164 lines
6.5 KiB
Plaintext
164 lines
6.5 KiB
Plaintext
SECure COMPuting with filters
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=============================
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Introduction
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------------
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A large number of system calls are exposed to every userland process
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with many of them going unused for the entire lifetime of the process.
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As system calls change and mature, bugs are found and eradicated. A
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certain subset of userland applications benefit by having a reduced set
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of available system calls. The resulting set reduces the total kernel
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surface exposed to the application. System call filtering is meant for
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use with those applications.
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Seccomp filtering provides a means for a process to specify a filter for
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incoming system calls. The filter is expressed as a Berkeley Packet
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Filter (BPF) program, as with socket filters, except that the data
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operated on is related to the system call being made: system call
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number and the system call arguments. This allows for expressive
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filtering of system calls using a filter program language with a long
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history of being exposed to userland and a straightforward data set.
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Additionally, BPF makes it impossible for users of seccomp to fall prey
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to time-of-check-time-of-use (TOCTOU) attacks that are common in system
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call interposition frameworks. BPF programs may not dereference
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pointers which constrains all filters to solely evaluating the system
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call arguments directly.
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What it isn't
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-------------
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System call filtering isn't a sandbox. It provides a clearly defined
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mechanism for minimizing the exposed kernel surface. It is meant to be
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a tool for sandbox developers to use. Beyond that, policy for logical
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behavior and information flow should be managed with a combination of
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other system hardening techniques and, potentially, an LSM of your
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choosing. Expressive, dynamic filters provide further options down this
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path (avoiding pathological sizes or selecting which of the multiplexed
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system calls in socketcall() is allowed, for instance) which could be
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construed, incorrectly, as a more complete sandboxing solution.
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Usage
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-----
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An additional seccomp mode is added and is enabled using the same
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prctl(2) call as the strict seccomp. If the architecture has
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CONFIG_HAVE_ARCH_SECCOMP_FILTER, then filters may be added as below:
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PR_SET_SECCOMP:
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Now takes an additional argument which specifies a new filter
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using a BPF program.
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The BPF program will be executed over struct seccomp_data
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reflecting the system call number, arguments, and other
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metadata. The BPF program must then return one of the
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acceptable values to inform the kernel which action should be
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taken.
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Usage:
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prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, prog);
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The 'prog' argument is a pointer to a struct sock_fprog which
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will contain the filter program. If the program is invalid, the
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call will return -1 and set errno to EINVAL.
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If fork/clone and execve are allowed by @prog, any child
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processes will be constrained to the same filters and system
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call ABI as the parent.
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Prior to use, the task must call prctl(PR_SET_NO_NEW_PRIVS, 1) or
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run with CAP_SYS_ADMIN privileges in its namespace. If these are not
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true, -EACCES will be returned. This requirement ensures that filter
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programs cannot be applied to child processes with greater privileges
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than the task that installed them.
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Additionally, if prctl(2) is allowed by the attached filter,
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additional filters may be layered on which will increase evaluation
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time, but allow for further decreasing the attack surface during
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execution of a process.
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The above call returns 0 on success and non-zero on error.
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Return values
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-------------
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A seccomp filter may return any of the following values. If multiple
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filters exist, the return value for the evaluation of a given system
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call will always use the highest precedent value. (For example,
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SECCOMP_RET_KILL will always take precedence.)
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In precedence order, they are:
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SECCOMP_RET_KILL:
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Results in the task exiting immediately without executing the
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system call. The exit status of the task (status & 0x7f) will
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be SIGSYS, not SIGKILL.
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SECCOMP_RET_TRAP:
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Results in the kernel sending a SIGSYS signal to the triggering
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task without executing the system call. The kernel will
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rollback the register state to just before the system call
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entry such that a signal handler in the task will be able to
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inspect the ucontext_t->uc_mcontext registers and emulate
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system call success or failure upon return from the signal
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handler.
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The SECCOMP_RET_DATA portion of the return value will be passed
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as si_errno.
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SIGSYS triggered by seccomp will have a si_code of SYS_SECCOMP.
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SECCOMP_RET_ERRNO:
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Results in the lower 16-bits of the return value being passed
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to userland as the errno without executing the system call.
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SECCOMP_RET_TRACE:
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When returned, this value will cause the kernel to attempt to
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notify a ptrace()-based tracer prior to executing the system
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call. If there is no tracer present, -ENOSYS is returned to
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userland and the system call is not executed.
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A tracer will be notified if it requests PTRACE_O_TRACESECCOMP
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using ptrace(PTRACE_SETOPTIONS). The tracer will be notified
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of a PTRACE_EVENT_SECCOMP and the SECCOMP_RET_DATA portion of
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the BPF program return value will be available to the tracer
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via PTRACE_GETEVENTMSG.
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SECCOMP_RET_ALLOW:
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Results in the system call being executed.
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If multiple filters exist, the return value for the evaluation of a
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given system call will always use the highest precedent value.
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Precedence is only determined using the SECCOMP_RET_ACTION mask. When
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multiple filters return values of the same precedence, only the
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SECCOMP_RET_DATA from the most recently installed filter will be
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returned.
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Pitfalls
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--------
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The biggest pitfall to avoid during use is filtering on system call
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number without checking the architecture value. Why? On any
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architecture that supports multiple system call invocation conventions,
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the system call numbers may vary based on the specific invocation. If
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the numbers in the different calling conventions overlap, then checks in
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the filters may be abused. Always check the arch value!
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Example
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-------
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The samples/seccomp/ directory contains both an x86-specific example
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and a more generic example of a higher level macro interface for BPF
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program generation.
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Adding architecture support
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-----------------------
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See arch/Kconfig for the authoritative requirements. In general, if an
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architecture supports both ptrace_event and seccomp, it will be able to
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support seccomp filter with minor fixup: SIGSYS support and seccomp return
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value checking. Then it must just add CONFIG_HAVE_ARCH_SECCOMP_FILTER
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to its arch-specific Kconfig.
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