C reproducer:
// autogenerated by syzkaller (http://github.com/google/syzkaller)

#define _GNU_SOURCE
#include <endian.h>
#include <sys/syscall.h>
#include <unistd.h>
#include <errno.h>
#include <sched.h>
#include <signal.h>
#include <stdarg.h>
#include <stdbool.h>
#include <stdio.h>
#include <sys/prctl.h>
#include <sys/resource.h>
#include <sys/time.h>
#include <sys/wait.h>

__attribute__((noreturn)) static void doexit(int status)
{
	volatile unsigned i;
	syscall(__NR_exit_group, status);
	for (i = 0;; i++) {
	}
}
#include <stdint.h>
#include <string.h>
#include <errno.h>
#include <stdarg.h>
#include <stdio.h>

const int kFailStatus = 67;
const int kRetryStatus = 69;

  static void fail(const char* msg, ...)
{
	int e = errno;
	va_list args;
	va_start(args, msg);
	vfprintf(stderr, msg, args);
	va_end(args);
	fprintf(stderr, " (errno %d)\n", e);
	doexit((e == ENOMEM || e == EAGAIN) ? kRetryStatus : kFailStatus);
}

static void loop();

static void sandbox_common()
{
	prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0);
	setpgrp();
	setsid();

	struct rlimit rlim;
	rlim.rlim_cur = rlim.rlim_max = 160 << 20;
	setrlimit(RLIMIT_AS, &rlim);
	rlim.rlim_cur = rlim.rlim_max = 8 << 20;
	setrlimit(RLIMIT_MEMLOCK, &rlim);
	rlim.rlim_cur = rlim.rlim_max = 136 << 20;
	setrlimit(RLIMIT_FSIZE, &rlim);
	rlim.rlim_cur = rlim.rlim_max = 1 << 20;
	setrlimit(RLIMIT_STACK, &rlim);
	rlim.rlim_cur = rlim.rlim_max = 0;
	setrlimit(RLIMIT_CORE, &rlim);

	if (unshare(CLONE_NEWNS)) {
	}
	if (unshare(CLONE_NEWIPC)) {
	}
	if (unshare(0x02000000)) {
	}
	if (unshare(CLONE_NEWUTS)) {
	}
	if (unshare(CLONE_SYSVSEM)) {
	}
}

static int do_sandbox_none(void)
{
	if (unshare(CLONE_NEWPID)) {
	}
	int pid = fork();
	if (pid < 0)
		fail("sandbox fork failed");
	if (pid)
		return pid;

	sandbox_common();
	if (unshare(CLONE_NEWNET)) {
	}
	loop();
	doexit(1);
}

uint64_t r[2] = {0xffffffffffffffff, 0xffffffffffffffff};
void loop()
{
	long res = 0;
memcpy((void*)0x20000040, "./file0", 8);
	res = syscall(__NR_open, 0x20000040, 0x43fd, 0xfffffffffffffffe);
	if (res != -1)
		r[0] = res;
memcpy((void*)0x20000075, "/lib/x86_64-linux-gnu/libc.so.6", 32);
	res = syscall(__NR_open, 0x20000075, 0, 0);
	if (res != -1)
		r[1] = res;
	syscall(__NR_close, r[1]);
memcpy((void*)0x20000180, "./file0", 8);
	syscall(__NR_open, 0x20000180, 0, 8);
	syscall(__NR_mmap, 0x20002000, 0x2000, 0x40000000005, 0x11, r[1], 0x4000);
*(uint64_t*)0x20000000 = 0;
	syscall(__NR_write, r[0], 0x20000000, 0xd996d7aa);
}

int main()
{
	syscall(__NR_mmap, 0x20000000, 0x1000000, 3, 0x32, -1, 0);
	int pid = do_sandbox_none();
	int status = 0;
	while (waitpid(pid, &status, __WALL) != pid) {}
	return 0;
}