linux/samples/seccomp/bpf-direct.c
Will Drewry 8ac270d1e2 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>
2012-04-14 11:13:22 +10:00

177 lines
4.6 KiB
C

/*
* 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;
}