[BUG] mm/hugetlb: UAF after handle_userfault

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In hugetlb_handle_userfault, we call i_mmap_unlock_read before handle_userfault,
and call i_mmap_lock_read after it to make calling code simpler.
But the mapping variable can be invalid as vma may be changed
after mmap lock is dropped during handle_userfault.

This bug is found by syzkaller with the following operations:

mmap$IORING_OFF_SQ_RING(&(0x7f0000400000/0xc00000)=nil, 0xc00000, 0x0, 0x44832, 0xffffffffffffffff, 0x0)
r0 = userfaultfd(0x0)
r1 = openat(0xffffffffffffff9c, &(0x7f00000000c0)='./file2\x00', 0x59042, 0x0)
ftruncate(r1, 0x8)
r2 = mmap$IORING_OFF_SQ_RING(&(0x7f0000400000/0xc00000)=nil, 0xc00000, 0x2000002, 0x44832, r1, 0x0)
syz_memcpy_off$IO_URING_METADATA_FLAGS(r2, 0x0, &(0x7f0000000080), 0x0, 0x4)
syz_memcpy_off$IO_URING_METADATA_FLAGS(r2, 0x118, &(0x7f0000000280)=0x1, 0x0, 0x4)
ioctl$UFFDIO_API(r0, 0xc018aa3f, &(0x7f0000000180))
mprotect(&(0x7f0000ffb000/0x2000)=nil, 0x2000, 0x0)
ioctl$UFFDIO_REGISTER(r0, 0xc020aa00, &(0x7f0000000080)={{&(0x7f0000800000/0x800000)=nil, 0x800000}, 0x1})
mprotect(&(0x7f0000971000/0x3000)=nil, 0x3000, 0x0)
ioctl$UFFDIO_ZEROPAGE(r0, 0xc020aa04, &(0x7f0000000000)={{&(0x7f0000b35000/0x1000)=nil, 0x1000}})
r3 = fsmount(0xffffffffffffffff, 0x1, 0x0)
syz_kvm_setup_cpu$arm64(0xffffffffffffffff, r3, &(0x7f0000dbc000/0x18000)=nil, &(0x7f00000000c0)=[{0x0, &(0x7f0000000040)="17eb9261c249c6672ae95a46cbb140cbc2b0a3b8778ee972d27c7edcf4af5d7071ceebe27cda5f06301fa270d201883b9244", 0x32}], 0x1, 0x0, &(0x7f0000000100), 0x1)
r4 = openat(0xffffffffffffff9c, &(0x7f00000000c0)='./file2\x00', 0x59042, 0x0)
ftruncate(r4, 0x8)
mmap$IORING_OFF_SQ_RING(&(0x7f0000400000/0xc00000)=nil, 0xc00000, 0x2000002, 0x44832, r4, 0x0)

KASAN report is here:

[  476.621494] ==================================================================
[  476.622067] BUG: KASAN: use-after-free in down_read+0x80/0x1d0
[  476.622530] Write of size 8 at addr ffff0000cce177e0 by task syz-executor/15650

[  476.623305] CPU: 3 PID: 15650 Comm: syz-executor Kdump: loaded Not tainted 6.0.0-rc5+ #4
[  476.624130] Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
[  476.624834] Call trace:
[  476.625135]  dump_backtrace+0xe4/0x124
[  476.625586]  show_stack+0x20/0x5c
[  476.625983]  dump_stack_lvl+0x68/0x84
[  476.626415]  print_address_description.constprop.0+0x78/0x360
[  476.627046]  print_report+0x108/0x130
[  476.627473]  kasan_report+0x84/0x120
[  476.627892]  kasan_check_range+0xe4/0x190
[  476.628351]  __kasan_check_write+0x28/0x3c
[  476.628818]  down_read+0x80/0x1d0
[  476.629210]  hugetlb_handle_userfault+0x120/0x1a0
[  476.629748]  hugetlb_no_page+0x618/0x6a0
[  476.630202]  hugetlb_fault+0x1d4/0x5f4
[  476.630642]  handle_mm_fault+0x23c/0x2d0
[  476.631102]  do_page_fault+0x1c0/0x5b0
[  476.631542]  do_translation_fault+0x8c/0xc0
[  476.632022]  do_mem_abort+0x68/0xfc
[  476.632444]  el0_da+0x40/0xd0
[  476.632810]  el0t_64_sync_handler+0x68/0xc0
[  476.633292]  el0t_64_sync+0x180/0x184

[  476.633953] Allocated by task 15650:
[  476.634368]  kasan_save_stack+0x2c/0x54
[  476.634819]  __kasan_slab_alloc+0x6c/0x90
[  476.635285]  kmem_cache_alloc_lru+0x100/0x290
[  476.635784]  hugetlbfs_alloc_inode+0xac/0x2a0
[  476.636287]  alloc_inode+0x44/0x124
[  476.636706]  new_inode+0x7c/0x170
[  476.637105]  hugetlbfs_get_inode+0x158/0x2a0
[  476.637601]  hugetlb_file_setup+0xcc/0x49c
[  476.638073]  ksys_mmap_pgoff+0xe8/0x270
[  476.638521]  __arm64_sys_mmap+0x8c/0xb0
[  476.638970]  invoke_syscall+0x68/0x1a0
[  476.639412]  el0_svc_common.constprop.0+0x1e4/0x220
[  476.639962]  do_el0_svc+0x3c/0x50
[  476.640358]  el0_svc+0x28/0xd0
[  476.640731]  el0t_64_sync_handler+0xb8/0xc0
[  476.641214]  el0t_64_sync+0x180/0x184

[  476.641868] Freed by task 0:
[  476.642222]  kasan_save_stack+0x2c/0x54
[  476.642673]  kasan_set_track+0x2c/0x40
[  476.643114]  kasan_set_free_info+0x28/0x50
[  476.643586]  ____kasan_slab_free+0x140/0x1d0
[  476.644076]  __kasan_slab_free+0x18/0x2c
[  476.644533]  kmem_cache_free+0x178/0x3f0
[  476.644990]  hugetlbfs_free_inode+0x24/0x30
[  476.645472]  i_callback+0x34/0x64
[  476.645874]  rcu_do_batch+0x240/0x660
[  476.646307]  rcu_core+0x1ec/0x270
[  476.646708]  rcu_core_si+0x18/0x24
[  476.647115]  __do_softirq+0x19c/0x540

[  476.647765] Last potentially related work creation:
[  476.648313]  kasan_save_stack+0x2c/0x54
[  476.648759]  __kasan_record_aux_stack+0xa0/0xd0
[  476.649276]  kasan_record_aux_stack_noalloc+0x14/0x20
[  476.649858]  call_rcu+0xbc/0x370
[  476.650249]  destroy_inode+0x9c/0xc4
[  476.650674]  evict+0x2cc/0x3a0
[  476.651048]  iput_final+0x144/0x314
[  476.651463]  iput.part.0+0x114/0x130
[  476.651887]  iput+0x38/0x50
[  476.652235]  dentry_unlink_inode+0x150/0x210
[  476.652729]  __dentry_kill+0x1f8/0x3a0
[  476.653168]  dentry_kill+0x390/0x500
[  476.653601]  dput+0x304/0x354
[  476.653967]  __fput+0x138/0x360
[  476.654352]  ____fput+0x18/0x24
[  476.654736]  task_work_run+0x17c/0x340
[  476.655181]  ptrace_notify+0x1d0/0x1e0
[  476.655625]  syscall_trace_exit+0x1e4/0x320
[  476.656107]  el0_svc_common.constprop.0+0xc8/0x220
[  476.656649]  do_el0_svc+0x3c/0x50
[  476.657047]  el0_svc+0x28/0xd0
[  476.657423]  el0t_64_sync_handler+0xb8/0xc0
[  476.657918]  el0t_64_sync+0x180/0x184

[  476.658567] Second to last potentially related work creation:
[  476.659196]  kasan_save_stack+0x2c/0x54
[  476.659644]  __kasan_record_aux_stack+0xa0/0xd0
[  476.660162]  kasan_record_aux_stack_noalloc+0x14/0x20
[  476.660728]  call_rcu+0xbc/0x370
[  476.661117]  destroy_inode+0x9c/0xc4
[  476.661544]  evict+0x2cc/0x3a0
[  476.661917]  iput_final+0x144/0x314
[  476.662334]  iput.part.0+0x114/0x130
[  476.662759]  iput+0x38/0x50
[  476.663107]  dentry_unlink_inode+0x150/0x210
[  476.663599]  __dentry_kill+0x1f8/0x3a0
[  476.664039]  dentry_kill+0x390/0x500
[  476.664463]  dput+0x304/0x354
[  476.664828]  __fput+0x138/0x360
[  476.665211]  ____fput+0x18/0x24
[  476.665600]  task_work_run+0x17c/0x340
[  476.666041]  do_notify_resume+0x230/0x320
[  476.666509]  el0_svc+0x8c/0xd0
[  476.666883]  el0t_64_sync_handler+0xb8/0xc0
[  476.667367]  el0t_64_sync+0x180/0x184

[  476.668016] The buggy address belongs to the object at ffff0000cce17600
                which belongs to the cache hugetlbfs_inode_cache of size 640
[  476.669451] The buggy address is located 480 bytes inside of
                640-byte region [ffff0000cce17600, ffff0000cce17880)

[  476.670958] The buggy address belongs to the physical page:
[  476.671575] page:0000000032d18387 refcount:1 mapcount:0 mapping:0000000000000000 index:0xffff0000cce14600 pfn:0x10ce14
[  476.672686] head:0000000032d18387 order:2 compound_mapcount:0 compound_pincount:0
[  476.673486] memcg:ffff0000c500ec01
[  476.673893] flags: 0x17ffff800010200(slab|head|node=0|zone=2|lastcpupid=0xfffff)
[  476.674692] raw: 017ffff800010200 fffffc00005f1300 dead000000000002 ffff0000c15b5680
[  476.675517] raw: ffff0000cce14600 000000008015000f 00000001ffffffff ffff0000c500ec01
[  476.676339] page dumped because: kasan: bad access detected

[  476.677170] Memory state around the buggy address:
[  476.677715]  ffff0000cce17680: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[  476.678489]  ffff0000cce17700: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[  476.679265] >ffff0000cce17780: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[  476.680034]                                                        ^
[  476.680725]  ffff0000cce17800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[  476.681498]  ffff0000cce17880: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[  476.682271] ==================================================================



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