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/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 struct TrapKey { TrapKeyTrapKey67 TrapKey(TrapFnc f, const void* a, bool s) : fnc(f), aux(a), safe(s) {} 68 TrapFnc fnc; 69 const void* aux; 70 bool safe; 71 bool operator<(const TrapKey&) const; 72 }; 73 typedef std::map<TrapKey, uint16_t> TrapIds; 74 75 // Our constructor is private. A shared global instance is created 76 // automatically as needed. 77 Trap(); 78 79 // The destructor is unimplemented. Don't ever attempt to destruct this 80 // object. It'll break subsequent system calls that trigger a SIGSYS. 81 ~Trap(); 82 83 // We only have a very small number of methods. We opt to make them static 84 // and have them internally call GetInstance(). This is a little more 85 // convenient than having each caller obtain short-lived reference to the 86 // singleton. 87 // It also gracefully deals with methods that should check for the singleton, 88 // but avoid instantiating it, if it doesn't exist yet 89 // (e.g. ErrorCodeFromTrapId()). 90 static Trap* GetInstance(); 91 static void SigSysAction(int nr, siginfo_t* info, void* void_context); 92 93 // Make sure that SigSys is not inlined in order to get slightly better crash 94 // dumps. 95 void SigSys(int nr, siginfo_t* info, void* void_context) 96 __attribute__((noinline)); 97 ErrorCode MakeTrapImpl(TrapFnc fnc, const void* aux, bool safe); 98 bool SandboxDebuggingAllowedByUser() const; 99 100 // We have a global singleton that handles all of our SIGSYS traps. This 101 // variable must never be deallocated after it has been set up initially, as 102 // there is no way to reset in-kernel BPF filters that generate SIGSYS 103 // events. 104 static Trap* global_trap_; 105 106 TrapIds trap_ids_; // Maps from TrapKeys to numeric ids 107 ErrorCode* trap_array_; // Array of ErrorCodes indexed by ids 108 size_t trap_array_size_; // Currently used size of array 109 size_t trap_array_capacity_; // Currently allocated capacity of array 110 bool has_unsafe_traps_; // Whether unsafe traps have been enabled 111 112 // Copying and assigning is unimplemented. It doesn't make sense for a 113 // singleton. 114 DISALLOW_COPY_AND_ASSIGN(Trap); 115 }; 116 117 } // namespace sandbox 118 119 #endif // SANDBOX_LINUX_SECCOMP_BPF_TRAP_H__ 120