| // Copyright (c) 2012 The Chromium Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style license that can be |
| // found in the LICENSE file. |
| |
| #include "sandbox/linux/bpf_dsl/verifier.h" |
| |
| #include <string.h> |
| |
| #include <limits> |
| |
| #include "sandbox/linux/bpf_dsl/bpf_dsl.h" |
| #include "sandbox/linux/bpf_dsl/bpf_dsl_impl.h" |
| #include "sandbox/linux/bpf_dsl/policy.h" |
| #include "sandbox/linux/bpf_dsl/policy_compiler.h" |
| #include "sandbox/linux/bpf_dsl/seccomp_macros.h" |
| #include "sandbox/linux/bpf_dsl/syscall_set.h" |
| #include "sandbox/linux/seccomp-bpf/errorcode.h" |
| #include "sandbox/linux/system_headers/linux_filter.h" |
| #include "sandbox/linux/system_headers/linux_seccomp.h" |
| |
| namespace sandbox { |
| namespace bpf_dsl { |
| |
| namespace { |
| |
| const uint64_t kLower32Bits = std::numeric_limits<uint32_t>::max(); |
| const uint64_t kUpper32Bits = static_cast<uint64_t>(kLower32Bits) << 32; |
| |
| struct State { |
| State(const std::vector<struct sock_filter>& p, |
| const struct arch_seccomp_data& d) |
| : program(p), data(d), ip(0), accumulator(0), acc_is_valid(false) {} |
| const std::vector<struct sock_filter>& program; |
| const struct arch_seccomp_data& data; |
| unsigned int ip; |
| uint32_t accumulator; |
| bool acc_is_valid; |
| |
| private: |
| DISALLOW_IMPLICIT_CONSTRUCTORS(State); |
| }; |
| |
| uint32_t EvaluateErrorCode(bpf_dsl::PolicyCompiler* compiler, |
| const ErrorCode& code, |
| const struct arch_seccomp_data& data) { |
| if (code.error_type() == ErrorCode::ET_SIMPLE || |
| code.error_type() == ErrorCode::ET_TRAP) { |
| return code.err(); |
| } else if (code.error_type() == ErrorCode::ET_COND) { |
| if (code.width() == ErrorCode::TP_32BIT && |
| (data.args[code.argno()] >> 32) && |
| (data.args[code.argno()] & 0xFFFFFFFF80000000ull) != |
| 0xFFFFFFFF80000000ull) { |
| return compiler->Unexpected64bitArgument().err(); |
| } |
| bool equal = (data.args[code.argno()] & code.mask()) == code.value(); |
| return EvaluateErrorCode(compiler, equal ? *code.passed() : *code.failed(), |
| data); |
| } else { |
| return SECCOMP_RET_INVALID; |
| } |
| } |
| |
| bool VerifyErrorCode(bpf_dsl::PolicyCompiler* compiler, |
| const std::vector<struct sock_filter>& program, |
| struct arch_seccomp_data* data, |
| const ErrorCode& root_code, |
| const ErrorCode& code, |
| const char** err) { |
| if (code.error_type() == ErrorCode::ET_SIMPLE || |
| code.error_type() == ErrorCode::ET_TRAP) { |
| const uint32_t computed_ret = Verifier::EvaluateBPF(program, *data, err); |
| if (*err) { |
| return false; |
| } |
| const uint32_t policy_ret = EvaluateErrorCode(compiler, root_code, *data); |
| if (computed_ret != policy_ret) { |
| // For efficiency's sake, we'd much rather compare "computed_ret" |
| // against "code.err()". This works most of the time, but it doesn't |
| // always work for nested conditional expressions. The test values |
| // that we generate on the fly to probe expressions can trigger |
| // code flow decisions in multiple nodes of the decision tree, and the |
| // only way to compute the correct error code in that situation is by |
| // calling EvaluateErrorCode(). |
| *err = "Exit code from BPF program doesn't match"; |
| return false; |
| } |
| } else if (code.error_type() == ErrorCode::ET_COND) { |
| if (code.argno() < 0 || code.argno() >= 6) { |
| *err = "Invalid argument number in error code"; |
| return false; |
| } |
| |
| // TODO(mdempsky): The test values generated here try to provide good |
| // coverage for generated BPF instructions while avoiding combinatorial |
| // explosion on large policies. Ideally we would instead take a fuzzing-like |
| // approach and generate a bounded number of test cases regardless of policy |
| // size. |
| |
| // Verify that we can check a value for simple equality. |
| data->args[code.argno()] = code.value(); |
| if (!VerifyErrorCode(compiler, program, data, root_code, *code.passed(), |
| err)) { |
| return false; |
| } |
| |
| // If mask ignores any bits, verify that setting those bits is still |
| // detected as equality. |
| uint64_t ignored_bits = ~code.mask(); |
| if (code.width() == ErrorCode::TP_32BIT) { |
| ignored_bits = static_cast<uint32_t>(ignored_bits); |
| } |
| if ((ignored_bits & kLower32Bits) != 0) { |
| data->args[code.argno()] = code.value() | (ignored_bits & kLower32Bits); |
| if (!VerifyErrorCode(compiler, program, data, root_code, *code.passed(), |
| err)) { |
| return false; |
| } |
| } |
| if ((ignored_bits & kUpper32Bits) != 0) { |
| data->args[code.argno()] = code.value() | (ignored_bits & kUpper32Bits); |
| if (!VerifyErrorCode(compiler, program, data, root_code, *code.passed(), |
| err)) { |
| return false; |
| } |
| } |
| |
| // Verify that changing bits included in the mask is detected as inequality. |
| if ((code.mask() & kLower32Bits) != 0) { |
| data->args[code.argno()] = code.value() ^ (code.mask() & kLower32Bits); |
| if (!VerifyErrorCode(compiler, program, data, root_code, *code.failed(), |
| err)) { |
| return false; |
| } |
| } |
| if ((code.mask() & kUpper32Bits) != 0) { |
| data->args[code.argno()] = code.value() ^ (code.mask() & kUpper32Bits); |
| if (!VerifyErrorCode(compiler, program, data, root_code, *code.failed(), |
| err)) { |
| return false; |
| } |
| } |
| |
| if (code.width() == ErrorCode::TP_32BIT) { |
| // For 32-bit system call arguments, we emit additional instructions to |
| // validate the upper 32-bits. Here we test that validation. |
| |
| // Arbitrary 64-bit values should be rejected. |
| data->args[code.argno()] = 1ULL << 32; |
| if (!VerifyErrorCode(compiler, program, data, root_code, |
| compiler->Unexpected64bitArgument(), err)) { |
| return false; |
| } |
| |
| // Upper 32-bits set without the MSB of the lower 32-bits set should be |
| // rejected too. |
| data->args[code.argno()] = kUpper32Bits; |
| if (!VerifyErrorCode(compiler, program, data, root_code, |
| compiler->Unexpected64bitArgument(), err)) { |
| return false; |
| } |
| } |
| } else { |
| *err = "Attempting to return invalid error code from BPF program"; |
| return false; |
| } |
| return true; |
| } |
| |
| void Ld(State* state, const struct sock_filter& insn, const char** err) { |
| if (BPF_SIZE(insn.code) != BPF_W || BPF_MODE(insn.code) != BPF_ABS || |
| insn.jt != 0 || insn.jf != 0) { |
| *err = "Invalid BPF_LD instruction"; |
| return; |
| } |
| if (insn.k < sizeof(struct arch_seccomp_data) && (insn.k & 3) == 0) { |
| // We only allow loading of properly aligned 32bit quantities. |
| memcpy(&state->accumulator, |
| reinterpret_cast<const char*>(&state->data) + insn.k, 4); |
| } else { |
| *err = "Invalid operand in BPF_LD instruction"; |
| return; |
| } |
| state->acc_is_valid = true; |
| return; |
| } |
| |
| void Jmp(State* state, const struct sock_filter& insn, const char** err) { |
| if (BPF_OP(insn.code) == BPF_JA) { |
| if (state->ip + insn.k + 1 >= state->program.size() || |
| state->ip + insn.k + 1 <= state->ip) { |
| compilation_failure: |
| *err = "Invalid BPF_JMP instruction"; |
| return; |
| } |
| state->ip += insn.k; |
| } else { |
| if (BPF_SRC(insn.code) != BPF_K || !state->acc_is_valid || |
| state->ip + insn.jt + 1 >= state->program.size() || |
| state->ip + insn.jf + 1 >= state->program.size()) { |
| goto compilation_failure; |
| } |
| switch (BPF_OP(insn.code)) { |
| case BPF_JEQ: |
| if (state->accumulator == insn.k) { |
| state->ip += insn.jt; |
| } else { |
| state->ip += insn.jf; |
| } |
| break; |
| case BPF_JGT: |
| if (state->accumulator > insn.k) { |
| state->ip += insn.jt; |
| } else { |
| state->ip += insn.jf; |
| } |
| break; |
| case BPF_JGE: |
| if (state->accumulator >= insn.k) { |
| state->ip += insn.jt; |
| } else { |
| state->ip += insn.jf; |
| } |
| break; |
| case BPF_JSET: |
| if (state->accumulator & insn.k) { |
| state->ip += insn.jt; |
| } else { |
| state->ip += insn.jf; |
| } |
| break; |
| default: |
| goto compilation_failure; |
| } |
| } |
| } |
| |
| uint32_t Ret(State*, const struct sock_filter& insn, const char** err) { |
| if (BPF_SRC(insn.code) != BPF_K) { |
| *err = "Invalid BPF_RET instruction"; |
| return 0; |
| } |
| return insn.k; |
| } |
| |
| void Alu(State* state, const struct sock_filter& insn, const char** err) { |
| if (BPF_OP(insn.code) == BPF_NEG) { |
| state->accumulator = -state->accumulator; |
| return; |
| } else { |
| if (BPF_SRC(insn.code) != BPF_K) { |
| *err = "Unexpected source operand in arithmetic operation"; |
| return; |
| } |
| switch (BPF_OP(insn.code)) { |
| case BPF_ADD: |
| state->accumulator += insn.k; |
| break; |
| case BPF_SUB: |
| state->accumulator -= insn.k; |
| break; |
| case BPF_MUL: |
| state->accumulator *= insn.k; |
| break; |
| case BPF_DIV: |
| if (!insn.k) { |
| *err = "Illegal division by zero"; |
| break; |
| } |
| state->accumulator /= insn.k; |
| break; |
| case BPF_MOD: |
| if (!insn.k) { |
| *err = "Illegal division by zero"; |
| break; |
| } |
| state->accumulator %= insn.k; |
| break; |
| case BPF_OR: |
| state->accumulator |= insn.k; |
| break; |
| case BPF_XOR: |
| state->accumulator ^= insn.k; |
| break; |
| case BPF_AND: |
| state->accumulator &= insn.k; |
| break; |
| case BPF_LSH: |
| if (insn.k > 32) { |
| *err = "Illegal shift operation"; |
| break; |
| } |
| state->accumulator <<= insn.k; |
| break; |
| case BPF_RSH: |
| if (insn.k > 32) { |
| *err = "Illegal shift operation"; |
| break; |
| } |
| state->accumulator >>= insn.k; |
| break; |
| default: |
| *err = "Invalid operator in arithmetic operation"; |
| break; |
| } |
| } |
| } |
| |
| } // namespace |
| |
| bool Verifier::VerifyBPF(bpf_dsl::PolicyCompiler* compiler, |
| const std::vector<struct sock_filter>& program, |
| const bpf_dsl::Policy& policy, |
| const char** err) { |
| *err = NULL; |
| for (uint32_t sysnum : SyscallSet::All()) { |
| // We ideally want to iterate over the full system call range and values |
| // just above and just below this range. This gives us the full result set |
| // of the "evaluators". |
| // On Intel systems, this can fail in a surprising way, as a cleared bit 30 |
| // indicates either i386 or x86-64; and a set bit 30 indicates x32. And |
| // unless we pay attention to setting this bit correctly, an early check in |
| // our BPF program will make us fail with a misleading error code. |
| struct arch_seccomp_data data = {static_cast<int>(sysnum), |
| static_cast<uint32_t>(SECCOMP_ARCH)}; |
| #if defined(__i386__) || defined(__x86_64__) |
| #if defined(__x86_64__) && defined(__ILP32__) |
| if (!(sysnum & 0x40000000u)) { |
| continue; |
| } |
| #else |
| if (sysnum & 0x40000000u) { |
| continue; |
| } |
| #endif |
| #endif |
| ErrorCode code = SyscallSet::IsValid(sysnum) |
| ? policy.EvaluateSyscall(sysnum)->Compile(compiler) |
| : policy.InvalidSyscall()->Compile(compiler); |
| if (!VerifyErrorCode(compiler, program, &data, code, code, err)) { |
| return false; |
| } |
| } |
| return true; |
| } |
| |
| uint32_t Verifier::EvaluateBPF(const std::vector<struct sock_filter>& program, |
| const struct arch_seccomp_data& data, |
| const char** err) { |
| *err = NULL; |
| if (program.size() < 1 || program.size() >= SECCOMP_MAX_PROGRAM_SIZE) { |
| *err = "Invalid program length"; |
| return 0; |
| } |
| for (State state(program, data); !*err; ++state.ip) { |
| if (state.ip >= program.size()) { |
| *err = "Invalid instruction pointer in BPF program"; |
| break; |
| } |
| const struct sock_filter& insn = program[state.ip]; |
| switch (BPF_CLASS(insn.code)) { |
| case BPF_LD: |
| Ld(&state, insn, err); |
| break; |
| case BPF_JMP: |
| Jmp(&state, insn, err); |
| break; |
| case BPF_RET: { |
| uint32_t r = Ret(&state, insn, err); |
| switch (r & SECCOMP_RET_ACTION) { |
| case SECCOMP_RET_TRAP: |
| case SECCOMP_RET_ERRNO: |
| case SECCOMP_RET_TRACE: |
| case SECCOMP_RET_ALLOW: |
| break; |
| case SECCOMP_RET_KILL: // We don't ever generate this |
| case SECCOMP_RET_INVALID: // Should never show up in BPF program |
| default: |
| *err = "Unexpected return code found in BPF program"; |
| return 0; |
| } |
| return r; |
| } |
| case BPF_ALU: |
| Alu(&state, insn, err); |
| break; |
| default: |
| *err = "Unexpected instruction in BPF program"; |
| break; |
| } |
| } |
| return 0; |
| } |
| |
| } // namespace bpf_dsl |
| } // namespace sandbox |
