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

[David Hildenbrand <david@redhat.com>]

On 22.01.25 07:11, 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>
> ---
>   mm/page_isolation.c | 9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index 7e04047977cf..2a38f429defb 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -83,7 +83,14 @@ static struct page *has_unmovable_pages(unsigned long start_pfn, unsigned long e
>   			unsigned int skip_pages;
>   
>   			if (PageHuge(page)) {
> -				if (!hugepage_migration_supported(folio_hstate(folio)))
> +				struct hstate *h;
> +
> +				/*
> +				 * The huge page may be freed so can not
> +				 * use folio_hstate() directly.
> +				 */
> +				h = size_to_hstate(folio_size(folio));
> +				if (h && !hugepage_migration_supported(h))
>   					return page;

So in case we trigger the race as described, we assume the page is 
movable (just freed to the buddy). Makes sense to me.

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb