1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3  * arch/arm64/kernel/probes/decode-insn.c
4  *
5  * Copyright (C) 2013 Linaro Limited.
6  */
7 
8 #include <linux/kernel.h>
9 #include <linux/kprobes.h>
10 #include <linux/module.h>
11 #include <linux/kallsyms.h>
12 #include <asm/insn.h>
13 #include <asm/sections.h>
14 
15 #include "decode-insn.h"
16 #include "simulate-insn.h"
17 
18 static bool __kprobes aarch64_insn_is_steppable(u32 insn)
19 {
20 	/*
21 	 * Branch instructions will write a new value into the PC which is
22 	 * likely to be relative to the XOL address and therefore invalid.
23 	 * Deliberate generation of an exception during stepping is also not
24 	 * currently safe. Lastly, MSR instructions can do any number of nasty
25 	 * things we can't handle during single-stepping.
26 	 */
27 	if (aarch64_get_insn_class(insn) == AARCH64_INSN_CLS_BR_SYS) {
28 		if (aarch64_insn_is_branch(insn) ||
29 		    aarch64_insn_is_msr_imm(insn) ||
30 		    aarch64_insn_is_msr_reg(insn) ||
31 		    aarch64_insn_is_exception(insn) ||
32 		    aarch64_insn_is_eret(insn) ||
33 		    aarch64_insn_is_eret_auth(insn))
34 			return false;
35 
36 		/*
37 		 * The MRS instruction may not return a correct value when
38 		 * executing in the single-stepping environment. We do make one
39 		 * exception, for reading the DAIF bits.
40 		 */
41 		if (aarch64_insn_is_mrs(insn))
42 			return aarch64_insn_extract_system_reg(insn)
43 			     != AARCH64_INSN_SPCLREG_DAIF;
44 
45 		/*
46 		 * The HINT instruction is steppable only if it is in whitelist
47 		 * and the rest of other such instructions are blocked for
48 		 * single stepping as they may cause exception or other
49 		 * unintended behaviour.
50 		 */
51 		if (aarch64_insn_is_hint(insn))
52 			return aarch64_insn_is_steppable_hint(insn);
53 
54 		return true;
55 	}
56 
57 	/*
58 	 * Instructions which load PC relative literals are not going to work
59 	 * when executed from an XOL slot. Instructions doing an exclusive
60 	 * load/store are not going to complete successfully when single-step
61 	 * exception handling happens in the middle of the sequence.
62 	 */
63 	if (aarch64_insn_uses_literal(insn) ||
64 	    aarch64_insn_is_exclusive(insn))
65 		return false;
66 
67 	return true;
68 }
69 
70 /* Return:
71  *   INSN_REJECTED     If instruction is one not allowed to kprobe,
72  *   INSN_GOOD         If instruction is supported and uses instruction slot,
73  *   INSN_GOOD_NO_SLOT If instruction is supported but doesn't use its slot.
74  */
75 enum probe_insn __kprobes
76 arm_probe_decode_insn(probe_opcode_t insn, struct arch_probe_insn *api)
77 {
78 	/*
79 	 * Instructions reading or modifying the PC won't work from the XOL
80 	 * slot.
81 	 */
82 	if (aarch64_insn_is_steppable(insn))
83 		return INSN_GOOD;
84 
85 	if (aarch64_insn_is_bcond(insn)) {
86 		api->handler = simulate_b_cond;
87 	} else if (aarch64_insn_is_cbz(insn) ||
88 	    aarch64_insn_is_cbnz(insn)) {
89 		api->handler = simulate_cbz_cbnz;
90 	} else if (aarch64_insn_is_tbz(insn) ||
91 	    aarch64_insn_is_tbnz(insn)) {
92 		api->handler = simulate_tbz_tbnz;
93 	} else if (aarch64_insn_is_adr_adrp(insn)) {
94 		api->handler = simulate_adr_adrp;
95 	} else if (aarch64_insn_is_b(insn) ||
96 	    aarch64_insn_is_bl(insn)) {
97 		api->handler = simulate_b_bl;
98 	} else if (aarch64_insn_is_br(insn) ||
99 	    aarch64_insn_is_blr(insn) ||
100 	    aarch64_insn_is_ret(insn)) {
101 		api->handler = simulate_br_blr_ret;
102 	} else if (aarch64_insn_is_ldr_lit(insn)) {
103 		api->handler = simulate_ldr_literal;
104 	} else if (aarch64_insn_is_ldrsw_lit(insn)) {
105 		api->handler = simulate_ldrsw_literal;
106 	} else {
107 		/*
108 		 * Instruction cannot be stepped out-of-line and we don't
109 		 * (yet) simulate it.
110 		 */
111 		return INSN_REJECTED;
112 	}
113 
114 	return INSN_GOOD_NO_SLOT;
115 }
116 
117 #ifdef CONFIG_KPROBES
118 static bool __kprobes
119 is_probed_address_atomic(kprobe_opcode_t *scan_start, kprobe_opcode_t *scan_end)
120 {
121 	while (scan_start >= scan_end) {
122 		/*
123 		 * atomic region starts from exclusive load and ends with
124 		 * exclusive store.
125 		 */
126 		if (aarch64_insn_is_store_ex(le32_to_cpu(*scan_start)))
127 			return false;
128 		else if (aarch64_insn_is_load_ex(le32_to_cpu(*scan_start)))
129 			return true;
130 		scan_start--;
131 	}
132 
133 	return false;
134 }
135 
136 enum probe_insn __kprobes
137 arm_kprobe_decode_insn(kprobe_opcode_t *addr, struct arch_specific_insn *asi)
138 {
139 	enum probe_insn decoded;
140 	probe_opcode_t insn = le32_to_cpu(*addr);
141 	probe_opcode_t *scan_end = NULL;
142 	unsigned long size = 0, offset = 0;
143 
144 	/*
145 	 * If there's a symbol defined in front of and near enough to
146 	 * the probe address assume it is the entry point to this
147 	 * code and use it to further limit how far back we search
148 	 * when determining if we're in an atomic sequence. If we could
149 	 * not find any symbol skip the atomic test altogether as we
150 	 * could otherwise end up searching irrelevant text/literals.
151 	 * KPROBES depends on KALLSYMS so this last case should never
152 	 * happen.
153 	 */
154 	if (kallsyms_lookup_size_offset((unsigned long) addr, &size, &offset)) {
155 		if (offset < (MAX_ATOMIC_CONTEXT_SIZE*sizeof(kprobe_opcode_t)))
156 			scan_end = addr - (offset / sizeof(kprobe_opcode_t));
157 		else
158 			scan_end = addr - MAX_ATOMIC_CONTEXT_SIZE;
159 	}
160 	decoded = arm_probe_decode_insn(insn, &asi->api);
161 
162 	if (decoded != INSN_REJECTED && scan_end)
163 		if (is_probed_address_atomic(addr - 1, scan_end))
164 			return INSN_REJECTED;
165 
166 	return decoded;
167 }
168 #endif
169