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_SANDBOX_BPF_H__
6 #define SANDBOX_LINUX_SECCOMP_BPF_SANDBOX_BPF_H__
19 #include "base/compiler_specific.h"
20 #include "base/memory/scoped_ptr.h"
21 #include "sandbox/linux/seccomp-bpf/die.h"
22 #include "sandbox/linux/seccomp-bpf/errorcode.h"
23 #include "sandbox/linux/seccomp-bpf/linux_seccomp.h"
24 #include "sandbox/sandbox_export.h"
28 // This must match the kernel's seccomp_data structure.
29 struct arch_seccomp_data {
32 uint64_t instruction_pointer;
43 class SandboxBPFPolicy;
44 class SandboxUnittestHelper;
47 class SANDBOX_EXPORT SandboxBPF {
50 STATUS_UNKNOWN, // Status prior to calling supportsSeccompSandbox()
51 STATUS_UNSUPPORTED, // The kernel does not appear to support sandboxing
52 STATUS_UNAVAILABLE, // Currently unavailable but might work again later
53 STATUS_AVAILABLE, // Sandboxing is available but not currently active
54 STATUS_ENABLED // The sandbox is now active
57 // Depending on the level of kernel support, seccomp-bpf may require the
58 // process to be single-threaded in order to enable it. When calling
59 // StartSandbox(), the program should indicate whether or not the sandbox
60 // should try and engage with multi-thread support.
61 enum SandboxThreadState {
63 PROCESS_SINGLE_THREADED, // The program is currently single-threaded.
64 // Note: PROCESS_MULTI_THREADED requires experimental kernel support that
65 // has not been contributed to upstream Linux.
66 PROCESS_MULTI_THREADED, // The program may be multi-threaded.
69 // A vector of BPF instructions that need to be installed as a filter
70 // program in the kernel.
71 typedef std::vector<struct sock_filter> Program;
73 // Constructors and destructors.
74 // NOTE: Setting a policy and starting the sandbox is a one-way operation.
75 // The kernel does not provide any option for unloading a loaded
76 // sandbox. Strictly speaking, that means we should disallow calling
77 // the destructor, if StartSandbox() has ever been called. In practice,
78 // this makes it needlessly complicated to operate on "Sandbox"
79 // objects. So, we instead opted to allow object destruction. But it
80 // should be noted that during its lifetime, the object probably made
81 // irreversible state changes to the runtime environment. These changes
82 // stay in effect even after the destructor has been run.
86 // Checks whether a particular system call number is valid on the current
87 // architecture. E.g. on ARM there's a non-contiguous range of private
89 static bool IsValidSyscallNumber(int sysnum);
91 // There are a lot of reasons why the Seccomp sandbox might not be available.
92 // This could be because the kernel does not support Seccomp mode, or it
93 // could be because another sandbox is already active.
94 // "proc_fd" should be a file descriptor for "/proc", or -1 if not
95 // provided by the caller.
96 static SandboxStatus SupportsSeccompSandbox(int proc_fd);
98 // The sandbox needs to be able to access files in "/proc/self". If this
99 // directory is not accessible when "startSandbox()" gets called, the caller
100 // can provide an already opened file descriptor by calling "set_proc_fd()".
101 // The sandbox becomes the new owner of this file descriptor and will
102 // eventually close it when "StartSandbox()" executes.
103 void set_proc_fd(int proc_fd);
105 // Set the BPF policy as |policy|. Ownership of |policy| is transfered here
106 // to the sandbox object.
107 void SetSandboxPolicy(SandboxBPFPolicy* policy);
109 // We can use ErrorCode to request calling of a trap handler. This method
110 // performs the required wrapping of the callback function into an
112 // The "aux" field can carry a pointer to arbitrary data. See EvaluateSyscall
113 // for a description of how to pass data from SetSandboxPolicy() to a Trap()
115 ErrorCode Trap(Trap::TrapFnc fnc, const void* aux);
117 // Calls a user-space trap handler and disables all sandboxing for system
118 // calls made from this trap handler.
119 // This feature is available only if explicitly enabled by the user having
120 // set the CHROME_SANDBOX_DEBUGGING environment variable.
121 // Returns an ET_INVALID ErrorCode, if called when not enabled.
122 // NOTE: This feature, by definition, disables all security features of
123 // the sandbox. It should never be used in production, but it can be
124 // very useful to diagnose code that is incompatible with the sandbox.
125 // If even a single system call returns "UnsafeTrap", the security of
126 // entire sandbox should be considered compromised.
127 ErrorCode UnsafeTrap(Trap::TrapFnc fnc, const void* aux);
129 // From within an UnsafeTrap() it is often useful to be able to execute
130 // the system call that triggered the trap. The ForwardSyscall() method
131 // makes this easy. It is more efficient than calling glibc's syscall()
132 // function, as it avoid the extra round-trip to the signal handler. And
133 // it automatically does the correct thing to report kernel-style error
134 // conditions, rather than setting errno. See the comments for TrapFnc for
135 // details. In other words, the return value from ForwardSyscall() is
136 // directly suitable as a return value for a trap handler.
137 static intptr_t ForwardSyscall(const struct arch_seccomp_data& args);
139 // We can also use ErrorCode to request evaluation of a conditional
140 // statement based on inspection of system call parameters.
141 // This method wrap an ErrorCode object around the conditional statement.
142 // Argument "argno" (1..6) will be compared to "value" using comparator
143 // "op". If the condition is true "passed" will be returned, otherwise
145 // If "is32bit" is set, the argument must in the range of 0x0..(1u << 32 - 1)
146 // If it is outside this range, the sandbox treats the system call just
147 // the same as any other ABI violation (i.e. it aborts with an error
149 ErrorCode Cond(int argno,
150 ErrorCode::ArgType is_32bit,
151 ErrorCode::Operation op,
153 const ErrorCode& passed,
154 const ErrorCode& failed);
156 // Kill the program and print an error message.
157 ErrorCode Kill(const char* msg);
159 // This is the main public entry point. It finds all system calls that
160 // need rewriting, sets up the resources needed by the sandbox, and
161 // enters Seccomp mode.
162 // The calling process must specify its current SandboxThreadState, as a way
163 // to tell the sandbox which type of kernel support it should engage.
164 // It is possible to stack multiple sandboxes by creating separate "Sandbox"
165 // objects and calling "StartSandbox()" on each of them. Please note, that
166 // this requires special care, though, as newly stacked sandboxes can never
167 // relax restrictions imposed by earlier sandboxes. Furthermore, installing
168 // a new policy requires making system calls, that might already be
170 // Finally, stacking does add more kernel overhead than having a single
171 // combined policy. So, it should only be used if there are no alternatives.
172 bool StartSandbox(SandboxThreadState thread_state) WARN_UNUSED_RESULT;
174 // Assembles a BPF filter program from the current policy. After calling this
175 // function, you must not call any other sandboxing function.
176 // Typically, AssembleFilter() is only used by unit tests and by sandbox
177 // internals. It should not be used by production code.
178 // For performance reasons, we normally only run the assembled BPF program
179 // through the verifier, iff the program was built in debug mode.
180 // But by setting "force_verification", the caller can request that the
181 // verifier is run unconditionally. This is useful for unittests.
182 Program* AssembleFilter(bool force_verification);
184 // Returns the fatal ErrorCode that is used to indicate that somebody
185 // attempted to pass a 64bit value in a 32bit system call argument.
186 // This method is primarily needed for testing purposes.
187 ErrorCode Unexpected64bitArgument();
190 friend class CodeGen;
191 friend class SandboxUnittestHelper;
192 friend class ErrorCode;
195 Range(uint32_t f, uint32_t t, const ErrorCode& e)
196 : from(f), to(t), err(e) {}
200 typedef std::vector<Range> Ranges;
201 typedef std::map<uint32_t, ErrorCode> ErrMap;
202 typedef std::set<ErrorCode, struct ErrorCode::LessThan> Conds;
204 // Get a file descriptor pointing to "/proc", if currently available.
205 int proc_fd() { return proc_fd_; }
207 // Creates a subprocess and runs "code_in_sandbox" inside of the specified
208 // policy. The caller has to make sure that "this" has not yet been
209 // initialized with any other policies.
210 bool RunFunctionInPolicy(void (*code_in_sandbox)(),
211 scoped_ptr<SandboxBPFPolicy> policy);
213 // Performs a couple of sanity checks to verify that the kernel supports the
214 // features that we need for successful sandboxing.
215 // The caller has to make sure that "this" has not yet been initialized with
216 // any other policies.
217 bool KernelSupportSeccompBPF();
219 // Verify that the current policy passes some basic sanity checks.
220 void PolicySanityChecks(SandboxBPFPolicy* policy);
222 // Assembles and installs a filter based on the policy that has previously
223 // been configured with SetSandboxPolicy().
224 void InstallFilter(SandboxThreadState thread_state);
226 // Verify the correctness of a compiled program by comparing it against the
227 // current policy. This function should only ever be called by unit tests and
228 // by the sandbox internals. It should not be used by production code.
229 void VerifyProgram(const Program& program, bool has_unsafe_traps);
231 // Finds all the ranges of system calls that need to be handled. Ranges are
232 // sorted in ascending order of system call numbers. There are no gaps in the
233 // ranges. System calls with identical ErrorCodes are coalesced into a single
235 void FindRanges(Ranges* ranges);
237 // Returns a BPF program snippet that implements a jump table for the
238 // given range of system call numbers. This function runs recursively.
239 Instruction* AssembleJumpTable(CodeGen* gen,
240 Ranges::const_iterator start,
241 Ranges::const_iterator stop);
243 // Returns a BPF program snippet that makes the BPF filter program exit
244 // with the given ErrorCode "err". N.B. the ErrorCode may very well be a
245 // conditional expression; if so, this function will recursively call
246 // CondExpression() and possibly RetExpression() to build a complex set of
248 Instruction* RetExpression(CodeGen* gen, const ErrorCode& err);
250 // Returns a BPF program that evaluates the conditional expression in
251 // "cond" and returns the appropriate value from the BPF filter program.
252 // This function recursively calls RetExpression(); it should only ever be
253 // called from RetExpression().
254 Instruction* CondExpression(CodeGen* gen, const ErrorCode& cond);
256 static SandboxStatus status_;
260 scoped_ptr<const SandboxBPFPolicy> policy_;
262 bool sandbox_has_started_;
264 DISALLOW_COPY_AND_ASSIGN(SandboxBPF);
267 } // namespace sandbox
269 #endif // SANDBOX_LINUX_SECCOMP_BPF_SANDBOX_BPF_H__