Re: [PATCH] mm/hugetlb: fix null ptr defer in hugetlb_vma_lock_write

["Yin, Fengwei" <nh26223@qq.com>]

On 1/11/2023 22:27, Rik van Riel wrote:
> On Wed, 2023-11-01 at 14:36 +0800, Edward Adam Davis wrote:
>> When obtaining resv_map from vma, it is necessary to simultaneously
>> determine
>> the flag HPAGE_RESV_OWNER of vm_private_data.
>> Only when they are met simultaneously, resv_map is valid.
>>
>> Reported-and-tested-by:
>> syzbot+6ada951e7c0f7bc8a71e@syzkaller.appspotmail.com
>> Fixes: bf4916922c60 ("hugetlbfs: extend hugetlb_vma_lock to private
>> VMAs")
>> Signed-off-by: Edward Adam Davis <eadavis@qq.com>
>> ---
>>   include/linux/hugetlb.h | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
>> index 47d25a5e1933..1a3ec1aee1a3 100644
>> --- a/include/linux/hugetlb.h
>> +++ b/include/linux/hugetlb.h
>> @@ -1265,9 +1265,11 @@ static inline bool __vma_shareable_lock(struct
>> vm_area_struct *vma)
>>          return (vma->vm_flags & VM_MAYSHARE) && vma->vm_private_data;
>>   }
>>   
>> +#define HPAGE_RESV_OWNER    (1UL << 0)
>>   static inline bool __vma_private_lock(struct vm_area_struct *vma)
>>   {
>> -       return (!(vma->vm_flags & VM_MAYSHARE)) && vma-
>>> vm_private_data;
>> +       return (!(vma->vm_flags & VM_MAYSHARE)) && vma-
>>> vm_private_data &&
>> +               ((unsigned long)vma->vm_private_data &
>> HPAGE_RESV_OWNER);
>>   }
I am wondering whether this line should be:
    return (!(vma->vm_flags & VM_MAYSHARE)) &&
	((unsigned long)vma->vm_private_data & HPAGE_RESV_OWNER);

or even:
    return (unsigned long)vma->vm_private_data & HPAGE_RESV_OWNER;

because mm/hugetlb.c has:
	"We know private mapping must have HPAGE_RESV_OWNER set."

> 
> This could be cleaned up a bit by moving the HPAGE_RESV_OWNER
> definition (and its friends) into hugetlb.h, as well as the
> is_vma_resv_set() helper function.
> 
> Then __vma_private_lock() can just call is_vma_resv_set(),
> and open coding a duplicate of the same code.
> 
> Not having duplicates of the code will make it much harder
> to "miss a spot" with future changes.
> 
> I am still struggling to find a place where we might leave
> HPAGE_RESV_OWNER behind on a pointer that is otherwise NULL,
> but if your tests show this fixes the issue, I'm all for it :)
> 
Like you said, vma->vm_private_data is cleared during fork().

But I saw following possible code path in child process:
hugetlb_wp()
     unmap_ref_private() if alloc_hugetlb_folio fails
         unmap_hugepage_range()
             __unmap_hugepage_range()
                 set_vma_resv_flags(vma, HPAGE_RESV_UNMAPPED)

vm_private_data become 0x2 which is none NULL without HPAGE_RESV_OWNER
bit.