Re: [PATCH] mm,madvise,hugetlb: check for 0-length range after end address adjustment

["Ricardo Cañuelo Navarro" <rcn@igalia.com>]

Hi all,

Some more context about the patch. The issue (WARNING in
madvise_vma_behavior) was found by a private syzbot instance, so I can't
share the link, but it can be triggered by an unprivileged user with
this reproducer:

--8<------------------------------------------------------------------

#define _GNU_SOURCE

#include <endian.h>
#include <errno.h>
#include <fcntl.h>
#include <pthread.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <time.h>
#include <unistd.h>

#include <linux/futex.h>

#ifndef __NR_userfaultfd
#define __NR_userfaultfd 323
#endif

static void sleep_ms(uint64_t ms)
{
  usleep(ms * 1000);
}

static uint64_t current_time_ms(void)
{
  struct timespec ts;
  if (clock_gettime(CLOCK_MONOTONIC, &ts))
    exit(1);
  return (uint64_t)ts.tv_sec * 1000 + (uint64_t)ts.tv_nsec / 1000000;
}

static void thread_start(void* (*fn)(void*), void* arg)
{
  pthread_t th;
  pthread_attr_t attr;
  pthread_attr_init(&attr);
  pthread_attr_setstacksize(&attr, 128 << 10);
  int i = 0;
  for (; i < 100; i++) {
    if (pthread_create(&th, &attr, fn, arg) == 0) {
      pthread_attr_destroy(&attr);
      return;
    }
    if (errno == EAGAIN) {
      usleep(50);
      continue;
    }
    break;
  }
  exit(1);
}

typedef struct {
  int state;
} event_t;

static void event_init(event_t* ev)
{
  ev->state = 0;
}

static void event_reset(event_t* ev)
{
  ev->state = 0;
}

static void event_set(event_t* ev)
{
  if (ev->state)
    exit(1);
  __atomic_store_n(&ev->state, 1, __ATOMIC_RELEASE);
  syscall(SYS_futex, &ev->state, FUTEX_WAKE | FUTEX_PRIVATE_FLAG, 1000000);
}

static void event_wait(event_t* ev)
{
  while (!__atomic_load_n(&ev->state, __ATOMIC_ACQUIRE))
    syscall(SYS_futex, &ev->state, FUTEX_WAIT | FUTEX_PRIVATE_FLAG, 0, 0);
}

static int event_isset(event_t* ev)
{
  return __atomic_load_n(&ev->state, __ATOMIC_ACQUIRE);
}

static int event_timedwait(event_t* ev, uint64_t timeout)
{
  uint64_t start = current_time_ms();
  uint64_t now = start;
  for (;;) {
    uint64_t remain = timeout - (now - start);
    struct timespec ts;
    ts.tv_sec = remain / 1000;
    ts.tv_nsec = (remain % 1000) * 1000 * 1000;
    syscall(SYS_futex, &ev->state, FUTEX_WAIT | FUTEX_PRIVATE_FLAG, 0, &ts);
    if (__atomic_load_n(&ev->state, __ATOMIC_ACQUIRE))
      return 1;
    now = current_time_ms();
    if (now - start > timeout)
      return 0;
  }
}

struct thread_t {
  int created, call;
  event_t ready, done;
};

static struct thread_t threads[16];
static void execute_call(int call);
static int running;

static void* thr(void* arg)
{
  struct thread_t* th = (struct thread_t*)arg;
  for (;;) {
    event_wait(&th->ready);
    event_reset(&th->ready);
    execute_call(th->call);
    __atomic_fetch_sub(&running, 1, __ATOMIC_RELAXED);
    event_set(&th->done);
  }
  return 0;
}

static void loop(void)
{
  if (write(1, "executing program\n", sizeof("executing program\n") - 1)) {
  }
  int i, call, thread;
  for (call = 0; call < 8; call++) {
	  for (thread = 0; thread < (int)(sizeof(threads) / sizeof(threads[0]));
	       thread++) {
		  struct thread_t* th = &threads[thread];
		  if (!th->created) {
			  th->created = 1;
			  event_init(&th->ready);
			  event_init(&th->done);
			  event_set(&th->done);
			  thread_start(thr, th);
		  }
		  if (!event_isset(&th->done))
			  continue;
		  event_reset(&th->done);
		  th->call = call;
		  __atomic_fetch_add(&running, 1, __ATOMIC_RELAXED);
		  event_set(&th->ready);
		  event_timedwait(&th->done, 50);
		  break;
	  }
  }
  for (i = 0; i < 100 && __atomic_load_n(&running, __ATOMIC_RELAXED); i++)
    sleep_ms(1);
}

uint64_t r[1] = {0xffffffffffffffff};

void execute_call(int call)
{
  intptr_t res = 0;
  switch (call) {
  case 0:
    *(uint64_t*)0x20000040 = 0x20000004;
    *(uint32_t*)0x20000048 = 4;
    *(uint32_t*)0x2000004c = 2;
    *(uint32_t*)0x20000050 = 0;
    syscall(__NR_mq_notify, /*mqd=*/-1, /*notif=*/0x20000040ul);
    break;
  case 1:
    syscall(__NR_mremap, /*addr=*/0x20002000ul, /*len=*/0x3000ul,
            /*newlen=*/0x4000ul, /*flags=MREMAP_FIXED|MREMAP_MAYMOVE*/ 3ul,
            /*newaddr=*/0x20422000ul);
    break;
  case 2:
    res = syscall(__NR_userfaultfd,
                  /*flags=UFFD_USER_MODE_ONLY|O_CLOEXEC*/ 0x80001ul);
    if (res != -1)
      r[0] = res;
    break;
  case 3:
    *(uint64_t*)0x200000c0 = 0xaa;
    *(uint64_t*)0x200000c8 = 0xc;
    *(uint64_t*)0x200000d0 = 0;
    syscall(__NR_ioctl, /*fd=*/r[0], /*cmd=*/0xc018aa3f, /*arg=*/0x200000c0ul);
    break;
  case 4:
    *(uint64_t*)0x20000140 = 0x200e2000;
    *(uint64_t*)0x20000148 = 0xc00000;
    *(uint64_t*)0x20000150 = 1;
    *(uint64_t*)0x20000158 = 0;
    syscall(__NR_ioctl, /*fd=*/r[0], /*cmd=*/0xc020aa00, /*arg=*/0x20000140ul);
    break;
  case 5:
    syscall(
        __NR_mmap, /*addr=*/0x20000000ul, /*len=*/0x400000ul,
        /*prot=PROT_GROWSUP|PROT_SEM|PROT_WRITE|PROT_READ|PROT_EXEC*/
        0x200000ful,
        /*flags=MAP_SYNC|MAP_NONBLOCK|MAP_HUGETLB|MAP_FIXED|MAP_ANONYMOUS|0x2*/
        0xd0032ul, /*fd=*/-1, /*offset=*/0ul);
    break;
  case 6:
    syscall(__NR_madvise, /*addr=*/0x20000000ul, /*len=*/0x600003ul,
            /*advice=MADV_DONTNEED*/ 4ul);
    break;
  case 7:
    syscall(
        __NR_mmap, /*addr=*/0x20000000ul, /*len=*/0xff5000ul, /*prot=*/0ul,
        /*flags=MAP_POPULATE|MAP_NORESERVE|MAP_NONBLOCK|MAP_HUGETLB|MAP_FIXED|0x2000000000821*/
        0x200000005c831ul, /*fd=*/-1, /*offset=*/0ul);
    break;
  }
}
int main(void)
{
  syscall(__NR_mmap, /*addr=*/0x1ffff000ul, /*len=*/0x1000ul, /*prot=*/0ul,
          /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/ 0x32ul, /*fd=*/-1,
          /*offset=*/0ul);
  syscall(__NR_mmap, /*addr=*/0x20000000ul, /*len=*/0x1000000ul,
          /*prot=PROT_WRITE|PROT_READ|PROT_EXEC*/ 7ul,
          /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/ 0x32ul, /*fd=*/-1,
          /*offset=*/0ul);
  syscall(__NR_mmap, /*addr=*/0x21000000ul, /*len=*/0x1000ul, /*prot=*/0ul,
          /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/ 0x32ul, /*fd=*/-1,
          /*offset=*/0ul);
  loop();
  return 0;
}

--8<------------------------------------------------------------------

Cheers,
Ricardo