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.
5 #ifndef SANDBOX_LINUX_SECCOMP_BPF_TRAP_H__
6 #define SANDBOX_LINUX_SECCOMP_BPF_TRAP_H__
13 #include "base/macros.h"
14 #include "sandbox/sandbox_export.h"
18 // This must match the kernel's seccomp_data structure.
19 struct arch_seccomp_data {
22 uint64_t instruction_pointer;
26 // The Trap class allows a BPF filter program to branch out to user space by
27 // raising a SIGSYS signal.
28 // N.B.: This class does not perform any synchronization operations. If
29 // modifications are made to any of the traps, it is the caller's
30 // responsibility to ensure that this happens in a thread-safe fashion.
31 // Preferably, that means that no other threads should be running at that
32 // time. For the purposes of our sandbox, this assertion should always be
33 // true. Threads are incompatible with the seccomp sandbox anyway.
34 class SANDBOX_EXPORT Trap {
36 // TrapFnc is a pointer to a function that handles Seccomp traps in
37 // user-space. The seccomp policy can request that a trap handler gets
38 // installed; it does so by returning a suitable ErrorCode() from the
39 // syscallEvaluator. See the ErrorCode() constructor for how to pass in
40 // the function pointer.
41 // Please note that TrapFnc is executed from signal context and must be
43 // http://pubs.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_04.html
44 // Also note that it follows the calling convention of native system calls.
45 // In other words, it reports an error by returning an exit code in the
46 // range -1..-4096. It should not set errno when reporting errors; on the
47 // other hand, accidentally modifying errno is harmless and the changes will
48 // be undone afterwards.
49 typedef intptr_t (*TrapFnc)(const struct arch_seccomp_data& args, void* aux);
51 // Registers a new trap handler and sets up the appropriate SIGSYS handler
53 // N.B.: This makes a permanent state change. Traps cannot be unregistered,
54 // as that would break existing BPF filters that are still active.
55 static uint16_t MakeTrap(TrapFnc fnc, const void* aux, bool safe);
57 // Enables support for unsafe traps in the SIGSYS signal handler. This is a
58 // one-way fuse. It works in conjunction with the BPF compiler emitting code
59 // that unconditionally allows system calls, if they have a magic return
60 // address (i.e. SandboxSyscall(-1)).
61 // Once unsafe traps are enabled, the sandbox is essentially compromised.
62 // But this is still a very useful feature for debugging purposes. Use with
63 // care. This feature is availably only if enabled by the user (see above).
64 // Returns "true", if unsafe traps were turned on.
65 static bool EnableUnsafeTrapsInSigSysHandler();
67 // Returns true if a safe trap handler is associated with a
68 // particular trap ID.
69 static bool IsSafeTrapId(uint16_t id);
73 TrapKey() : fnc(NULL), aux(NULL), safe(false) {}
74 TrapKey(TrapFnc f, const void* a, bool s) : fnc(f), aux(a), safe(s) {}
78 bool operator<(const TrapKey&) const;
80 typedef std::map<TrapKey, uint16_t> TrapIds;
82 // Our constructor is private. A shared global instance is created
83 // automatically as needed.
86 // The destructor is unimplemented. Don't ever attempt to destruct this
87 // object. It'll break subsequent system calls that trigger a SIGSYS.
90 // We only have a very small number of methods. We opt to make them static
91 // and have them internally call GetInstance(). This is a little more
92 // convenient than having each caller obtain short-lived reference to the
94 // It also gracefully deals with methods that should check for the singleton,
95 // but avoid instantiating it, if it doesn't exist yet
96 // (e.g. ErrorCodeFromTrapId()).
97 static Trap* GetInstance();
98 static void SigSysAction(int nr, siginfo_t* info, void* void_context);
100 // Make sure that SigSys is not inlined in order to get slightly better crash
102 void SigSys(int nr, siginfo_t* info, void* void_context)
103 __attribute__((noinline));
104 uint16_t MakeTrapImpl(TrapFnc fnc, const void* aux, bool safe);
105 bool SandboxDebuggingAllowedByUser() const;
107 // We have a global singleton that handles all of our SIGSYS traps. This
108 // variable must never be deallocated after it has been set up initially, as
109 // there is no way to reset in-kernel BPF filters that generate SIGSYS
111 static Trap* global_trap_;
113 TrapIds trap_ids_; // Maps from TrapKeys to numeric ids
114 TrapKey* trap_array_; // Array of TrapKeys indexed by ids
115 size_t trap_array_size_; // Currently used size of array
116 size_t trap_array_capacity_; // Currently allocated capacity of array
117 bool has_unsafe_traps_; // Whether unsafe traps have been enabled
119 // Copying and assigning is unimplemented. It doesn't make sense for a
121 DISALLOW_COPY_AND_ASSIGN(Trap);
124 } // namespace sandbox
126 #endif // SANDBOX_LINUX_SECCOMP_BPF_TRAP_H__