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>
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: Kevin F. Haggerty <haggertk@lineageos.org>
This commit is contained in:
Will Drewry
Francescodario Cuzzocrea
parent
4a9c54e896
commit
8297eb92f9
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@ -1,4 +1,4 @@
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# Makefile for Linux samples code
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obj-$(CONFIG_SAMPLES) += kobject/ kprobes/ tracepoints/ trace_events/ \
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hw_breakpoint/ kfifo/ kdb/ hidraw/ rpmsg/
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hw_breakpoint/ kfifo/ kdb/ hidraw/ rpmsg/ seccomp/
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@ -0,0 +1,38 @@
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# kbuild trick to avoid linker error. Can be omitted if a module is built.
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obj- := dummy.o
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hostprogs-$(CONFIG_SECCOMP) := bpf-fancy dropper
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bpf-fancy-objs := bpf-fancy.o bpf-helper.o
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HOSTCFLAGS_bpf-fancy.o += -I$(objtree)/usr/include
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HOSTCFLAGS_bpf-fancy.o += -idirafter $(objtree)/include
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HOSTCFLAGS_bpf-helper.o += -I$(objtree)/usr/include
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HOSTCFLAGS_bpf-helper.o += -idirafter $(objtree)/include
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HOSTCFLAGS_dropper.o += -I$(objtree)/usr/include
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HOSTCFLAGS_dropper.o += -idirafter $(objtree)/include
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dropper-objs := dropper.o
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# bpf-direct.c is x86-only.
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ifeq ($(SRCARCH),x86)
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# List of programs to build
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hostprogs-$(CONFIG_SECCOMP) += bpf-direct
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bpf-direct-objs := bpf-direct.o
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endif
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HOSTCFLAGS_bpf-direct.o += -I$(objtree)/usr/include
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HOSTCFLAGS_bpf-direct.o += -idirafter $(objtree)/include
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# Try to match the kernel target.
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ifeq ($(CONFIG_64BIT),)
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HOSTCFLAGS_bpf-direct.o += -m32
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HOSTCFLAGS_dropper.o += -m32
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HOSTCFLAGS_bpf-helper.o += -m32
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HOSTCFLAGS_bpf-fancy.o += -m32
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HOSTLOADLIBES_bpf-direct += -m32
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HOSTLOADLIBES_bpf-fancy += -m32
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HOSTLOADLIBES_dropper += -m32
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endif
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# Tell kbuild to always build the programs
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always := $(hostprogs-y)
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@ -0,0 +1,192 @@
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/*
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* Seccomp filter example for x86 (32-bit and 64-bit) with BPF macros
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*
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* Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
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* Author: Will Drewry <wad@chromium.org>
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*
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* The code may be used by anyone for any purpose,
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* and can serve as a starting point for developing
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* applications using prctl(PR_SET_SECCOMP, 2, ...).
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*/
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#if defined(__i386__) || defined(__x86_64__)
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#define SUPPORTED_ARCH 1
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#endif
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#if defined(SUPPORTED_ARCH)
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#define __USE_GNU 1
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#define _GNU_SOURCE 1
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#include <linux/types.h>
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#include <linux/filter.h>
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#include <linux/seccomp.h>
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#include <linux/unistd.h>
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#include <signal.h>
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#include <stdio.h>
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#include <stddef.h>
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#include <string.h>
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#include <sys/prctl.h>
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#include <unistd.h>
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#define syscall_arg(_n) (offsetof(struct seccomp_data, args[_n]))
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#define syscall_nr (offsetof(struct seccomp_data, nr))
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#if defined(__i386__)
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#define REG_RESULT REG_EAX
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#define REG_SYSCALL REG_EAX
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#define REG_ARG0 REG_EBX
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#define REG_ARG1 REG_ECX
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#define REG_ARG2 REG_EDX
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#define REG_ARG3 REG_ESI
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#define REG_ARG4 REG_EDI
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#define REG_ARG5 REG_EBP
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#elif defined(__x86_64__)
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#define REG_RESULT REG_RAX
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#define REG_SYSCALL REG_RAX
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#define REG_ARG0 REG_RDI
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#define REG_ARG1 REG_RSI
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#define REG_ARG2 REG_RDX
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#define REG_ARG3 REG_R10
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#define REG_ARG4 REG_R8
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#define REG_ARG5 REG_R9
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#else
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#error Unsupported platform
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#endif
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#ifndef PR_SET_NO_NEW_PRIVS
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#define PR_SET_NO_NEW_PRIVS 38
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#endif
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#ifndef SYS_SECCOMP
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#define SYS_SECCOMP 1
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#endif
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static void emulator(int nr, siginfo_t *info, void *void_context)
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{
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ucontext_t *ctx = (ucontext_t *)(void_context);
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int syscall;
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char *buf;
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ssize_t bytes;
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size_t len;
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if (info->si_code != SYS_SECCOMP)
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return;
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if (!ctx)
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return;
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syscall = ctx->uc_mcontext.gregs[REG_SYSCALL];
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buf = (char *) ctx->uc_mcontext.gregs[REG_ARG1];
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len = (size_t) ctx->uc_mcontext.gregs[REG_ARG2];
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if (syscall != __NR_write)
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return;
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if (ctx->uc_mcontext.gregs[REG_ARG0] != STDERR_FILENO)
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return;
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/* Redirect stderr messages to stdout. Doesn't handle EINTR, etc */
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ctx->uc_mcontext.gregs[REG_RESULT] = -1;
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if (write(STDOUT_FILENO, "[ERR] ", 6) > 0) {
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bytes = write(STDOUT_FILENO, buf, len);
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ctx->uc_mcontext.gregs[REG_RESULT] = bytes;
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}
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return;
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}
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static int install_emulator(void)
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{
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struct sigaction act;
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sigset_t mask;
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memset(&act, 0, sizeof(act));
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sigemptyset(&mask);
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sigaddset(&mask, SIGSYS);
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act.sa_sigaction = &emulator;
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act.sa_flags = SA_SIGINFO;
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if (sigaction(SIGSYS, &act, NULL) < 0) {
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perror("sigaction");
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return -1;
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}
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if (sigprocmask(SIG_UNBLOCK, &mask, NULL)) {
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perror("sigprocmask");
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return -1;
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}
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return 0;
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}
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static int install_filter(void)
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{
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struct sock_filter filter[] = {
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/* Grab the system call number */
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BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_nr),
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/* Jump table for the allowed syscalls */
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BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_rt_sigreturn, 0, 1),
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BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
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#ifdef __NR_sigreturn
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BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_sigreturn, 0, 1),
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BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
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#endif
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BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit_group, 0, 1),
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BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
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BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit, 0, 1),
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BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
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BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_read, 1, 0),
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BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_write, 3, 2),
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/* Check that read is only using stdin. */
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BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)),
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BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDIN_FILENO, 4, 0),
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BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
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/* Check that write is only using stdout */
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BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)),
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BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDOUT_FILENO, 1, 0),
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/* Trap attempts to write to stderr */
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BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDERR_FILENO, 1, 2),
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BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
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BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_TRAP),
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BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
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};
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struct sock_fprog prog = {
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.len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
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.filter = filter,
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};
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if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
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perror("prctl(NO_NEW_PRIVS)");
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return 1;
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}
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if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog)) {
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perror("prctl");
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return 1;
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}
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return 0;
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}
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#define payload(_c) (_c), sizeof((_c))
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int main(int argc, char **argv)
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{
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char buf[4096];
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ssize_t bytes = 0;
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if (install_emulator())
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return 1;
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if (install_filter())
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return 1;
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syscall(__NR_write, STDOUT_FILENO,
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payload("OHAI! WHAT IS YOUR NAME? "));
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bytes = syscall(__NR_read, STDIN_FILENO, buf, sizeof(buf));
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syscall(__NR_write, STDOUT_FILENO, payload("HELLO, "));
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syscall(__NR_write, STDOUT_FILENO, buf, bytes);
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syscall(__NR_write, STDERR_FILENO,
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payload("Error message going to STDERR\n"));
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return 0;
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}
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#else /* SUPPORTED_ARCH */
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/*
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* This sample is x86-only. Since kernel samples are compiled with the
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* host toolchain, a non-x86 host will result in using only the main()
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* below.
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*/
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int main(void)
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{
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return 1;
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}
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#endif /* SUPPORTED_ARCH */
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