arch/riscv/kernel/cfi.c

   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  */
  14 static 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  */
  65 enum bug_trap_type handle_cfi_failure(struct pt_regs *regs)
  66 {
  67         unsigned long target;
  68         u32 type;
  69
  70         if (!is_cfi_trap(regs->epc))
  71                 return BUG_TRAP_TYPE_NONE;
  72
  73         if (!decode_cfi_insn(regs, &target, &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
  80 struct bpf_insn;
  81
  82 /* Must match bpf_func_t / DEFINE_BPF_PROG_RUN() */
  83 extern 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; */
  93 asm (
  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 */
 105 extern 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; */
 110 asm (
 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
 121 u32 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