1// SPDX-License-Identifier: GPL-2.0
2/*
3 * Clang Control Flow Integrity (CFI) support.
4 *
5 * Copyright (C) 2022 Google LLC
6 */
7#include <linux/string.h>
8#include <linux/cfi.h>
9#include <asm/insn.h>
10#include <asm/insn-eval.h>
11
12/*
13 * Returns the target address and the expected type when regs->ip points
14 * to a compiler-generated CFI trap.
15 */
16static bool decode_cfi_insn(struct pt_regs *regs, unsigned long *target,
17 u32 *type)
18{
19 char buffer[MAX_INSN_SIZE];
20 struct insn insn;
21 int offset = 0;
22
23 *target = *type = 0;
24
25 /*
26 * The compiler generates the following instruction sequence
27 * for indirect call checks:
28 *
29 *   movl -<id>, %r10d ; 6 bytes
30 * addl -<pos>(%reg), %r10d; 4 bytes
31 * je .Ltmp1 ; 2 bytes
32 * ud2 ; <- regs->ip
33 * .Ltmp1:
34 *
35 * We can decode the expected type and the target address from the
36 * movl/addl instructions.
37 */
38 if (copy_from_kernel_nofault(dst: buffer, src: (void *)regs->ip - 12, MAX_INSN_SIZE))
39 return false;
40 if (insn_decode_kernel(&insn, &buffer[offset]))
41 return false;
42 if (insn.opcode.value != 0xBA)
43 return false;
44
45 *type = -(u32)insn.immediate.value;
46
47 if (copy_from_kernel_nofault(dst: buffer, src: (void *)regs->ip - 6, MAX_INSN_SIZE))
48 return false;
49 if (insn_decode_kernel(&insn, &buffer[offset]))
50 return false;
51 if (insn.opcode.value != 0x3)
52 return false;
53
54 /* Read the target address from the register. */
55 offset = insn_get_modrm_rm_off(insn: &insn, regs);
56 if (offset < 0)
57 return false;
58
59 *target = *(unsigned long *)((void *)regs + offset);
60
61 return true;
62}
63
64/*
65 * Checks if a ud2 trap is because of a CFI failure, and handles the trap
66 * if needed. Returns a bug_trap_type value similarly to report_bug.
67 */
68enum bug_trap_type handle_cfi_failure(struct pt_regs *regs)
69{
70 unsigned long target, addr = regs->ip;
71 u32 type;
72
73 switch (cfi_mode) {
74 case CFI_KCFI:
75 if (!is_cfi_trap(addr))
76 return BUG_TRAP_TYPE_NONE;
77
78 if (!decode_cfi_insn(regs, target: &target, type: &type))
79 return report_cfi_failure_noaddr(regs, addr);
80
81 break;
82
83 case CFI_FINEIBT:
84 if (!decode_fineibt_insn(regs, &target, &type))
85 return BUG_TRAP_TYPE_NONE;
86
87 break;
88
89 default:
90 return BUG_TRAP_TYPE_NONE;
91 }
92
93 return report_cfi_failure(regs, addr, &target, type);
94}
95
96/*
97 * Ensure that __kcfi_typeid_ symbols are emitted for functions that may
98 * not be indirectly called with all configurations.
99 */
100__ADDRESSABLE(__memcpy)
101

source code of linux/arch/x86/kernel/cfi.c