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_ERRORCODE_H__
6 #define SANDBOX_LINUX_SECCOMP_BPF_ERRORCODE_H__
8 #include "sandbox/linux/seccomp-bpf/trap.h"
9 #include "sandbox/sandbox_export.h"
13 // This class holds all the possible values that can be returned by a sandbox
15 // We can either wrap a symbolic ErrorCode (i.e. ERR_XXX enum values), an
16 // errno value (in the range 0..4095), a pointer to a TrapFnc callback
17 // handling a SECCOMP_RET_TRAP trap, or a complex constraint.
18 // All of the commonly used values are stored in the "err_" field. So, code
19 // that is using the ErrorCode class typically operates on a single 32bit
21 class SANDBOX_EXPORT ErrorCode {
24 // Allow this system call. The value of ERR_ALLOWED is pretty much
25 // completely arbitrary. But we want to pick it so that is is unlikely
26 // to be passed in accidentally, when the user intended to return an
27 // "errno" (see below) value instead.
28 ERR_ALLOWED = 0x04000000,
30 // If the progress is being ptraced with PTRACE_O_TRACESECCOMP, then the
31 // tracer will be notified of a PTRACE_EVENT_SECCOMP and allowed to change
32 // or skip the system call. The lower 16 bits of err will be available to
33 // the tracer via PTRACE_GETEVENTMSG.
34 ERR_TRACE = 0x08000000,
36 // Deny the system call with a particular "errno" value.
37 // N.B.: It is also possible to return "0" here. That would normally
38 // indicate success, but it won't actually run the system call.
39 // This is very different from return ERR_ALLOWED.
42 // MIPS only supports errno up to 1133
45 // TODO(markus): Android only supports errno up to 255
46 // (crbug.com/181647).
51 // While BPF filter programs always operate on 32bit quantities, the kernel
52 // always sees system call arguments as 64bit values. This statement is true
53 // no matter whether the host system is natively operating in 32bit or 64bit.
54 // The BPF compiler hides the fact that BPF instructions cannot directly
55 // access 64bit quantities. But policies are still advised to specify whether
56 // a system call expects a 32bit or a 64bit quantity.
58 // When passed as an argument to SandboxBPF::Cond(), TP_32BIT requests that
59 // the conditional test should operate on the 32bit part of the system call
61 // On 64bit architectures, this verifies that user space did not pass
62 // a 64bit value as an argument to the system call. If it did, that will be
63 // interpreted as an attempt at breaking the sandbox and results in the
64 // program getting terminated.
65 // In other words, only perform a 32bit test, if you are sure this
66 // particular system call would never legitimately take a 64bit
68 // Implementation detail: TP_32BIT does two things. 1) it restricts the
69 // conditional test to operating on the LSB only, and 2) it adds code to
70 // the BPF filter program verifying that the MSB the kernel received from
71 // user space is either 0, or 0xFFFFFFFF; the latter is acceptable, iff bit
72 // 31 was set in the system call argument. It deals with 32bit arguments
73 // having been sign extended.
76 // When passed as an argument to SandboxBPF::Cond(), TP_64BIT requests that
77 // the conditional test should operate on the full 64bit argument. It is
78 // generally harmless to perform a 64bit test on 32bit systems, as the
79 // kernel will always see the top 32 bits of all arguments as zero'd out.
80 // This approach has the desirable property that for tests of pointer
81 // values, we can always use TP_64BIT no matter the host architecture.
82 // But of course, that also means, it is possible to write conditional
83 // policies that turn into no-ops on 32bit systems; this is by design.
89 // Test whether the system call argument is equal to the operand.
92 // Tests a system call argument against a bit mask.
93 // The "ALL_BITS" variant performs this test: "arg & mask == mask"
94 // This implies that a mask of zero always results in a passing test.
95 // The "ANY_BITS" variant performs this test: "arg & mask != 0"
96 // This implies that a mask of zero always results in a failing test.
108 // We allow the default constructor, as it makes the ErrorCode class
109 // much easier to use. But if we ever encounter an invalid ErrorCode
110 // when compiling a BPF filter, we deliberately generate an invalid
111 // program that will get flagged both by our Verifier class and by
114 explicit ErrorCode(int err);
116 // For all practical purposes, ErrorCodes are treated as if they were
117 // structs. The copy constructor and assignment operator are trivial and
118 // we do not need to explicitly specify them.
119 // Most notably, it is in fact perfectly OK to directly copy the passed_ and
120 // failed_ field. They only ever get set by our private constructor, and the
121 // callers handle life-cycle management for these objects.
126 bool Equals(const ErrorCode& err) const;
127 bool LessThan(const ErrorCode& err) const;
129 uint32_t err() const { return err_; }
130 ErrorType error_type() const { return error_type_; }
132 bool safe() const { return safe_; }
134 uint64_t mask() const { return mask_; }
135 uint64_t value() const { return value_; }
136 int argno() const { return argno_; }
137 ArgType width() const { return width_; }
138 const ErrorCode* passed() const { return passed_; }
139 const ErrorCode* failed() const { return failed_; }
142 bool operator()(const ErrorCode& a, const ErrorCode& b) const {
143 return a.LessThan(b);
148 friend class CodeGen;
149 friend class SandboxBPF;
152 // If we are wrapping a callback, we must assign a unique id. This id is
153 // how the kernel tells us which one of our different SECCOMP_RET_TRAP
154 // cases has been triggered.
155 ErrorCode(Trap::TrapFnc fnc, const void* aux, bool safe);
157 // Some system calls require inspection of arguments. This constructor
158 // allows us to specify additional constraints.
163 const ErrorCode* passed,
164 const ErrorCode* failed);
166 ErrorType error_type_;
169 // Fields needed for SECCOMP_RET_TRAP callbacks
171 Trap::TrapFnc fnc_; // Callback function and arg, if trap was
172 void* aux_; // triggered by the kernel's BPF filter.
173 bool safe_; // Keep sandbox active while calling fnc_()
176 // Fields needed when inspecting additional arguments.
178 uint64_t mask_; // Mask that we are comparing under.
179 uint64_t value_; // Value that we are comparing with.
180 int argno_; // Syscall arg number that we are inspecting.
181 ArgType width_; // Whether we are looking at a 32/64bit value.
182 const ErrorCode* passed_; // Value to be returned if comparison passed,
183 const ErrorCode* failed_; // or if it failed.
187 // 32bit field used for all possible types of ErrorCode values. This is
188 // the value that uniquely identifies any ErrorCode and it (typically) can
189 // be emitted directly into a BPF filter program.
193 } // namespace sandbox
195 #endif // SANDBOX_LINUX_SECCOMP_BPF_ERRORCODE_H__