1// SPDX-License-Identifier: GPL-2.0
2/*
3 * Clang Control Flow Integrity (CFI) support.
4 *
5 * Copyright (C) 2023 Google LLC
6 */
7#include <linux/cfi.h>
8#include <asm/insn.h>
9
10/*
11 * Returns the target address and the expected type when regs->epc points
12 * to a compiler-generated CFI trap.
13 */
14static bool decode_cfi_insn(struct pt_regs *regs, unsigned long *target,
15 u32 *type)
16{
17 unsigned long *regs_ptr = (unsigned long *)regs;
18 int rs1_num;
19 u32 insn;
20
21 *target = *type = 0;
22
23 /*
24 * The compiler generates the following instruction sequence
25 * for indirect call checks:
26 *
27 *   lw t1, -4(<reg>)
28 * lui t2, <hi20>
29 * addiw t2, t2, <lo12>
30 * beq t1, t2, .Ltmp1
31 * ebreak ; <- regs->epc
32 * .Ltmp1:
33 * jalr <reg>
34 *
35 * We can read the expected type and the target address from the
36 * registers passed to the beq/jalr instructions.
37 */
38 if (get_kernel_nofault(insn, (void *)regs->epc - 4))
39 return false;
40 if (!riscv_insn_is_beq(insn))
41 return false;
42
43 *type = (u32)regs_ptr[RV_EXTRACT_RS1_REG(insn)];
44
45 if (get_kernel_nofault(insn, (void *)regs->epc) ||
46 get_kernel_nofault(insn, (void *)regs->epc + GET_INSN_LENGTH(insn)))
47 return false;
48
49 if (riscv_insn_is_jalr(insn))
50 rs1_num = RV_EXTRACT_RS1_REG(insn);
51 else if (riscv_insn_is_c_jalr(insn))
52 rs1_num = RVC_EXTRACT_C2_RS1_REG(insn);
53 else
54 return false;
55
56 *target = regs_ptr[rs1_num];
57
58 return true;
59}
60
61/*
62 * Checks if the ebreak trap is because of a CFI failure, and handles the trap
63 * if needed. Returns a bug_trap_type value similarly to report_bug.
64 */
65enum bug_trap_type handle_cfi_failure(struct pt_regs *regs)
66{
67 unsigned long target;
68 u32 type;
69
70 if (!is_cfi_trap(addr: regs->epc))
71 return BUG_TRAP_TYPE_NONE;
72
73 if (!decode_cfi_insn(regs, target: &target, type: &type))
74 return report_cfi_failure_noaddr(regs, regs->epc);
75
76 return report_cfi_failure(regs, regs->epc, &target, type);
77}
78
79#ifdef CONFIG_CFI_CLANG
80struct bpf_insn;
81
82/* Must match bpf_func_t / DEFINE_BPF_PROG_RUN() */
83extern unsigned int __bpf_prog_runX(const void *ctx,
84 const struct bpf_insn *insn);
85
86/*
87 * Force a reference to the external symbol so the compiler generates
88 * __kcfi_typid.
89 */
90__ADDRESSABLE(__bpf_prog_runX);
91
92/* u32 __ro_after_init cfi_bpf_hash = __kcfi_typeid___bpf_prog_runX; */
93asm (
94" .pushsection .data..ro_after_init,\"aw\",@progbits \n"
95" .type cfi_bpf_hash,@object \n"
96" .globl cfi_bpf_hash \n"
97" .p2align 2, 0x0 \n"
98"cfi_bpf_hash: \n"
99" .word __kcfi_typeid___bpf_prog_runX \n"
100" .size cfi_bpf_hash, 4 \n"
101" .popsection \n"
102);
103
104/* Must match bpf_callback_t */
105extern u64 __bpf_callback_fn(u64, u64, u64, u64, u64);
106
107__ADDRESSABLE(__bpf_callback_fn);
108
109/* u32 __ro_after_init cfi_bpf_subprog_hash = __kcfi_typeid___bpf_callback_fn; */
110asm (
111" .pushsection .data..ro_after_init,\"aw\",@progbits \n"
112" .type cfi_bpf_subprog_hash,@object \n"
113" .globl cfi_bpf_subprog_hash \n"
114" .p2align 2, 0x0 \n"
115"cfi_bpf_subprog_hash: \n"
116" .word __kcfi_typeid___bpf_callback_fn \n"
117" .size cfi_bpf_subprog_hash, 4 \n"
118" .popsection \n"
119);
120
121u32 cfi_get_func_hash(void *func)
122{
123 u32 hash;
124
125 if (get_kernel_nofault(hash, func - cfi_get_offset()))
126 return 0;
127
128 return hash;
129}
130#endif
131

source code of linux/arch/riscv/kernel/cfi.c