED_TF); 170 171 return 1; 172 } 173 174 void set_task_blockstep(struct task_struct *task, bool on) 175 { 176 unsigned long debugctl; 177 178 /* 179 * Ensure irq/preemption can't change debugctl in between. 180 * Note also that both TIF_BLOCKSTEP and debugctl should 181 * be changed atomically wrt preemption. 182 * 183 * NOTE: this means that set/clear TIF_BLOCKSTEP is only safe if 184 * task is current or it can't be running, otherwise we can race 185 * with __switch_to_xtra(). We rely on ptrace_freeze_traced(). 186 */ 187 local_irq_disable(); 188 debugctl = get_debugctlmsr(); 189 if (on) { 190 debugctl |= DEBUGCTLMSR_BTF; 191 set_tsk_thread_flag(task, TIF_BLOCKSTEP); 192 } else { 193 debugctl &= ~DEBUGCTLMSR_BTF; 194 clear_tsk_thread_flag(task, TIF_BLOCKSTEP); 195 } 196 if (task == current) 197 update_debugctlmsr(debugctl); 198 local_irq_enable(); 199 } 200 201 /* 202 * Enable single or block step. 203 */ 204 static void enable_step(struct task_struct *child, bool block) 205 { 206 /* 207 * Make sure block stepping (BTF) is not enabled unless it should be. 208 * Note that we don't try to worry about any is_setting_trap_flag() 209 * instructions after the first when using block stepping. 210 * So no one should try to use debugger block stepping in a program 211 * that uses user-mode single stepping itself. 212 */ 213 if (enable_single_step(child) && block) 214 set_task_blockstep(child, true); 215 else if (test_tsk_thread_flag(child, TIF_BLOCKSTEP)) 216 set_task_blockstep(child, false); 217 } 218 219 void user_enable_single_step(struct task_struct *child) 220 { 221 enable_step(child, 0); 222 } 223 224 void user_enable_block_step(struct task_struct *child) 225 { 226 enable_step(child, 1); 227 } 228 229 void user_disable_single_step(struct task_struct *child) 230 { 231 /* 232 * Make sure block stepping (BTF) is disabled. 233 */ 234 if (test_tsk_thread_flag(child, TIF_BLOCKSTEP)) 235 set_task_blockstep(child, false); 236 237 /* Always clear TIF_SINGLESTEP... */ 238 clear_tsk_thread_flag(child, TIF_SINGLESTEP); 239 clear_task_syscall_work(child, SYSCALL_EXIT_TRAP); 240 241 /* But touch TF only if it was set by us.. */ 242 if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF)) 243 task_pt_regs(child)->flags &= ~X86_EFLAGS_TF; 244 } 245