The patch titled Subject: mm/hugetlb: fix kernel NULL pointer dereference when replacing free hugetlb folios has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-hugetlb-fix-kernel-null-pointer-dereference-when-replacing-free-hugetlb-folios.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Ge Yang yangge1116@126.com Subject: mm/hugetlb: fix kernel NULL pointer dereference when replacing free hugetlb folios Date: Thu, 22 May 2025 11:22:17 +0800

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.

Link: https://lkml.kernel.org/r/1747884137-26685-1-git-send-email-yangge1116@126.c... Fixes: 04f13d241b8b ("mm: replace free hugepage folios after migration") Signed-off-by: Ge Yang yangge1116@126.com Reviewed-by: Muchun Song muchun.song@linux.dev Reviewed-by: Oscar Salvador osalvador@suse.de Cc: Baolin Wang baolin.wang@linux.alibaba.com Cc: Barry Song 21cnbao@gmail.com Cc: David Hildenbrand david@redhat.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/hugetlb.c | 8 ++++++++ 1 file changed, 8 insertions(+)

--- a/mm/hugetlb.c~mm-hugetlb-fix-kernel-null-pointer-dereference-when-replacing-free-hugetlb-folios +++ a/mm/hugetlb.c @@ -2949,12 +2949,20 @@ int replace_free_hugepage_folios(unsigne

while (start_pfn < end_pfn) { folio = pfn_folio(start_pfn); + + /* + * The folio might have been dissolved from under our feet, so make sure + * to carefully check the state under the lock. + */ + spin_lock_irq(&hugetlb_lock); if (folio_test_hugetlb(folio)) { h = folio_hstate(folio); } else { + spin_unlock_irq(&hugetlb_lock); start_pfn++; continue; } + spin_unlock_irq(&hugetlb_lock);

if (!folio_ref_count(folio)) { ret = alloc_and_dissolve_hugetlb_folio(h, folio, _

Patches currently in -mm which might be from yangge1116@126.com are

mm-hugetlb-fix-kernel-null-pointer-dereference-when-replacing-free-hugetlb-folios.patch