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Documentation: prctl/seccomp_filter

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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>
This commit is contained in:
Will Drewry 2012-04-12 16:48:04 -05:00 committed by James Morris
parent c6cfbeb402
commit 8ac270d1e2
8 changed files with 875 additions and 1 deletions
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163
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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.

2
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# 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/

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# 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)

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/*
* 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;
}

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/*
* 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;
}

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/*
* 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);
}

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/*
* 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__ */

68
samples/seccomp/dropper.c Normal file
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@ -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;
}