1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3 * Copyright (C) 2022 - Google LLC
4 * Author: Ard Biesheuvel <ardb@google.com>
5 */
6
7 #include <linux/bug.h>
8 #include <linux/errno.h>
9 #include <linux/init.h>
10 #include <linux/linkage.h>
11 #include <linux/printk.h>
12 #include <linux/types.h>
13
14 #include <asm/cacheflush.h>
15 #include <asm/scs.h>
16
17 //
18 // This minimal DWARF CFI parser is partially based on the code in
19 // arch/arc/kernel/unwind.c, and on the document below:
20 // https://refspecs.linuxbase.org/LSB_4.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html
21 //
22
23 #define DW_CFA_nop 0x00
24 #define DW_CFA_set_loc 0x01
25 #define DW_CFA_advance_loc1 0x02
26 #define DW_CFA_advance_loc2 0x03
27 #define DW_CFA_advance_loc4 0x04
28 #define DW_CFA_offset_extended 0x05
29 #define DW_CFA_restore_extended 0x06
30 #define DW_CFA_undefined 0x07
31 #define DW_CFA_same_value 0x08
32 #define DW_CFA_register 0x09
33 #define DW_CFA_remember_state 0x0a
34 #define DW_CFA_restore_state 0x0b
35 #define DW_CFA_def_cfa 0x0c
36 #define DW_CFA_def_cfa_register 0x0d
37 #define DW_CFA_def_cfa_offset 0x0e
38 #define DW_CFA_def_cfa_expression 0x0f
39 #define DW_CFA_expression 0x10
40 #define DW_CFA_offset_extended_sf 0x11
41 #define DW_CFA_def_cfa_sf 0x12
42 #define DW_CFA_def_cfa_offset_sf 0x13
43 #define DW_CFA_val_offset 0x14
44 #define DW_CFA_val_offset_sf 0x15
45 #define DW_CFA_val_expression 0x16
46 #define DW_CFA_lo_user 0x1c
47 #define DW_CFA_negate_ra_state 0x2d
48 #define DW_CFA_GNU_args_size 0x2e
49 #define DW_CFA_GNU_negative_offset_extended 0x2f
50 #define DW_CFA_hi_user 0x3f
51
52 extern const u8 __eh_frame_start[], __eh_frame_end[];
53
54 enum {
55 PACIASP = 0xd503233f,
56 AUTIASP = 0xd50323bf,
57 SCS_PUSH = 0xf800865e,
58 SCS_POP = 0xf85f8e5e,
59 };
60
scs_patch_loc(u64 loc)61 static void __always_inline scs_patch_loc(u64 loc)
62 {
63 u32 insn = le32_to_cpup((void *)loc);
64
65 switch (insn) {
66 case PACIASP:
67 *(u32 *)loc = cpu_to_le32(SCS_PUSH);
68 break;
69 case AUTIASP:
70 *(u32 *)loc = cpu_to_le32(SCS_POP);
71 break;
72 default:
73 /*
74 * While the DW_CFA_negate_ra_state directive is guaranteed to
75 * appear right after a PACIASP/AUTIASP instruction, it may
76 * also appear after a DW_CFA_restore_state directive that
77 * restores a state that is only partially accurate, and is
78 * followed by DW_CFA_negate_ra_state directive to toggle the
79 * PAC bit again. So we permit other instructions here, and ignore
80 * them.
81 */
82 return;
83 }
84 dcache_clean_pou(loc, loc + sizeof(u32));
85 }
86
87 /*
88 * Skip one uleb128/sleb128 encoded quantity from the opcode stream. All bytes
89 * except the last one have bit #7 set.
90 */
skip_xleb128(const u8 ** opcode,int size)91 static int __always_inline skip_xleb128(const u8 **opcode, int size)
92 {
93 u8 c;
94
95 do {
96 c = *(*opcode)++;
97 size--;
98 } while (c & BIT(7));
99
100 return size;
101 }
102
103 struct eh_frame {
104 /*
105 * The size of this frame if 0 < size < U32_MAX, 0 terminates the list.
106 */
107 u32 size;
108
109 /*
110 * The first frame is a Common Information Entry (CIE) frame, followed
111 * by one or more Frame Description Entry (FDE) frames. In the former
112 * case, this field is 0, otherwise it is the negated offset relative
113 * to the associated CIE frame.
114 */
115 u32 cie_id_or_pointer;
116
117 union {
118 struct { // CIE
119 u8 version;
120 u8 augmentation_string[];
121 };
122
123 struct { // FDE
124 s32 initial_loc;
125 s32 range;
126 u8 opcodes[];
127 };
128 };
129 };
130
scs_handle_fde_frame(const struct eh_frame * frame,bool fde_has_augmentation_data,int code_alignment_factor,bool dry_run)131 static int noinstr scs_handle_fde_frame(const struct eh_frame *frame,
132 bool fde_has_augmentation_data,
133 int code_alignment_factor,
134 bool dry_run)
135 {
136 int size = frame->size - offsetof(struct eh_frame, opcodes) + 4;
137 u64 loc = (u64)offset_to_ptr(&frame->initial_loc);
138 const u8 *opcode = frame->opcodes;
139
140 if (fde_has_augmentation_data) {
141 int l;
142
143 // assume single byte uleb128_t
144 if (WARN_ON(*opcode & BIT(7)))
145 return -ENOEXEC;
146
147 l = *opcode++;
148 opcode += l;
149 size -= l + 1;
150 }
151
152 /*
153 * Starting from 'loc', apply the CFA opcodes that advance the location
154 * pointer, and identify the locations of the PAC instructions.
155 */
156 while (size-- > 0) {
157 switch (*opcode++) {
158 case DW_CFA_nop:
159 case DW_CFA_remember_state:
160 case DW_CFA_restore_state:
161 break;
162
163 case DW_CFA_advance_loc1:
164 loc += *opcode++ * code_alignment_factor;
165 size--;
166 break;
167
168 case DW_CFA_advance_loc2:
169 loc += *opcode++ * code_alignment_factor;
170 loc += (*opcode++ << 8) * code_alignment_factor;
171 size -= 2;
172 break;
173
174 case DW_CFA_def_cfa:
175 case DW_CFA_offset_extended:
176 size = skip_xleb128(&opcode, size);
177 fallthrough;
178 case DW_CFA_def_cfa_offset:
179 case DW_CFA_def_cfa_offset_sf:
180 case DW_CFA_def_cfa_register:
181 case DW_CFA_same_value:
182 case DW_CFA_restore_extended:
183 case 0x80 ... 0xbf:
184 size = skip_xleb128(&opcode, size);
185 break;
186
187 case DW_CFA_negate_ra_state:
188 if (!dry_run)
189 scs_patch_loc(loc - 4);
190 break;
191
192 case 0x40 ... 0x7f:
193 // advance loc
194 loc += (opcode[-1] & 0x3f) * code_alignment_factor;
195 break;
196
197 case 0xc0 ... 0xff:
198 break;
199
200 default:
201 pr_err("unhandled opcode: %02x in FDE frame %lx\n", opcode[-1], (uintptr_t)frame);
202 return -ENOEXEC;
203 }
204 }
205 return 0;
206 }
207
scs_patch(const u8 eh_frame[],int size)208 int noinstr scs_patch(const u8 eh_frame[], int size)
209 {
210 const u8 *p = eh_frame;
211
212 while (size > 4) {
213 const struct eh_frame *frame = (const void *)p;
214 bool fde_has_augmentation_data = true;
215 int code_alignment_factor = 1;
216 int ret;
217
218 if (frame->size == 0 ||
219 frame->size == U32_MAX ||
220 frame->size > size)
221 break;
222
223 if (frame->cie_id_or_pointer == 0) {
224 const u8 *p = frame->augmentation_string;
225
226 /* a 'z' in the augmentation string must come first */
227 fde_has_augmentation_data = *p == 'z';
228
229 /*
230 * The code alignment factor is a uleb128 encoded field
231 * but given that the only sensible values are 1 or 4,
232 * there is no point in decoding the whole thing.
233 */
234 p += strlen(p) + 1;
235 if (!WARN_ON(*p & BIT(7)))
236 code_alignment_factor = *p;
237 } else {
238 ret = scs_handle_fde_frame(frame,
239 fde_has_augmentation_data,
240 code_alignment_factor,
241 true);
242 if (ret)
243 return ret;
244 scs_handle_fde_frame(frame, fde_has_augmentation_data,
245 code_alignment_factor, false);
246 }
247
248 p += sizeof(frame->size) + frame->size;
249 size -= sizeof(frame->size) + frame->size;
250 }
251 return 0;
252 }
253
scs_patch_vmlinux(void)254 asmlinkage void __init scs_patch_vmlinux(void)
255 {
256 if (!should_patch_pac_into_scs())
257 return;
258
259 WARN_ON(scs_patch(__eh_frame_start, __eh_frame_end - __eh_frame_start));
260 icache_inval_all_pou();
261 isb();
262 }
263