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

[Oscar Salvador <osalvador@suse.de>]

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.

Reviewed-by: Oscar Salvador <osalvador@suse.de>


-- 
Oscar Salvador
SUSE Labs