Will Drewry | 8ac270d | 2012-04-12 16:48:04 -0500 | [diff] [blame] | 1 | SECure COMPuting with filters |
| 2 | ============================= |
| 3 | |
| 4 | Introduction |
| 5 | ------------ |
| 6 | |
| 7 | A large number of system calls are exposed to every userland process |
| 8 | with many of them going unused for the entire lifetime of the process. |
| 9 | As system calls change and mature, bugs are found and eradicated. A |
| 10 | certain subset of userland applications benefit by having a reduced set |
| 11 | of available system calls. The resulting set reduces the total kernel |
| 12 | surface exposed to the application. System call filtering is meant for |
| 13 | use with those applications. |
| 14 | |
| 15 | Seccomp filtering provides a means for a process to specify a filter for |
| 16 | incoming system calls. The filter is expressed as a Berkeley Packet |
| 17 | Filter (BPF) program, as with socket filters, except that the data |
| 18 | operated on is related to the system call being made: system call |
| 19 | number and the system call arguments. This allows for expressive |
| 20 | filtering of system calls using a filter program language with a long |
| 21 | history of being exposed to userland and a straightforward data set. |
| 22 | |
| 23 | Additionally, BPF makes it impossible for users of seccomp to fall prey |
| 24 | to time-of-check-time-of-use (TOCTOU) attacks that are common in system |
| 25 | call interposition frameworks. BPF programs may not dereference |
| 26 | pointers which constrains all filters to solely evaluating the system |
| 27 | call arguments directly. |
| 28 | |
| 29 | What it isn't |
| 30 | ------------- |
| 31 | |
| 32 | System call filtering isn't a sandbox. It provides a clearly defined |
| 33 | mechanism for minimizing the exposed kernel surface. It is meant to be |
| 34 | a tool for sandbox developers to use. Beyond that, policy for logical |
| 35 | behavior and information flow should be managed with a combination of |
| 36 | other system hardening techniques and, potentially, an LSM of your |
| 37 | choosing. Expressive, dynamic filters provide further options down this |
| 38 | path (avoiding pathological sizes or selecting which of the multiplexed |
| 39 | system calls in socketcall() is allowed, for instance) which could be |
| 40 | construed, incorrectly, as a more complete sandboxing solution. |
| 41 | |
| 42 | Usage |
| 43 | ----- |
| 44 | |
| 45 | An additional seccomp mode is added and is enabled using the same |
| 46 | prctl(2) call as the strict seccomp. If the architecture has |
| 47 | CONFIG_HAVE_ARCH_SECCOMP_FILTER, then filters may be added as below: |
| 48 | |
| 49 | PR_SET_SECCOMP: |
| 50 | Now takes an additional argument which specifies a new filter |
| 51 | using a BPF program. |
| 52 | The BPF program will be executed over struct seccomp_data |
| 53 | reflecting the system call number, arguments, and other |
| 54 | metadata. The BPF program must then return one of the |
| 55 | acceptable values to inform the kernel which action should be |
| 56 | taken. |
| 57 | |
| 58 | Usage: |
| 59 | prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, prog); |
| 60 | |
| 61 | The 'prog' argument is a pointer to a struct sock_fprog which |
| 62 | will contain the filter program. If the program is invalid, the |
| 63 | call will return -1 and set errno to EINVAL. |
| 64 | |
| 65 | If fork/clone and execve are allowed by @prog, any child |
| 66 | processes will be constrained to the same filters and system |
| 67 | call ABI as the parent. |
| 68 | |
| 69 | Prior to use, the task must call prctl(PR_SET_NO_NEW_PRIVS, 1) or |
| 70 | run with CAP_SYS_ADMIN privileges in its namespace. If these are not |
| 71 | true, -EACCES will be returned. This requirement ensures that filter |
| 72 | programs cannot be applied to child processes with greater privileges |
| 73 | than the task that installed them. |
| 74 | |
| 75 | Additionally, if prctl(2) is allowed by the attached filter, |
| 76 | additional filters may be layered on which will increase evaluation |
| 77 | time, but allow for further decreasing the attack surface during |
| 78 | execution of a process. |
| 79 | |
| 80 | The above call returns 0 on success and non-zero on error. |
| 81 | |
| 82 | Return values |
| 83 | ------------- |
| 84 | A seccomp filter may return any of the following values. If multiple |
| 85 | filters exist, the return value for the evaluation of a given system |
| 86 | call will always use the highest precedent value. (For example, |
| 87 | SECCOMP_RET_KILL will always take precedence.) |
| 88 | |
| 89 | In precedence order, they are: |
| 90 | |
| 91 | SECCOMP_RET_KILL: |
| 92 | Results in the task exiting immediately without executing the |
| 93 | system call. The exit status of the task (status & 0x7f) will |
| 94 | be SIGSYS, not SIGKILL. |
| 95 | |
| 96 | SECCOMP_RET_TRAP: |
| 97 | Results in the kernel sending a SIGSYS signal to the triggering |
| 98 | task without executing the system call. The kernel will |
| 99 | rollback the register state to just before the system call |
| 100 | entry such that a signal handler in the task will be able to |
| 101 | inspect the ucontext_t->uc_mcontext registers and emulate |
| 102 | system call success or failure upon return from the signal |
| 103 | handler. |
| 104 | |
| 105 | The SECCOMP_RET_DATA portion of the return value will be passed |
| 106 | as si_errno. |
| 107 | |
| 108 | SIGSYS triggered by seccomp will have a si_code of SYS_SECCOMP. |
| 109 | |
| 110 | SECCOMP_RET_ERRNO: |
| 111 | Results in the lower 16-bits of the return value being passed |
| 112 | to userland as the errno without executing the system call. |
| 113 | |
| 114 | SECCOMP_RET_TRACE: |
| 115 | When returned, this value will cause the kernel to attempt to |
| 116 | notify a ptrace()-based tracer prior to executing the system |
| 117 | call. If there is no tracer present, -ENOSYS is returned to |
| 118 | userland and the system call is not executed. |
| 119 | |
| 120 | A tracer will be notified if it requests PTRACE_O_TRACESECCOMP |
| 121 | using ptrace(PTRACE_SETOPTIONS). The tracer will be notified |
| 122 | of a PTRACE_EVENT_SECCOMP and the SECCOMP_RET_DATA portion of |
| 123 | the BPF program return value will be available to the tracer |
| 124 | via PTRACE_GETEVENTMSG. |
| 125 | |
| 126 | SECCOMP_RET_ALLOW: |
| 127 | Results in the system call being executed. |
| 128 | |
| 129 | If multiple filters exist, the return value for the evaluation of a |
| 130 | given system call will always use the highest precedent value. |
| 131 | |
| 132 | Precedence is only determined using the SECCOMP_RET_ACTION mask. When |
| 133 | multiple filters return values of the same precedence, only the |
| 134 | SECCOMP_RET_DATA from the most recently installed filter will be |
| 135 | returned. |
| 136 | |
| 137 | Pitfalls |
| 138 | -------- |
| 139 | |
| 140 | The biggest pitfall to avoid during use is filtering on system call |
| 141 | number without checking the architecture value. Why? On any |
| 142 | architecture that supports multiple system call invocation conventions, |
| 143 | the system call numbers may vary based on the specific invocation. If |
| 144 | the numbers in the different calling conventions overlap, then checks in |
| 145 | the filters may be abused. Always check the arch value! |
| 146 | |
| 147 | Example |
| 148 | ------- |
| 149 | |
| 150 | The samples/seccomp/ directory contains both an x86-specific example |
| 151 | and a more generic example of a higher level macro interface for BPF |
| 152 | program generation. |
| 153 | |
| 154 | |
| 155 | |
| 156 | Adding architecture support |
| 157 | ----------------------- |
| 158 | |
| 159 | See arch/Kconfig for the authoritative requirements. In general, if an |
| 160 | architecture supports both ptrace_event and seccomp, it will be able to |
| 161 | support seccomp filter with minor fixup: SIGSYS support and seccomp return |
| 162 | value checking. Then it must just add CONFIG_HAVE_ARCH_SECCOMP_FILTER |
| 163 | to its arch-specific Kconfig. |