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 */
16 static 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 -4(%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(buffer, (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(buffer, (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, 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 */
68 enum bug_trap_type handle_cfi_failure(struct pt_regs *regs)
69 {
70 unsigned long target;
71 u32 type;
72
73 if (!is_cfi_trap(regs->ip))
74 return BUG_TRAP_TYPE_NONE;
75
76 if (!decode_cfi_insn(regs, &target, &type))
77 return report_cfi_failure_noaddr(regs, regs->ip);
78
79 return report_cfi_failure(regs, regs->ip, &target, type);
80 }
81
82 /*
83 * Ensure that __kcfi_typeid_ symbols are emitted for functions that may
84 * not be indirectly called with all configurations.
85 */
86 __ADDRESSABLE(__memcpy)
87
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