Documentation: prctl/seccomp_filter

Documents how system call filtering using Berkeley Packet
Filter programs works and how it may be used.
Includes an example for x86 and a semi-generic
example using a macro-based code generator.

Acked-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Will Drewry <wad@chromium.org>
Acked-by: Kees Cook <keescook@chromium.org>

v18: - added acked by
     - update no new privs numbers
v17: - remove @compat note and add Pitfalls section for arch checking
       (keescook@chromium.org)
v16: -
v15: -
v14: - rebase/nochanges
v13: - rebase on to 88ebdda615
v12: - comment on the ptrace_event use
     - update arch support comment
     - note the behavior of SECCOMP_RET_DATA when there are multiple filters
       (keescook@chromium.org)
     - lots of samples/ clean up incl 64-bit bpf-direct support
       (markus@chromium.org)
     - rebase to linux-next
v11: - overhaul return value language, updates (keescook@chromium.org)
     - comment on do_exit(SIGSYS)
v10: - update for SIGSYS
     - update for new seccomp_data layout
     - update for ptrace option use
v9: - updated bpf-direct.c for SIGILL
v8: - add PR_SET_NO_NEW_PRIVS to the samples.
v7: - updated for all the new stuff in v7: TRAP, TRACE
    - only talk about PR_SET_SECCOMP now
    - fixed bad JLE32 check (coreyb@linux.vnet.ibm.com)
    - adds dropper.c: a simple system call disabler
v6: - tweak the language to note the requirement of
      PR_SET_NO_NEW_PRIVS being called prior to use. (luto@mit.edu)
v5: - update sample to use system call arguments
    - adds a "fancy" example using a macro-based generator
    - cleaned up bpf in the sample
    - update docs to mention arguments
    - fix prctl value (eparis@redhat.com)
    - language cleanup (rdunlap@xenotime.net)
v4: - update for no_new_privs use
    - minor tweaks
v3: - call out BPF <-> Berkeley Packet Filter (rdunlap@xenotime.net)
    - document use of tentative always-unprivileged
    - guard sample compilation for i386 and x86_64
v2: - move code to samples (corbet@lwn.net)
Signed-off-by: James Morris <james.l.morris@oracle.com>

Documentation/prctl/seccomp_filter.txt

@@ -0,0 +1,163 @@
+		SECure COMPuting with filters
+		=============================
+
+Introduction
+------------
+
+A large number of system calls are exposed to every userland process
+with many of them going unused for the entire lifetime of the process.
+As system calls change and mature, bugs are found and eradicated.  A
+certain subset of userland applications benefit by having a reduced set
+of available system calls.  The resulting set reduces the total kernel
+surface exposed to the application.  System call filtering is meant for
+use with those applications.
+
+Seccomp filtering provides a means for a process to specify a filter for
+incoming system calls.  The filter is expressed as a Berkeley Packet
+Filter (BPF) program, as with socket filters, except that the data
+operated on is related to the system call being made: system call
+number and the system call arguments.  This allows for expressive
+filtering of system calls using a filter program language with a long
+history of being exposed to userland and a straightforward data set.
+
+Additionally, BPF makes it impossible for users of seccomp to fall prey
+to time-of-check-time-of-use (TOCTOU) attacks that are common in system
+call interposition frameworks.  BPF programs may not dereference
+pointers which constrains all filters to solely evaluating the system
+call arguments directly.
+
+What it isn't
+-------------
+
+System call filtering isn't a sandbox.  It provides a clearly defined
+mechanism for minimizing the exposed kernel surface.  It is meant to be
+a tool for sandbox developers to use.  Beyond that, policy for logical
+behavior and information flow should be managed with a combination of
+other system hardening techniques and, potentially, an LSM of your
+choosing.  Expressive, dynamic filters provide further options down this
+path (avoiding pathological sizes or selecting which of the multiplexed
+system calls in socketcall() is allowed, for instance) which could be
+construed, incorrectly, as a more complete sandboxing solution.
+
+Usage
+-----
+
+An additional seccomp mode is added and is enabled using the same
+prctl(2) call as the strict seccomp.  If the architecture has
+CONFIG_HAVE_ARCH_SECCOMP_FILTER, then filters may be added as below:
+
+PR_SET_SECCOMP:
+	Now takes an additional argument which specifies a new filter
+	using a BPF program.
+	The BPF program will be executed over struct seccomp_data
+	reflecting the system call number, arguments, and other
+	metadata.  The BPF program must then return one of the
+	acceptable values to inform the kernel which action should be
+	taken.
+
+	Usage:
+		prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, prog);
+
+	The 'prog' argument is a pointer to a struct sock_fprog which
+	will contain the filter program.  If the program is invalid, the
+	call will return -1 and set errno to EINVAL.
+
+	If fork/clone and execve are allowed by @prog, any child
+	processes will be constrained to the same filters and system
+	call ABI as the parent.
+
+	Prior to use, the task must call prctl(PR_SET_NO_NEW_PRIVS, 1) or
+	run with CAP_SYS_ADMIN privileges in its namespace.  If these are not
+	true, -EACCES will be returned.  This requirement ensures that filter
+	programs cannot be applied to child processes with greater privileges
+	than the task that installed them.
+
+	Additionally, if prctl(2) is allowed by the attached filter,
+	additional filters may be layered on which will increase evaluation
+	time, but allow for further decreasing the attack surface during
+	execution of a process.
+
+The above call returns 0 on success and non-zero on error.
+
+Return values
+-------------
+A seccomp filter may return any of the following values. If multiple
+filters exist, the return value for the evaluation of a given system
+call will always use the highest precedent value. (For example,
+SECCOMP_RET_KILL will always take precedence.)
+
+In precedence order, they are:
+
+SECCOMP_RET_KILL:
+	Results in the task exiting immediately without executing the
+	system call.  The exit status of the task (status & 0x7f) will
+	be SIGSYS, not SIGKILL.
+
+SECCOMP_RET_TRAP:
+	Results in the kernel sending a SIGSYS signal to the triggering
+	task without executing the system call.  The kernel will
+	rollback the register state to just before the system call
+	entry such that a signal handler in the task will be able to
+	inspect the ucontext_t->uc_mcontext registers and emulate
+	system call success or failure upon return from the signal
+	handler.
+
+	The SECCOMP_RET_DATA portion of the return value will be passed
+	as si_errno.
+
+	SIGSYS triggered by seccomp will have a si_code of SYS_SECCOMP.
+
+SECCOMP_RET_ERRNO:
+	Results in the lower 16-bits of the return value being passed
+	to userland as the errno without executing the system call.
+
+SECCOMP_RET_TRACE:
+	When returned, this value will cause the kernel to attempt to
+	notify a ptrace()-based tracer prior to executing the system
+	call.  If there is no tracer present, -ENOSYS is returned to
+	userland and the system call is not executed.
+
+	A tracer will be notified if it requests PTRACE_O_TRACESECCOMP
+	using ptrace(PTRACE_SETOPTIONS).  The tracer will be notified
+	of a PTRACE_EVENT_SECCOMP and the SECCOMP_RET_DATA portion of
+	the BPF program return value will be available to the tracer
+	via PTRACE_GETEVENTMSG.
+
+SECCOMP_RET_ALLOW:
+	Results in the system call being executed.
+
+If multiple filters exist, the return value for the evaluation of a
+given system call will always use the highest precedent value.
+
+Precedence is only determined using the SECCOMP_RET_ACTION mask.  When
+multiple filters return values of the same precedence, only the
+SECCOMP_RET_DATA from the most recently installed filter will be
+returned.
+
+Pitfalls
+--------
+
+The biggest pitfall to avoid during use is filtering on system call
+number without checking the architecture value.  Why?  On any
+architecture that supports multiple system call invocation conventions,
+the system call numbers may vary based on the specific invocation.  If
+the numbers in the different calling conventions overlap, then checks in
+the filters may be abused.  Always check the arch value!
+
+Example
+-------
+
+The samples/seccomp/ directory contains both an x86-specific example
+and a more generic example of a higher level macro interface for BPF
+program generation.
+
+
+
+Adding architecture support
+-----------------------
+
+See arch/Kconfig for the authoritative requirements.  In general, if an
+architecture supports both ptrace_event and seccomp, it will be able to
+support seccomp filter with minor fixup: SIGSYS support and seccomp return
+value checking.  Then it must just add CONFIG_HAVE_ARCH_SECCOMP_FILTER
+to its arch-specific Kconfig.

samples/Makefile

@@ -1,4 +1,4 @@
 # Makefile for Linux samples code
 
 obj-$(CONFIG_SAMPLES)	+= kobject/ kprobes/ tracepoints/ trace_events/ \
-			   hw_breakpoint/ kfifo/ kdb/ hidraw/ rpmsg/
+			   hw_breakpoint/ kfifo/ kdb/ hidraw/ rpmsg/ seccomp/

samples/seccomp/Makefile

@@ -0,0 +1,38 @@
+# kbuild trick to avoid linker error. Can be omitted if a module is built.
+obj- := dummy.o
+
+hostprogs-$(CONFIG_SECCOMP) := bpf-fancy dropper
+bpf-fancy-objs := bpf-fancy.o bpf-helper.o
+
+HOSTCFLAGS_bpf-fancy.o += -I$(objtree)/usr/include
+HOSTCFLAGS_bpf-fancy.o += -idirafter $(objtree)/include
+HOSTCFLAGS_bpf-helper.o += -I$(objtree)/usr/include
+HOSTCFLAGS_bpf-helper.o += -idirafter $(objtree)/include
+
+HOSTCFLAGS_dropper.o += -I$(objtree)/usr/include
+HOSTCFLAGS_dropper.o += -idirafter $(objtree)/include
+dropper-objs := dropper.o
+
+# bpf-direct.c is x86-only.
+ifeq ($(SRCARCH),x86)
+# List of programs to build
+hostprogs-$(CONFIG_SECCOMP) += bpf-direct
+bpf-direct-objs := bpf-direct.o
+endif
+
+HOSTCFLAGS_bpf-direct.o += -I$(objtree)/usr/include
+HOSTCFLAGS_bpf-direct.o += -idirafter $(objtree)/include
+
+# Try to match the kernel target.
+ifeq ($(CONFIG_64BIT),)
+HOSTCFLAGS_bpf-direct.o += -m32
+HOSTCFLAGS_dropper.o += -m32
+HOSTCFLAGS_bpf-helper.o += -m32
+HOSTCFLAGS_bpf-fancy.o += -m32
+HOSTLOADLIBES_bpf-direct += -m32
+HOSTLOADLIBES_bpf-fancy += -m32
+HOSTLOADLIBES_dropper += -m32
+endif
+
+# Tell kbuild to always build the programs
+always := $(hostprogs-y)

samples/seccomp/bpf-direct.c

@@ -0,0 +1,176 @@
+/*
+ * Seccomp filter example for x86 (32-bit and 64-bit) with BPF macros
+ *
+ * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
+ * Author: Will Drewry <wad@chromium.org>
+ *
+ * The code may be used by anyone for any purpose,
+ * and can serve as a starting point for developing
+ * applications using prctl(PR_SET_SECCOMP, 2, ...).
+ */
+#define __USE_GNU 1
+#define _GNU_SOURCE 1
+
+#include <linux/types.h>
+#include <linux/filter.h>
+#include <linux/seccomp.h>
+#include <linux/unistd.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stddef.h>
+#include <string.h>
+#include <sys/prctl.h>
+#include <unistd.h>
+
+#define syscall_arg(_n) (offsetof(struct seccomp_data, args[_n]))
+#define syscall_nr (offsetof(struct seccomp_data, nr))
+
+#if defined(__i386__)
+#define REG_RESULT	REG_EAX
+#define REG_SYSCALL	REG_EAX
+#define REG_ARG0	REG_EBX
+#define REG_ARG1	REG_ECX
+#define REG_ARG2	REG_EDX
+#define REG_ARG3	REG_ESI
+#define REG_ARG4	REG_EDI
+#define REG_ARG5	REG_EBP
+#elif defined(__x86_64__)
+#define REG_RESULT	REG_RAX
+#define REG_SYSCALL	REG_RAX
+#define REG_ARG0	REG_RDI
+#define REG_ARG1	REG_RSI
+#define REG_ARG2	REG_RDX
+#define REG_ARG3	REG_R10
+#define REG_ARG4	REG_R8
+#define REG_ARG5	REG_R9
+#else
+#error Unsupported platform
+#endif
+
+#ifndef PR_SET_NO_NEW_PRIVS
+#define PR_SET_NO_NEW_PRIVS 38
+#endif
+
+#ifndef SYS_SECCOMP
+#define SYS_SECCOMP 1
+#endif
+
+static void emulator(int nr, siginfo_t *info, void *void_context)
+{
+	ucontext_t *ctx = (ucontext_t *)(void_context);
+	int syscall;
+	char *buf;
+	ssize_t bytes;
+	size_t len;
+	if (info->si_code != SYS_SECCOMP)
+		return;
+	if (!ctx)
+		return;
+	syscall = ctx->uc_mcontext.gregs[REG_SYSCALL];
+	buf = (char *) ctx->uc_mcontext.gregs[REG_ARG1];
+	len = (size_t) ctx->uc_mcontext.gregs[REG_ARG2];
+
+	if (syscall != __NR_write)
+		return;
+	if (ctx->uc_mcontext.gregs[REG_ARG0] != STDERR_FILENO)
+		return;
+	/* Redirect stderr messages to stdout. Doesn't handle EINTR, etc */
+	ctx->uc_mcontext.gregs[REG_RESULT] = -1;
+	if (write(STDOUT_FILENO, "[ERR] ", 6) > 0) {
+		bytes = write(STDOUT_FILENO, buf, len);
+		ctx->uc_mcontext.gregs[REG_RESULT] = bytes;
+	}
+	return;
+}
+
+static int install_emulator(void)
+{
+	struct sigaction act;
+	sigset_t mask;
+	memset(&act, 0, sizeof(act));
+	sigemptyset(&mask);
+	sigaddset(&mask, SIGSYS);
+
+	act.sa_sigaction = &emulator;
+	act.sa_flags = SA_SIGINFO;
+	if (sigaction(SIGSYS, &act, NULL) < 0) {
+		perror("sigaction");
+		return -1;
+	}
+	if (sigprocmask(SIG_UNBLOCK, &mask, NULL)) {
+		perror("sigprocmask");
+		return -1;
+	}
+	return 0;
+}
+
+static int install_filter(void)
+{
+	struct sock_filter filter[] = {
+		/* Grab the system call number */
+		BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_nr),
+		/* Jump table for the allowed syscalls */
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_rt_sigreturn, 0, 1),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+#ifdef __NR_sigreturn
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_sigreturn, 0, 1),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+#endif
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit_group, 0, 1),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit, 0, 1),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_read, 1, 0),
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_write, 3, 2),
+
+		/* Check that read is only using stdin. */
+		BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)),
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDIN_FILENO, 4, 0),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
+
+		/* Check that write is only using stdout */
+		BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)),
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDOUT_FILENO, 1, 0),
+		/* Trap attempts to write to stderr */
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDERR_FILENO, 1, 2),
+
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_TRAP),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
+	};
+	struct sock_fprog prog = {
+		.len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
+		.filter = filter,
+	};
+
+	if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
+		perror("prctl(NO_NEW_PRIVS)");
+		return 1;
+	}
+
+
+	if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog)) {
+		perror("prctl");
+		return 1;
+	}
+	return 0;
+}
+
+#define payload(_c) (_c), sizeof((_c))
+int main(int argc, char **argv)
+{
+	char buf[4096];
+	ssize_t bytes = 0;
+	if (install_emulator())
+		return 1;
+	if (install_filter())
+		return 1;
+	syscall(__NR_write, STDOUT_FILENO,
+		payload("OHAI! WHAT IS YOUR NAME? "));
+	bytes = syscall(__NR_read, STDIN_FILENO, buf, sizeof(buf));
+	syscall(__NR_write, STDOUT_FILENO, payload("HELLO, "));
+	syscall(__NR_write, STDOUT_FILENO, buf, bytes);
+	syscall(__NR_write, STDERR_FILENO,
+		payload("Error message going to STDERR\n"));
+	return 0;
+}

samples/seccomp/bpf-fancy.c

@@ -0,0 +1,102 @@
+/*
+ * Seccomp BPF example using a macro-based generator.
+ *
+ * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
+ * Author: Will Drewry <wad@chromium.org>
+ *
+ * The code may be used by anyone for any purpose,
+ * and can serve as a starting point for developing
+ * applications using prctl(PR_ATTACH_SECCOMP_FILTER).
+ */
+
+#include <linux/filter.h>
+#include <linux/seccomp.h>
+#include <linux/unistd.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/prctl.h>
+#include <unistd.h>
+
+#include "bpf-helper.h"
+
+#ifndef PR_SET_NO_NEW_PRIVS
+#define PR_SET_NO_NEW_PRIVS 38
+#endif
+
+int main(int argc, char **argv)
+{
+	struct bpf_labels l;
+	static const char msg1[] = "Please type something: ";
+	static const char msg2[] = "You typed: ";
+	char buf[256];
+	struct sock_filter filter[] = {
+		/* TODO: LOAD_SYSCALL_NR(arch) and enforce an arch */
+		LOAD_SYSCALL_NR,
+		SYSCALL(__NR_exit, ALLOW),
+		SYSCALL(__NR_exit_group, ALLOW),
+		SYSCALL(__NR_write, JUMP(&l, write_fd)),
+		SYSCALL(__NR_read, JUMP(&l, read)),
+		DENY,  /* Don't passthrough into a label */
+
+		LABEL(&l, read),
+		ARG(0),
+		JNE(STDIN_FILENO, DENY),
+		ARG(1),
+		JNE((unsigned long)buf, DENY),
+		ARG(2),
+		JGE(sizeof(buf), DENY),
+		ALLOW,
+
+		LABEL(&l, write_fd),
+		ARG(0),
+		JEQ(STDOUT_FILENO, JUMP(&l, write_buf)),
+		JEQ(STDERR_FILENO, JUMP(&l, write_buf)),
+		DENY,
+
+		LABEL(&l, write_buf),
+		ARG(1),
+		JEQ((unsigned long)msg1, JUMP(&l, msg1_len)),
+		JEQ((unsigned long)msg2, JUMP(&l, msg2_len)),
+		JEQ((unsigned long)buf, JUMP(&l, buf_len)),
+		DENY,
+
+		LABEL(&l, msg1_len),
+		ARG(2),
+		JLT(sizeof(msg1), ALLOW),
+		DENY,
+
+		LABEL(&l, msg2_len),
+		ARG(2),
+		JLT(sizeof(msg2), ALLOW),
+		DENY,
+
+		LABEL(&l, buf_len),
+		ARG(2),
+		JLT(sizeof(buf), ALLOW),
+		DENY,
+	};
+	struct sock_fprog prog = {
+		.filter = filter,
+		.len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
+	};
+	ssize_t bytes;
+	bpf_resolve_jumps(&l, filter, sizeof(filter)/sizeof(*filter));
+
+	if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
+		perror("prctl(NO_NEW_PRIVS)");
+		return 1;
+	}
+
+	if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog)) {
+		perror("prctl(SECCOMP)");
+		return 1;
+	}
+	syscall(__NR_write, STDOUT_FILENO, msg1, strlen(msg1));
+	bytes = syscall(__NR_read, STDIN_FILENO, buf, sizeof(buf)-1);
+	bytes = (bytes > 0 ? bytes : 0);
+	syscall(__NR_write, STDERR_FILENO, msg2, strlen(msg2));
+	syscall(__NR_write, STDERR_FILENO, buf, bytes);
+	/* Now get killed */
+	syscall(__NR_write, STDERR_FILENO, msg2, strlen(msg2)+2);
+	return 0;
+}

samples/seccomp/bpf-helper.c

@@ -0,0 +1,89 @@
+/*
+ * Seccomp BPF helper functions
+ *
+ * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
+ * Author: Will Drewry <wad@chromium.org>
+ *
+ * The code may be used by anyone for any purpose,
+ * and can serve as a starting point for developing
+ * applications using prctl(PR_ATTACH_SECCOMP_FILTER).
+ */
+
+#include <stdio.h>
+#include <string.h>
+
+#include "bpf-helper.h"
+
+int bpf_resolve_jumps(struct bpf_labels *labels,
+		      struct sock_filter *filter, size_t count)
+{
+	struct sock_filter *begin = filter;
+	__u8 insn = count - 1;
+
+	if (count < 1)
+		return -1;
+	/*
+	* Walk it once, backwards, to build the label table and do fixups.
+	* Since backward jumps are disallowed by BPF, this is easy.
+	*/
+	filter += insn;
+	for (; filter >= begin; --insn, --filter) {
+		if (filter->code != (BPF_JMP+BPF_JA))
+			continue;
+		switch ((filter->jt<<8)|filter->jf) {
+		case (JUMP_JT<<8)|JUMP_JF:
+			if (labels->labels[filter->k].location == 0xffffffff) {
+				fprintf(stderr, "Unresolved label: '%s'\n",
+					labels->labels[filter->k].label);
+				return 1;
+			}
+			filter->k = labels->labels[filter->k].location -
+				    (insn + 1);
+			filter->jt = 0;
+			filter->jf = 0;
+			continue;
+		case (LABEL_JT<<8)|LABEL_JF:
+			if (labels->labels[filter->k].location != 0xffffffff) {
+				fprintf(stderr, "Duplicate label use: '%s'\n",
+					labels->labels[filter->k].label);
+				return 1;
+			}
+			labels->labels[filter->k].location = insn;
+			filter->k = 0; /* fall through */
+			filter->jt = 0;
+			filter->jf = 0;
+			continue;
+		}
+	}
+	return 0;
+}
+
+/* Simple lookup table for labels. */
+__u32 seccomp_bpf_label(struct bpf_labels *labels, const char *label)
+{
+	struct __bpf_label *begin = labels->labels, *end;
+	int id;
+	if (labels->count == 0) {
+		begin->label = label;
+		begin->location = 0xffffffff;
+		labels->count++;
+		return 0;
+	}
+	end = begin + labels->count;
+	for (id = 0; begin < end; ++begin, ++id) {
+		if (!strcmp(label, begin->label))
+			return id;
+	}
+	begin->label = label;
+	begin->location = 0xffffffff;
+	labels->count++;
+	return id;
+}
+
+void seccomp_bpf_print(struct sock_filter *filter, size_t count)
+{
+	struct sock_filter *end = filter + count;
+	for ( ; filter < end; ++filter)
+		printf("{ code=%u,jt=%u,jf=%u,k=%u },\n",
+			filter->code, filter->jt, filter->jf, filter->k);
+}

samples/seccomp/bpf-helper.h

@@ -0,0 +1,238 @@
+/*
+ * Example wrapper around BPF macros.
+ *
+ * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
+ * Author: Will Drewry <wad@chromium.org>
+ *
+ * The code may be used by anyone for any purpose,
+ * and can serve as a starting point for developing
+ * applications using prctl(PR_SET_SECCOMP, 2, ...).
+ *
+ * No guarantees are provided with respect to the correctness
+ * or functionality of this code.
+ */
+#ifndef __BPF_HELPER_H__
+#define __BPF_HELPER_H__
+
+#include <asm/bitsperlong.h>	/* for __BITS_PER_LONG */
+#include <endian.h>
+#include <linux/filter.h>
+#include <linux/seccomp.h>	/* for seccomp_data */
+#include <linux/types.h>
+#include <linux/unistd.h>
+#include <stddef.h>
+
+#define BPF_LABELS_MAX 256
+struct bpf_labels {
+	int count;
+	struct __bpf_label {
+		const char *label;
+		__u32 location;
+	} labels[BPF_LABELS_MAX];
+};
+
+int bpf_resolve_jumps(struct bpf_labels *labels,
+		      struct sock_filter *filter, size_t count);
+__u32 seccomp_bpf_label(struct bpf_labels *labels, const char *label);
+void seccomp_bpf_print(struct sock_filter *filter, size_t count);
+
+#define JUMP_JT 0xff
+#define JUMP_JF 0xff
+#define LABEL_JT 0xfe
+#define LABEL_JF 0xfe
+
+#define ALLOW \
+	BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW)
+#define DENY \
+	BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL)
+#define JUMP(labels, label) \
+	BPF_JUMP(BPF_JMP+BPF_JA, FIND_LABEL((labels), (label)), \
+		 JUMP_JT, JUMP_JF)
+#define LABEL(labels, label) \
+	BPF_JUMP(BPF_JMP+BPF_JA, FIND_LABEL((labels), (label)), \
+		 LABEL_JT, LABEL_JF)
+#define SYSCALL(nr, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (nr), 0, 1), \
+	jt
+
+/* Lame, but just an example */
+#define FIND_LABEL(labels, label) seccomp_bpf_label((labels), #label)
+
+#define EXPAND(...) __VA_ARGS__
+/* Map all width-sensitive operations */
+#if __BITS_PER_LONG == 32
+
+#define JEQ(x, jt) JEQ32(x, EXPAND(jt))
+#define JNE(x, jt) JNE32(x, EXPAND(jt))
+#define JGT(x, jt) JGT32(x, EXPAND(jt))
+#define JLT(x, jt) JLT32(x, EXPAND(jt))
+#define JGE(x, jt) JGE32(x, EXPAND(jt))
+#define JLE(x, jt) JLE32(x, EXPAND(jt))
+#define JA(x, jt) JA32(x, EXPAND(jt))
+#define ARG(i) ARG_32(i)
+#define LO_ARG(idx) offsetof(struct seccomp_data, args[(idx)])
+
+#elif __BITS_PER_LONG == 64
+
+/* Ensure that we load the logically correct offset. */
+#if __BYTE_ORDER == __LITTLE_ENDIAN
+#define ENDIAN(_lo, _hi) _lo, _hi
+#define LO_ARG(idx) offsetof(struct seccomp_data, args[(idx)])
+#define HI_ARG(idx) offsetof(struct seccomp_data, args[(idx)]) + sizeof(__u32)
+#elif __BYTE_ORDER == __BIG_ENDIAN
+#define ENDIAN(_lo, _hi) _hi, _lo
+#define LO_ARG(idx) offsetof(struct seccomp_data, args[(idx)]) + sizeof(__u32)
+#define HI_ARG(idx) offsetof(struct seccomp_data, args[(idx)])
+#else
+#error "Unknown endianness"
+#endif
+
+union arg64 {
+	struct {
+		__u32 ENDIAN(lo32, hi32);
+	};
+	__u64 u64;
+};
+
+#define JEQ(x, jt) \
+	JEQ64(((union arg64){.u64 = (x)}).lo32, \
+	      ((union arg64){.u64 = (x)}).hi32, \
+	      EXPAND(jt))
+#define JGT(x, jt) \
+	JGT64(((union arg64){.u64 = (x)}).lo32, \
+	      ((union arg64){.u64 = (x)}).hi32, \
+	      EXPAND(jt))
+#define JGE(x, jt) \
+	JGE64(((union arg64){.u64 = (x)}).lo32, \
+	      ((union arg64){.u64 = (x)}).hi32, \
+	      EXPAND(jt))
+#define JNE(x, jt) \
+	JNE64(((union arg64){.u64 = (x)}).lo32, \
+	      ((union arg64){.u64 = (x)}).hi32, \
+	      EXPAND(jt))
+#define JLT(x, jt) \
+	JLT64(((union arg64){.u64 = (x)}).lo32, \
+	      ((union arg64){.u64 = (x)}).hi32, \
+	      EXPAND(jt))
+#define JLE(x, jt) \
+	JLE64(((union arg64){.u64 = (x)}).lo32, \
+	      ((union arg64){.u64 = (x)}).hi32, \
+	      EXPAND(jt))
+
+#define JA(x, jt) \
+	JA64(((union arg64){.u64 = (x)}).lo32, \
+	       ((union arg64){.u64 = (x)}).hi32, \
+	       EXPAND(jt))
+#define ARG(i) ARG_64(i)
+
+#else
+#error __BITS_PER_LONG value unusable.
+#endif
+
+/* Loads the arg into A */
+#define ARG_32(idx) \
+	BPF_STMT(BPF_LD+BPF_W+BPF_ABS, LO_ARG(idx))
+
+/* Loads hi into A and lo in X */
+#define ARG_64(idx) \
+	BPF_STMT(BPF_LD+BPF_W+BPF_ABS, LO_ARG(idx)), \
+	BPF_STMT(BPF_ST, 0), /* lo -> M[0] */ \
+	BPF_STMT(BPF_LD+BPF_W+BPF_ABS, HI_ARG(idx)), \
+	BPF_STMT(BPF_ST, 1) /* hi -> M[1] */
+
+#define JEQ32(value, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (value), 0, 1), \
+	jt
+
+#define JNE32(value, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (value), 1, 0), \
+	jt
+
+/* Checks the lo, then swaps to check the hi. A=lo,X=hi */
+#define JEQ64(lo, hi, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 5), \
+	BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (lo), 0, 2), \
+	BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \
+	jt, \
+	BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */
+
+#define JNE64(lo, hi, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 5, 0), \
+	BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (lo), 2, 0), \
+	BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \
+	jt, \
+	BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */
+
+#define JA32(value, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, (value), 0, 1), \
+	jt
+
+#define JA64(lo, hi, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, (hi), 3, 0), \
+	BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
+	BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, (lo), 0, 2), \
+	BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \
+	jt, \
+	BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */
+
+#define JGE32(value, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (value), 0, 1), \
+	jt
+
+#define JLT32(value, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (value), 1, 0), \
+	jt
+
+/* Shortcut checking if hi > arg.hi. */
+#define JGE64(lo, hi, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (hi), 4, 0), \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 5), \
+	BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
+	BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (lo), 0, 2), \
+	BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \
+	jt, \
+	BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */
+
+#define JLT64(lo, hi, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (hi), 0, 4), \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 5), \
+	BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
+	BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (lo), 2, 0), \
+	BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \
+	jt, \
+	BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */
+
+#define JGT32(value, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (value), 0, 1), \
+	jt
+
+#define JLE32(value, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (value), 1, 0), \
+	jt
+
+/* Check hi > args.hi first, then do the GE checking */
+#define JGT64(lo, hi, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (hi), 4, 0), \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 5), \
+	BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
+	BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (lo), 0, 2), \
+	BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \
+	jt, \
+	BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */
+
+#define JLE64(lo, hi, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (hi), 6, 0), \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 3), \
+	BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
+	BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (lo), 2, 0), \
+	BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \
+	jt, \
+	BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */
+
+#define LOAD_SYSCALL_NR \
+	BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \
+		 offsetof(struct seccomp_data, nr))
+
+#endif  /* __BPF_HELPER_H__ */

samples/seccomp/dropper.c

@@ -0,0 +1,68 @@
+/*
+ * Naive system call dropper built on seccomp_filter.
+ *
+ * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
+ * Author: Will Drewry <wad@chromium.org>
+ *
+ * The code may be used by anyone for any purpose,
+ * and can serve as a starting point for developing
+ * applications using prctl(PR_SET_SECCOMP, 2, ...).
+ *
+ * When run, returns the specified errno for the specified
+ * system call number against the given architecture.
+ *
+ * Run this one as root as PR_SET_NO_NEW_PRIVS is not called.
+ */
+
+#include <errno.h>
+#include <linux/audit.h>
+#include <linux/filter.h>
+#include <linux/seccomp.h>
+#include <linux/unistd.h>
+#include <stdio.h>
+#include <stddef.h>
+#include <stdlib.h>
+#include <sys/prctl.h>
+#include <unistd.h>
+
+static int install_filter(int nr, int arch, int error)
+{
+	struct sock_filter filter[] = {
+		BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
+			 (offsetof(struct seccomp_data, arch))),
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, arch, 0, 3),
+		BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
+			 (offsetof(struct seccomp_data, nr))),
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, nr, 0, 1),
+		BPF_STMT(BPF_RET+BPF_K,
+			 SECCOMP_RET_ERRNO|(error & SECCOMP_RET_DATA)),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+	};
+	struct sock_fprog prog = {
+		.len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
+		.filter = filter,
+	};
+	if (prctl(PR_SET_SECCOMP, 2, &prog)) {
+		perror("prctl");
+		return 1;
+	}
+	return 0;
+}
+
+int main(int argc, char **argv)
+{
+	if (argc < 5) {
+		fprintf(stderr, "Usage:\n"
+			"dropper <syscall_nr> <arch> <errno> <prog> [<args>]\n"
+			"Hint:	AUDIT_ARCH_I386: 0x%X\n"
+			"	AUDIT_ARCH_X86_64: 0x%X\n"
+			"\n", AUDIT_ARCH_I386, AUDIT_ARCH_X86_64);
+		return 1;
+	}
+	if (install_filter(strtol(argv[1], NULL, 0), strtol(argv[2], NULL, 0),
+			   strtol(argv[3], NULL, 0)))
+		return 1;
+	execv(argv[4], &argv[4]);
+	printf("Failed to execv\n");
+	return 255;
+}