// 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.
#ifndef SANDBOX_LINUX_SECCOMP_BPF_TRAP_H__
#define SANDBOX_LINUX_SECCOMP_BPF_TRAP_H__
#include <signal.h>
#include <stdint.h>
#include <map>
#include <vector>
#include "base/basictypes.h"
#include "sandbox/linux/sandbox_export.h"
namespace sandbox {
class ErrorCode;
// The Trap class allows a BPF filter program to branch out to user space by
// raising a SIGSYS signal.
// N.B.: This class does not perform any synchronization operations. If
// modifications are made to any of the traps, it is the caller's
// responsibility to ensure that this happens in a thread-safe fashion.
// Preferably, that means that no other threads should be running at that
// time. For the purposes of our sandbox, this assertion should always be
// true. Threads are incompatible with the seccomp sandbox anyway.
class SANDBOX_EXPORT Trap {
public:
// TrapFnc is a pointer to a function that handles Seccomp traps in
// user-space. The seccomp policy can request that a trap handler gets
// installed; it does so by returning a suitable ErrorCode() from the
// syscallEvaluator. See the ErrorCode() constructor for how to pass in
// the function pointer.
// Please note that TrapFnc is executed from signal context and must be
// async-signal safe:
// http://pubs.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_04.html
// Also note that it follows the calling convention of native system calls.
// In other words, it reports an error by returning an exit code in the
// range -1..-4096. It should not set errno when reporting errors; on the
// other hand, accidentally modifying errno is harmless and the changes will
// be undone afterwards.
typedef intptr_t (*TrapFnc)(const struct arch_seccomp_data& args, void* aux);
// Registers a new trap handler and sets up the appropriate SIGSYS handler
// as needed.
// N.B.: This makes a permanent state change. Traps cannot be unregistered,
// as that would break existing BPF filters that are still active.
static ErrorCode MakeTrap(TrapFnc fnc, const void* aux, bool safe);
// Enables support for unsafe traps in the SIGSYS signal handler. This is a
// one-way fuse. It works in conjunction with the BPF compiler emitting code
// that unconditionally allows system calls, if they have a magic return
// address (i.e. SandboxSyscall(-1)).
// Once unsafe traps are enabled, the sandbox is essentially compromised.
// But this is still a very useful feature for debugging purposes. Use with
// care. This feature is availably only if enabled by the user (see above).
// Returns "true", if unsafe traps were turned on.
static bool EnableUnsafeTrapsInSigSysHandler();
// Returns the ErrorCode associate with a particular trap id.
static ErrorCode ErrorCodeFromTrapId(uint16_t id);
private:
// The destructor is unimplemented. Don't ever attempt to destruct this
// object. It'll break subsequent system calls that trigger a SIGSYS.
~Trap();
struct TrapKey {
TrapKey(TrapFnc f, const void* a, bool s) : fnc(f), aux(a), safe(s) {}
TrapFnc fnc;
const void* aux;
bool safe;
bool operator<(const TrapKey&) const;
};
typedef std::map<TrapKey, uint16_t> TrapIds;
// We only have a very small number of methods. We opt to make them static
// and have them internally call GetInstance(). This is a little more
// convenient than having each caller obtain short-lived reference to the
// singleton.
// It also gracefully deals with methods that should check for the singleton,
// but avoid instantiating it, if it doesn't exist yet
// (e.g. ErrorCodeFromTrapId()).
static Trap* GetInstance();
static void SigSysAction(int nr, siginfo_t* info, void* void_context);
// Make sure that SigSys is not inlined in order to get slightly better crash
// dumps.
void SigSys(int nr, siginfo_t* info, void* void_context)
__attribute__((noinline));
ErrorCode MakeTrapImpl(TrapFnc fnc, const void* aux, bool safe);
bool SandboxDebuggingAllowedByUser() const;
// We have a global singleton that handles all of our SIGSYS traps. This
// variable must never be deallocated after it has been set up initially, as
// there is no way to reset in-kernel BPF filters that generate SIGSYS
// events.
static Trap* global_trap_;
TrapIds trap_ids_; // Maps from TrapKeys to numeric ids
ErrorCode* trap_array_; // Array of ErrorCodes indexed by ids
size_t trap_array_size_; // Currently used size of array
size_t trap_array_capacity_; // Currently allocated capacity of array
bool has_unsafe_traps_; // Whether unsafe traps have been enabled
// Our constructor is private. A shared global instance is created
// automatically as needed.
// Copying and assigning is unimplemented. It doesn't make sense for a
// singleton.
DISALLOW_IMPLICIT_CONSTRUCTORS(Trap);
};
} // namespace sandbox
#endif // SANDBOX_LINUX_SECCOMP_BPF_TRAP_H__