Re: [PATCH] mm: page_isolation: avoid call folio_hstate() without hugetlb_lock

[Liu Shixin <liushixin2@huawei.com>]

On 2025/1/23 17:55, Oscar Salvador wrote:
> On Wed, Jan 22, 2025 at 02:11:51PM +0800, Liu Shixin wrote:
>> I found a NULL pointer dereference as followed:
>>
>>  BUG: kernel NULL pointer dereference, address: 0000000000000028
>>  #PF: supervisor read access in kernel mode
>>  #PF: error_code(0x0000) - not-present page
>>  PGD 0 P4D 0
>>  Oops: Oops: 0000 [#1] SMP PTI
>>  CPU: 5 UID: 0 PID: 5964 Comm: sh Kdump: loaded Not tainted 6.13.0-dirty #20
>>  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.
>>  RIP: 0010:has_unmovable_pages+0x184/0x360
>>  ...
>>  Call Trace:
>>   <TASK>
>>   set_migratetype_isolate+0xd1/0x180
>>   start_isolate_page_range+0xd2/0x170
>>   alloc_contig_range_noprof+0x101/0x660
>>   alloc_contig_pages_noprof+0x238/0x290
>>   alloc_gigantic_folio.isra.0+0xb6/0x1f0
>>   only_alloc_fresh_hugetlb_folio.isra.0+0xf/0x60
>>   alloc_pool_huge_folio+0x80/0xf0
>>   set_max_huge_pages+0x211/0x490
>>   __nr_hugepages_store_common+0x5f/0xe0
>>   nr_hugepages_store+0x77/0x80
>>   kernfs_fop_write_iter+0x118/0x200
>>   vfs_write+0x23c/0x3f0
>>   ksys_write+0x62/0xe0
>>   do_syscall_64+0x5b/0x170
>>   entry_SYSCALL_64_after_hwframe+0x76/0x7e
>>
>> As has_unmovable_pages() call folio_hstate() without hugetlb_lock, there
>> is a race to free the HugeTLB page between PageHuge() and folio_hstate().
>> There is no need to add hugetlb_lock here as the HugeTLB page can be freed
>> in lot of places. So it's enough to unfold folio_hstate() and add a check
>> to avoid NULL pointer dereference for hugepage_migration_supported().
>>
>> Fixes: 464c7ffbcb16 ("mm/hugetlb: filter out hugetlb pages if HUGEPAGE migration is not supported.")
>> Signed-off-by: Liu Shixin <liushixin2@huawei.com>
> I wonder whether we should place a comment in hugepage_migration_supported stating
> that the hstate _must_ be valid, as we do not perform any sanity check further
> down the road.
Most of the functions in hugetlb.h imply that hstate is valid, and in fact it does.
So maybe it's enough to comment just in the special caller.
>
> Reviewed-by: Oscar Salvador <osalvador@suse.de>
Thanks for the review.

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