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Date: Tue, 17 Apr 2012 22:28:06 -0400
From: Paul Gortmaker <paul.gortmaker@...driver.com>
To: Will Drewry <wad@...omium.org>
Cc: linux-kernel@...r.kernel.org, linux-man@...r.kernel.org,
linux-security-module@...r.kernel.org, linux-arch@...r.kernel.org,
linux-doc@...r.kernel.org, kernel-hardening@...ts.openwall.com,
netdev@...r.kernel.org, x86@...nel.org, arnd@...db.de, davem@...emloft.net,
hpa@...or.com, mingo@...hat.com, oleg@...hat.com, peterz@...radead.org,
rdunlap@...otime.net, mcgrathr@...omium.org, tglx@...utronix.de, luto@....edu,
eparis@...hat.com, serge.hallyn@...onical.com, djm@...drot.org,
scarybeasts@...il.com, indan@....nu, pmoore@...hat.com,
akpm@...ux-foundation.org, corbet@....net, eric.dumazet@...il.com,
markus@...omium.org, coreyb@...ux.vnet.ibm.com, keescook@...omium.org,
jmorris@...ei.org
Subject: Re: [PATCH v18 15/15] Documentation: prctl/seccomp_filter
On Thu, Apr 12, 2012 at 5:48 PM, Will Drewry <wad@...omium.org> wrote:
> 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.
Hi guys, that is quite the To/CC list. I didn't have the heart to add
yet another address (linux-next) to it...
Can you have a look at this link? It appears to be breaking
the i386 allmodconfig builds in linux-next.
http://kisskb.ellerman.id.au/kisskb/buildresult/6123842/
"samples/seccomp/bpf-direct.c:47:2: error: #error Unsupported platform"
Thanks,
Paul.
--
>
> Acked-by: Eric Paris <eparis@...hat.com>
> Signed-off-by: Will Drewry <wad@...omium.org>
>
> v18: - added acked by
> - update no new privs numbers
> v17: - remove @compat note and add Pitfalls section for arch checking
> (keescook@...omium.org)
> v16: -
> v15: -
> v14: - rebase/nochanges
> v13: - rebase on to 88ebdda6159ffc15699f204c33feb3e431bf9bdc
> v12: - comment on the ptrace_event use
> - update arch support comment
> - note the behavior of SECCOMP_RET_DATA when there are multiple filters
> (keescook@...omium.org)
> - lots of samples/ clean up incl 64-bit bpf-direct support
> (markus@...omium.org)
> - rebase to linux-next
> v11: - overhaul return value language, updates (keescook@...omium.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@...ux.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@....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@...hat.com)
> - language cleanup (rdunlap@...otime.net)
> v4: - update for no_new_privs use
> - minor tweaks
> v3: - call out BPF <-> Berkeley Packet Filter (rdunlap@...otime.net)
> - document use of tentative always-unprivileged
> - guard sample compilation for i386 and x86_64
> v2: - move code to samples (corbet@....net)
> ---
> Documentation/prctl/seccomp_filter.txt | 163 ++++++++++++++++++++++
> samples/Makefile | 2 +-
> samples/seccomp/Makefile | 38 +++++
> samples/seccomp/bpf-direct.c | 176 +++++++++++++++++++++++
> samples/seccomp/bpf-fancy.c | 102 ++++++++++++++
> samples/seccomp/bpf-helper.c | 89 ++++++++++++
> samples/seccomp/bpf-helper.h | 238 ++++++++++++++++++++++++++++++++
> samples/seccomp/dropper.c | 68 +++++++++
> 8 files changed, 875 insertions(+), 1 deletions(-)
> create mode 100644 Documentation/prctl/seccomp_filter.txt
> create mode 100644 samples/seccomp/Makefile
> create mode 100644 samples/seccomp/bpf-direct.c
> create mode 100644 samples/seccomp/bpf-fancy.c
> create mode 100644 samples/seccomp/bpf-helper.c
> create mode 100644 samples/seccomp/bpf-helper.h
> create mode 100644 samples/seccomp/dropper.c
>
> diff --git a/Documentation/prctl/seccomp_filter.txt b/Documentation/prctl/seccomp_filter.txt
> new file mode 100644
> index 0000000..597c3c5
> --- /dev/null
> +++ b/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.
> diff --git a/samples/Makefile b/samples/Makefile
> index 2f75851..5ef08bb 100644
> --- a/samples/Makefile
> +++ b/samples/Makefile
> @@ -1,4 +1,4 @@
> # Makefile for Linux samples code
>
> obj-$(CONFIG_SAMPLES) += kobject/ kprobes/ tracepoints/ trace_events/ \
> - hw_breakpoint/ kfifo/ kdb/ hidraw/ rpmsg/
> + hw_breakpoint/ kfifo/ kdb/ hidraw/ rpmsg/ seccomp/
> diff --git a/samples/seccomp/Makefile b/samples/seccomp/Makefile
> new file mode 100644
> index 0000000..e8fe0f5
> --- /dev/null
> +++ b/samples/seccomp/Makefile
> @@ -0,0 +1,38 @@
> +# kbuild trick to avoid linker error. Can be omitted if a module is built.
> +obj- := dummy.o
> +
> +hostprogs-$(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)
> diff --git a/samples/seccomp/bpf-direct.c b/samples/seccomp/bpf-direct.c
> new file mode 100644
> index 0000000..26f523e
> --- /dev/null
> +++ b/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@...omium.org>
> + * Author: Will Drewry <wad@...omium.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;
> +}
> diff --git a/samples/seccomp/bpf-fancy.c b/samples/seccomp/bpf-fancy.c
> new file mode 100644
> index 0000000..8eb483a
> --- /dev/null
> +++ b/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@...omium.org>
> + * Author: Will Drewry <wad@...omium.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;
> +}
> diff --git a/samples/seccomp/bpf-helper.c b/samples/seccomp/bpf-helper.c
> new file mode 100644
> index 0000000..579cfe3
> --- /dev/null
> +++ b/samples/seccomp/bpf-helper.c
> @@ -0,0 +1,89 @@
> +/*
> + * Seccomp BPF helper functions
> + *
> + * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@...omium.org>
> + * Author: Will Drewry <wad@...omium.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);
> +}
> diff --git a/samples/seccomp/bpf-helper.h b/samples/seccomp/bpf-helper.h
> new file mode 100644
> index 0000000..643279d
> --- /dev/null
> +++ b/samples/seccomp/bpf-helper.h
> @@ -0,0 +1,238 @@
> +/*
> + * Example wrapper around BPF macros.
> + *
> + * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@...omium.org>
> + * Author: Will Drewry <wad@...omium.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__ */
> diff --git a/samples/seccomp/dropper.c b/samples/seccomp/dropper.c
> new file mode 100644
> index 0000000..c69c347
> --- /dev/null
> +++ b/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@...omium.org>
> + * Author: Will Drewry <wad@...omium.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;
> +}
> --
> 1.7.5.4
>
> --
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