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seccomp_filter.c
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seccomp_filter.c
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#include <unistd.h>
#include <stdbool.h>
#include <signal.h>
#include <stddef.h>
#include <sys/prctl.h>
#include <sys/ptrace.h>
#include <linux/seccomp.h>
#include <linux/filter.h>
#include <linux/limits.h>
#include <linux/bpf.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include <sys/syscall.h>
#include "zdtmtst.h"
const char *test_doc = "Check that SECCOMP_MODE_FILTER is restored";
const char *test_author = "Tycho Andersen <tycho.andersen@canonical.com>";
int get_seccomp_mode(pid_t pid)
{
FILE *f;
char buf[PATH_MAX];
sprintf(buf, "/proc/%d/status", pid);
f = fopen(buf, "r+");
if (!f) {
pr_perror("fopen failed");
return -1;
}
while (NULL != fgets(buf, sizeof(buf), f)) {
int mode;
if (sscanf(buf, "Seccomp:\t%d", &mode) != 1)
continue;
fclose(f);
return mode;
}
fclose(f);
return -1;
}
int filter_syscall(int syscall_nr)
{
struct sock_filter filter[] = {
BPF_STMT(BPF_LD+BPF_W+BPF_ABS, offsetof(struct seccomp_data, nr)),
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, syscall_nr, 0, 1),
BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
};
struct sock_fprog bpf_prog = {
.len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
.filter = filter,
};
if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &bpf_prog) < 0) {
pr_err("prctl failed");
return -1;
}
return 0;
}
int main(int argc, char ** argv)
{
pid_t pid;
int mode, status;
int sk_pair[2], sk, ret;
char c = 'K';
test_init(argc, argv);
if (socketpair(PF_LOCAL, SOCK_SEQPACKET, 0, sk_pair)) {
pr_perror("socketpair");
return -1;
}
pid = fork();
if (pid < 0) {
pr_perror("fork");
return -1;
}
if (pid == 0) {
sk = sk_pair[1];
close(sk_pair[0]);
/*
* Let's install a few filters separately to make sure the
* chaining actually works.
*/
if (filter_syscall(__NR_ptrace) < 0)
_exit(1);
if (filter_syscall(__NR_getpid) < 0)
_exit(1);
/* FIXME: seccomp requires a task to be root in its user ns in
* order to install filters for security reasons, so that
* unprivileged parents cannot take over privileged childen.
* However, we restore euids before we restore seccomp filters,
* so if someone does a setuid(1000) here, the restore will
* fail. We need to reorder some things so that the other creds
* restore takes place after seccomp state is set; except that
* the tasks need to be ptraced so the seccomp filters
* potentially don't kill the task for calling setuid().
*/
zdtm_seccomp = 1;
test_msg("SECCOMP_MODE_FILTER is enabled\n");
if (write(sk, &c, 1) != 1) {
pr_perror("write");
_exit(1);
}
if (read(sk, &c, 1) != 1) {
pr_perror("read");
_exit(1);
}
/* We expect to be killed by our policy above. */
ptrace(PTRACE_TRACEME);
syscall(__NR_exit, 0);
}
sk = sk_pair[0];
close(sk_pair[1]);
if ((ret = read(sk, &c, 1)) != 1) {
pr_perror("read %d", ret);
goto err;
}
test_daemon();
test_waitsig();
mode = get_seccomp_mode(pid);
if (write(sk, &c, 1) != 1) {
pr_perror("write");
goto err;
}
if (waitpid(pid, &status, 0) != pid) {
pr_perror("waitpid");
exit(1);
}
if (WTERMSIG(status) != SIGSYS) {
pr_perror("expected SIGSYS, got %d\n", WTERMSIG(status));
exit(1);
}
if (mode != SECCOMP_MODE_FILTER) {
fail("seccomp mode mismatch %d\n", mode);
return 1;
}
pass();
return 0;
err:
kill(pid, SIGKILL);
return 1;
}