linux-kernel - Re: perf_event_open+clone = unkillable process

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Date:   Fri, 1 Feb 2019 18:06:03 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        jolsa@...hat.com, Namhyung Kim <namhyung@...nel.org>,
        luca abeni <luca.abeni@...tannapisa.it>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     syzkaller <syzkaller@...glegroups.com>
Subject: Re: perf_event_open+clone = unkillable process

On Fri, Feb 1, 2019 at 5:48 PM Dmitry Vyukov <dvyukov@...gle.com> wrote:
>
> Hello,
>
> The following program creates an unkillable process that eats CPU.
> /proc/pid/stack is empty, I am not sure what other info I can provide.
>
> Tested is on upstream commit 4aa9fc2a435abe95a1e8d7f8c7b3d6356514b37a.
> Config is attached.

Looking through other reproducers that create unkillable processes, I
think I found a much simpler reproducer (below). It's single threaded
and just setups SIGBUS handler and does timer_create+timer_settime to
send repeated SIGBUS. The resulting process can't be killed with
SIGKILL.
+Thomas for timers.


// autogenerated by syzkaller (https://github.com/google/syzkaller)

#define _GNU_SOURCE

#include <endian.h>
#include <setjmp.h>
#include <signal.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>

static __thread int skip_segv;
static __thread jmp_buf segv_env;

static void segv_handler(int sig, siginfo_t* info, void* ctx)
{
  uintptr_t addr = (uintptr_t)info->si_addr;
  const uintptr_t prog_start = 1 << 20;
  const uintptr_t prog_end = 100 << 20;
  if (__atomic_load_n(&skip_segv, __ATOMIC_RELAXED) &&
      (addr < prog_start || addr > prog_end)) {
    _longjmp(segv_env, 1);
  }
  exit(sig);
}

static void install_segv_handler(void)
{
  struct sigaction sa;
  memset(&sa, 0, sizeof(sa));
  sa.sa_handler = SIG_IGN;
  syscall(SYS_rt_sigaction, 0x20, &sa, NULL, 8);
  syscall(SYS_rt_sigaction, 0x21, &sa, NULL, 8);
  memset(&sa, 0, sizeof(sa));
  sa.sa_sigaction = segv_handler;
  sa.sa_flags = SA_NODEFER | SA_SIGINFO;
  sigaction(SIGSEGV, &sa, NULL);
  sigaction(SIGBUS, &sa, NULL);
}

int main(void)
{
  syscall(__NR_mmap, 0x20000000, 0x1000000, 3, 0x32, -1, 0);
  install_segv_handler();
  int tid = syscall(__NR_gettid);
  *(uint64_t*)0x20000000 = 0;
  *(uint32_t*)0x20000008 = 7;
  *(uint32_t*)0x2000000c = 4;
  *(uint32_t*)0x20000010 = tid;
  syscall(__NR_timer_create, 0, 0x20000000, 0x20000080);
  *(uint64_t*)0x2004a000 = 0;
  *(uint64_t*)0x2004a008 = 1;
  *(uint64_t*)0x2004a010 = 7;
  *(uint64_t*)0x2004a018 = 0xe4c;
  syscall(__NR_timer_settime, 0, 3, 0x2004a000, 0);
  return 0;
}