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().
184 	 */
185 	local_irq_disable();
186 	debugctl = get_debugctlmsr();
187 	if (on) {
188 		debugctl |= DEBUGCTLMSR_BTF;
189 		set_tsk_thread_flag(task, TIF_BLOCKSTEP);
190 	} else {
191 		debugctl &= ~DEBUGCTLMSR_BTF;
192 		clear_tsk_thread_flag(task, TIF_BLOCKSTEP);
193 	}
194 	if (task == current)
195 		update_debugctlmsr(debugctl);
196 	local_irq_enable();
197 }
198 
199 /*
200  * Enable single or block step.
201  */
enable_step(struct task_struct * child,bool block)202 static void enable_step(struct task_struct *child, bool block)
203 {
204 	/*
205 	 * Make sure block stepping (BTF) is not enabled unless it should be.
206 	 * Note that we don't try to worry about any is_setting_trap_flag()
207 	 * instructions after the first when using block stepping.
208 	 * So no one should try to use debugger block stepping in a program
209 	 * that uses user-mode single stepping itself.
210 	 */
211 	if (enable_single_step(child) && block)
212 		set_task_blockstep(child, true);
213 	else if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
214 		set_task_blockstep(child, false);
215 }
216 
user_enable_single_step(struct task_struct * child)217 void user_enable_single_step(struct task_struct *child)
218 {
219 	enable_step(child, 0);
220 }
221 
user_enable_block_step(struct task_struct * child)222 void user_enable_block_step(struct task_struct *child)
223 {
224 	enable_step(child, 1);
225 }
226 
user_disable_single_step(struct task_struct * child)227 void user_disable_single_step(struct task_struct *child)
228 {
229 	/*
230 	 * Make sure block stepping (BTF) is disabled.
231 	 */
232 	if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
233 		set_task_blockstep(child, false);
234 
235 	/* Always clear TIF_SINGLESTEP... */
236 	clear_tsk_thread_flag(child, TIF_SINGLESTEP);
237 	clear_task_syscall_work(child, SYSCALL_EXIT_TRAP);
238 
239 	/* But touch TF only if it was set by us.. */
240 	if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF))
241 		task_pt_regs(child)->flags &= ~X86_EFLAGS_TF;
242 }
243