mm/hugetlb: fix kernel NULL pointer dereference when replacing free hugetlb folios

In the Linux kernel, the following vulnerability has been resolved:

mm/hugetlb: fix kernel NULL pointer dereference when replacing free hugetlb folios

A kernel crash was observed when replacing free hugetlb folios:

BUG: kernel NULL pointer dereference, address: 0000000000000028
PGD 0 P4D 0
Oops: Oops: 0000 [#1] SMP NOPTI
CPU: 28 UID: 0 PID: 29639 Comm: test_cma.sh Tainted 6.15.0-rc6-zp #41 PREEMPT(voluntary)
RIP: 0010:alloc_and_dissolve_hugetlb_folio+0x1d/0x1f0
RSP: 0018:ffffc9000b30fa90 EFLAGS: 00010286
RAX: 0000000000000000 RBX: 0000000000342cca RCX: ffffea0043000000
RDX: ffffc9000b30fb08 RSI: ffffea0043000000 RDI: 0000000000000000
RBP: ffffc9000b30fb20 R08: 0000000000001000 R09: 0000000000000000
R10: ffff88886f92eb00 R11: 0000000000000000 R12: ffffea0043000000
R13: 0000000000000000 R14: 00000000010c0200 R15: 0000000000000004
FS: 00007fcda5f14740(0000) GS:ffff8888ec1d8000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000028 CR3: 0000000391402000 CR4: 0000000000350ef0
Call Trace:
<TASK>
replace_free_hugepage_folios+0xb6/0x100
alloc_contig_range_noprof+0x18a/0x590
? srso_return_thunk+0x5/0x5f
? down_read+0x12/0xa0
? srso_return_thunk+0x5/0x5f
cma_range_alloc.constprop.0+0x131/0x290
__cma_alloc+0xcf/0x2c0
cma_alloc_write+0x43/0xb0
simple_attr_write_xsigned.constprop.0.isra.0+0xb2/0x110
debugfs_attr_write+0x46/0x70
full_proxy_write+0x62/0xa0
vfs_write+0xf8/0x420
? srso_return_thunk+0x5/0x5f
? filp_flush+0x86/0xa0
? srso_return_thunk+0x5/0x5f
? filp_close+0x1f/0x30
? srso_return_thunk+0x5/0x5f
? do_dup2+0xaf/0x160
? srso_return_thunk+0x5/0x5f
ksys_write+0x65/0xe0
do_syscall_64+0x64/0x170
entry_SYSCALL_64_after_hwframe+0x76/0x7e

There is a potential race between __update_and_free_hugetlb_folio() and
replace_free_hugepage_folios():

CPU1 CPU2
__update_and_free_hugetlb_folio replace_free_hugepage_folios
folio_test_hugetlb(folio)
-- It's still hugetlb folio.

__folio_clear_hugetlb(folio)
hugetlb_free_folio(folio)
h = folio_hstate(folio)
-- Here, h is NULL pointer

When the above race condition occurs, folio_hstate(folio) returns NULL,
and subsequent access to this NULL pointer will cause the system to crash.
To resolve this issue, execute folio_hstate(folio) under the protection
of the hugetlb_lock lock, ensuring that folio_hstate(folio) does not
return NULL.