sandbox/linux/seccomp-bpf/errorcode.h

   1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #ifndef SANDBOX_LINUX_SECCOMP_BPF_ERRORCODE_H__
   6 #define SANDBOX_LINUX_SECCOMP_BPF_ERRORCODE_H__
   7
   8 #include "sandbox/linux/seccomp-bpf/linux_seccomp.h"
   9 #include "sandbox/linux/seccomp-bpf/trap.h"
  10 #include "sandbox/sandbox_export.h"
  11
  12 namespace sandbox {
  13
  14 struct arch_seccomp_data;
  15
  16 // This class holds all the possible values that can be returned by a sandbox
  17 // policy.
  18 // We can either wrap a symbolic ErrorCode (i.e. ERR_XXX enum values), an
  19 // errno value (in the range 0..4095), a pointer to a TrapFnc callback
  20 // handling a SECCOMP_RET_TRAP trap, or a complex constraint.
  21 // All of the commonly used values are stored in the "err_" field. So, code
  22 // that is using the ErrorCode class typically operates on a single 32bit
  23 // field.
  24 class SANDBOX_EXPORT ErrorCode {
  25  public:
  26   enum {
  27     // Allow this system call. The value of ERR_ALLOWED is pretty much
  28     // completely arbitrary. But we want to pick it so that is is unlikely
  29     // to be passed in accidentally, when the user intended to return an
  30     // "errno" (see below) value instead.
  31     ERR_ALLOWED = 0x04000000,
  32
  33     // If the progress is being ptraced with PTRACE_O_TRACESECCOMP, then the
  34     // tracer will be notified of a PTRACE_EVENT_SECCOMP and allowed to change
  35     // or skip the system call.  The lower 16 bits of err will be available to
  36     // the tracer via PTRACE_GETEVENTMSG.
  37     ERR_TRACE   = 0x08000000,
  38
  39     // Deny the system call with a particular "errno" value.
  40     // N.B.: It is also possible to return "0" here. That would normally
  41     //       indicate success, but it won't actually run the system call.
  42     //       This is very different from return ERR_ALLOWED.
  43     ERR_MIN_ERRNO = 0,
  44     // TODO(markus): Android only supports errno up to 255
  45     // (crbug.com/181647).
  46     ERR_MAX_ERRNO = 4095,
  47   };
  48
  49   // While BPF filter programs always operate on 32bit quantities, the kernel
  50   // always sees system call arguments as 64bit values. This statement is true
  51   // no matter whether the host system is natively operating in 32bit or 64bit.
  52   // The BPF compiler hides the fact that BPF instructions cannot directly
  53   // access 64bit quantities. But policies are still advised to specify whether
  54   // a system call expects a 32bit or a 64bit quantity.
  55   enum ArgType {
  56     // When passed as an argument to SandboxBPF::Cond(), TP_32BIT requests that
  57     // the conditional test should operate on the 32bit part of the system call
  58     // argument.
  59     // On 64bit architectures, this verifies that user space did not pass
  60     // a 64bit value as an argument to the system call. If it did, that will be
  61     // interpreted as an attempt at breaking the sandbox and results in the
  62     // program getting terminated.
  63     // In other words, only perform a 32bit test, if you are sure this
  64     // particular system call would never legitimately take a 64bit
  65     // argument.
  66     // Implementation detail: TP_32BIT does two things. 1) it restricts the
  67     // conditional test to operating on the LSB only, and 2) it adds code to
  68     // the BPF filter program verifying that the MSB  the kernel received from
  69     // user space is either 0, or 0xFFFFFFFF; the latter is acceptable, iff bit
  70     // 31 was set in the system call argument. It deals with 32bit arguments
  71     // having been sign extended.
  72     TP_32BIT,
  73
  74     // When passed as an argument to SandboxBPF::Cond(), TP_64BIT requests that
  75     // the conditional test should operate on the full 64bit argument. It is
  76     // generally harmless to perform a 64bit test on 32bit systems, as the
  77     // kernel will always see the top 32 bits of all arguments as zero'd out.
  78     // This approach has the desirable property that for tests of pointer
  79     // values, we can always use TP_64BIT no matter the host architecture.
  80     // But of course, that also means, it is possible to write conditional
  81     // policies that turn into no-ops on 32bit systems; this is by design.
  82     TP_64BIT,
  83   };
  84
  85   enum Operation {
  86     // Test whether the system call argument is equal to the operand.
  87     OP_EQUAL,
  88
  89     // Test whether the system call argument is greater (or equal) to the
  90     // operand. Please note that all tests always operate on unsigned
  91     // values. You can generally emulate signed tests, if that's what you
  92     // need.
  93     // TODO(markus): Check whether we should automatically emulate signed
  94     //               operations.
  95     OP_GREATER_UNSIGNED,
  96     OP_GREATER_EQUAL_UNSIGNED,
  97
  98     // Tests a system call argument against a bit mask.
  99     // The "ALL_BITS" variant performs this test: "arg & mask == mask"
 100     // This implies that a mask of zero always results in a passing test.
 101     // The "ANY_BITS" variant performs this test: "arg & mask != 0"
 102     // This implies that a mask of zero always results in a failing test.
 103     OP_HAS_ALL_BITS,
 104     OP_HAS_ANY_BITS,
 105
 106     // Total number of operations.
 107     OP_NUM_OPS,
 108   };
 109
 110   enum ErrorType {
 111     ET_INVALID,
 112     ET_SIMPLE,
 113     ET_TRAP,
 114     ET_COND,
 115   };
 116
 117   // We allow the default constructor, as it makes the ErrorCode class
 118   // much easier to use. But if we ever encounter an invalid ErrorCode
 119   // when compiling a BPF filter, we deliberately generate an invalid
 120   // program that will get flagged both by our Verifier class and by
 121   // the Linux kernel.
 122   ErrorCode() : error_type_(ET_INVALID), err_(SECCOMP_RET_INVALID) {}
 123   explicit ErrorCode(int err);
 124
 125   // For all practical purposes, ErrorCodes are treated as if they were
 126   // structs. The copy constructor and assignment operator are trivial and
 127   // we do not need to explicitly specify them.
 128   // Most notably, it is in fact perfectly OK to directly copy the passed_ and
 129   // failed_ field. They only ever get set by our private constructor, and the
 130   // callers handle life-cycle management for these objects.
 131
 132   // Destructor
 133   ~ErrorCode() {}
 134
 135   bool Equals(const ErrorCode& err) const;
 136   bool LessThan(const ErrorCode& err) const;
 137
 138   uint32_t err() const { return err_; }
 139   ErrorType error_type() const { return error_type_; }
 140
 141   bool safe() const { return safe_; }
 142
 143   uint64_t value() const { return value_; }
 144   int argno() const { return argno_; }
 145   ArgType width() const { return width_; }
 146   Operation op() const { return op_; }
 147   const ErrorCode* passed() const { return passed_; }
 148   const ErrorCode* failed() const { return failed_; }
 149
 150   struct LessThan {
 151     bool operator()(const ErrorCode& a, const ErrorCode& b) const {
 152       return a.LessThan(b);
 153     }
 154   };
 155
 156  private:
 157   friend class CodeGen;
 158   friend class SandboxBPF;
 159   friend class Trap;
 160
 161   // If we are wrapping a callback, we must assign a unique id. This id is
 162   // how the kernel tells us which one of our different SECCOMP_RET_TRAP
 163   // cases has been triggered.
 164   ErrorCode(Trap::TrapFnc fnc, const void* aux, bool safe, uint16_t id);
 165
 166   // Some system calls require inspection of arguments. This constructor
 167   // allows us to specify additional constraints.
 168   ErrorCode(int argno,
 169             ArgType width,
 170             Operation op,
 171             uint64_t value,
 172             const ErrorCode* passed,
 173             const ErrorCode* failed);
 174
 175   ErrorType error_type_;
 176
 177   union {
 178     // Fields needed for SECCOMP_RET_TRAP callbacks
 179     struct {
 180       Trap::TrapFnc fnc_;  // Callback function and arg, if trap was
 181       void* aux_;          //   triggered by the kernel's BPF filter.
 182       bool safe_;          // Keep sandbox active while calling fnc_()
 183     };
 184
 185     // Fields needed when inspecting additional arguments.
 186     struct {
 187       uint64_t value_;           // Value that we are comparing with.
 188       int argno_;                // Syscall arg number that we are inspecting.
 189       ArgType width_;            // Whether we are looking at a 32/64bit value.
 190       Operation op_;             // Comparison operation.
 191       const ErrorCode* passed_;  // Value to be returned if comparison passed,
 192       const ErrorCode* failed_;  //   or if it failed.
 193     };
 194   };
 195
 196   // 32bit field used for all possible types of ErrorCode values. This is
 197   // the value that uniquely identifies any ErrorCode and it (typically) can
 198   // be emitted directly into a BPF filter program.
 199   uint32_t err_;
 200 };
 201
 202 }  // namespace sandbox
 203
 204 #endif  // SANDBOX_LINUX_SECCOMP_BPF_ERRORCODE_H__