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
convert_ip_to_linear(struct task_struct * child,struct pt_regs * regs)12 unsigned 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
is_setting_trap_flag(struct task_struct * child,struct pt_regs * regs)56 static 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 */
enable_single_step(struct task_struct * child)111 static 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
set_task_blockstep(struct task_struct * task,bool on)172 void 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 */
enable_step(struct task_struct * child,bool block)203 static 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
user_enable_single_step(struct task_struct * child)218 void user_enable_single_step(struct task_struct *child)
219 {
220 enable_step(child, 0);
221 }
222
user_enable_block_step(struct task_struct * child)223 void user_enable_block_step(struct task_struct *child)
224 {
225 enable_step(child, 1);
226 }
227
user_disable_single_step(struct task_struct * child)228 void 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