sandbox/linux/seccomp-bpf/trap.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_TRAP_H__
   6 #define SANDBOX_LINUX_SECCOMP_BPF_TRAP_H__
   7
   8 #include <signal.h>
   9 #include <stdint.h>
  10
  11 #include <map>
  12 #include <vector>
  13
  14 #include "base/basictypes.h"
  15 #include "sandbox/linux/sandbox_export.h"
  16
  17 namespace sandbox {
  18
  19 class ErrorCode;
  20
  21 // The Trap class allows a BPF filter program to branch out to user space by
  22 // raising a SIGSYS signal.
  23 // N.B.: This class does not perform any synchronization operations. If
  24 //   modifications are made to any of the traps, it is the caller's
  25 //   responsibility to ensure that this happens in a thread-safe fashion.
  26 //   Preferably, that means that no other threads should be running at that
  27 //   time. For the purposes of our sandbox, this assertion should always be
  28 //   true. Threads are incompatible with the seccomp sandbox anyway.
  29 class SANDBOX_EXPORT Trap {
  30  public:
  31   // TrapFnc is a pointer to a function that handles Seccomp traps in
  32   // user-space. The seccomp policy can request that a trap handler gets
  33   // installed; it does so by returning a suitable ErrorCode() from the
  34   // syscallEvaluator. See the ErrorCode() constructor for how to pass in
  35   // the function pointer.
  36   // Please note that TrapFnc is executed from signal context and must be
  37   // async-signal safe:
  38   // http://pubs.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_04.html
  39   // Also note that it follows the calling convention of native system calls.
  40   // In other words, it reports an error by returning an exit code in the
  41   // range -1..-4096. It should not set errno when reporting errors; on the
  42   // other hand, accidentally modifying errno is harmless and the changes will
  43   // be undone afterwards.
  44   typedef intptr_t (*TrapFnc)(const struct arch_seccomp_data& args, void* aux);
  45
  46   // Registers a new trap handler and sets up the appropriate SIGSYS handler
  47   // as needed.
  48   // N.B.: This makes a permanent state change. Traps cannot be unregistered,
  49   //   as that would break existing BPF filters that are still active.
  50   static ErrorCode MakeTrap(TrapFnc fnc, const void* aux, bool safe);
  51
  52   // Enables support for unsafe traps in the SIGSYS signal handler. This is a
  53   // one-way fuse. It works in conjunction with the BPF compiler emitting code
  54   // that unconditionally allows system calls, if they have a magic return
  55   // address (i.e. SandboxSyscall(-1)).
  56   // Once unsafe traps are enabled, the sandbox is essentially compromised.
  57   // But this is still a very useful feature for debugging purposes. Use with
  58   // care. This feature is availably only if enabled by the user (see above).
  59   // Returns "true", if unsafe traps were turned on.
  60   static bool EnableUnsafeTrapsInSigSysHandler();
  61
  62   // Returns the ErrorCode associate with a particular trap id.
  63   static ErrorCode ErrorCodeFromTrapId(uint16_t id);
  64
  65  private:
  66   // The destructor is unimplemented. Don't ever attempt to destruct this
  67   // object. It'll break subsequent system calls that trigger a SIGSYS.
  68   ~Trap();
  69
  70   struct TrapKey {
  71     TrapKey(TrapFnc f, const void* a, bool s) : fnc(f), aux(a), safe(s) {}
  72     TrapFnc fnc;
  73     const void* aux;
  74     bool safe;
  75     bool operator<(const TrapKey&) const;
  76   };
  77   typedef std::map<TrapKey, uint16_t> TrapIds;
  78
  79   // We only have a very small number of methods. We opt to make them static
  80   // and have them internally call GetInstance(). This is a little more
  81   // convenient than having each caller obtain short-lived reference to the
  82   // singleton.
  83   // It also gracefully deals with methods that should check for the singleton,
  84   // but avoid instantiating it, if it doesn't exist yet
  85   // (e.g. ErrorCodeFromTrapId()).
  86   static Trap* GetInstance();
  87   static void SigSysAction(int nr, siginfo_t* info, void* void_context);
  88
  89   // Make sure that SigSys is not inlined in order to get slightly better crash
  90   // dumps.
  91   void SigSys(int nr, siginfo_t* info, void* void_context)
  92       __attribute__((noinline));
  93   ErrorCode MakeTrapImpl(TrapFnc fnc, const void* aux, bool safe);
  94   bool SandboxDebuggingAllowedByUser() const;
  95
  96   // We have a global singleton that handles all of our SIGSYS traps. This
  97   // variable must never be deallocated after it has been set up initially, as
  98   // there is no way to reset in-kernel BPF filters that generate SIGSYS
  99   // events.
 100   static Trap* global_trap_;
 101
 102   TrapIds trap_ids_;            // Maps from TrapKeys to numeric ids
 103   ErrorCode* trap_array_;       // Array of ErrorCodes indexed by ids
 104   size_t trap_array_size_;      // Currently used size of array
 105   size_t trap_array_capacity_;  // Currently allocated capacity of array
 106   bool has_unsafe_traps_;       // Whether unsafe traps have been enabled
 107
 108   // Our constructor is private. A shared global instance is created
 109   // automatically as needed.
 110   // Copying and assigning is unimplemented. It doesn't make sense for a
 111   // singleton.
 112   DISALLOW_IMPLICIT_CONSTRUCTORS(Trap);
 113 };
 114
 115 }  // namespace sandbox
 116
 117 #endif  // SANDBOX_LINUX_SECCOMP_BPF_TRAP_H__