Re: [PATCH v1] mm/hugetlb: fix refs calculation from unaligned @vaddr

[Joao Martins <joao.m.martins@oracle.com>]

On 7/13/21 9:29 PM, Mike Kravetz wrote:
> On 7/13/21 8:24 AM, Joao Martins wrote:
>> commit 82e5d378b0e47 ("mm/hugetlb: refactor subpage recording")
>> refactored the count of subpages but missed an edge case when @vaddr is
>> not aligned to PAGE_SIZE e.g. when close to vma->vm_end. It would then
>> errousnly set @refs to 0 and record_subpages_vmas() wouldn't set the
>>  @pages array element to its value, consequently causing the reported
>> null-deref by syzbot.
>>
>> Fix it by aligning down @vaddr by PAGE_SIZE in @refs calculation.
> 
> Thanks for finding and fixing!
> 
>>
>> Reported-by: syzbot+a3fcd59df1b372066f5a@syzkaller.appspotmail.com
>> Fixes: 82e5d378b0e47 ("mm/hugetlb: refactor subpage recording")
>> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
>> ---
>> An alternate approach is to have record_subpages_vmas() iterate while
>> addr < vm_end and renaming @refs to nr_pages (which would limit how many
>> pages we should store). But I felt that this approach would be slightly
>> more convoluted?
> 
> I prefer the approach you have taken in this patch.
> 
OK.

>>
>> Side-Note: I could add a WARN_ON_ONCE(!refs) and/or create an
>> helper like vma_pages() but with a ulong addr argument e.g.
>> vma_pages_from(vma, vaddr).
> 
> IIUC, the only way refs could be zero is if there was error in
> caluclations within this routine.  Correct?

Right. Albeit vaddr initialization is originally set with gup() starting address.

> IMO, the only reason to add a warning would be if there are any assumptions
> based on things outside this routine which could cause refs to be zero.  
> 
/me nods

>> The syzbot repro no longer reproduces after this patch. Additionally, ran
>> the libhugetlbfs tests (which were passing without this), gup_test and an
>> extra gup_test extension that take an offset to exercise gup() starting
>> address not being page aligned.
>> ---
>>  mm/hugetlb.c | 5 +++--
>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index 924553aa8f78..dfc940d5221d 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -5440,8 +5440,9 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>>  			continue;
>>  		}
>>  
>> -		refs = min3(pages_per_huge_page(h) - pfn_offset,
>> -			    (vma->vm_end - vaddr) >> PAGE_SHIFT, remainder);
>> +		/* vaddr may not be aligned to PAGE_SIZE */
>> +		refs = min3(pages_per_huge_page(h) - pfn_offset, remainder,
>> +		    (vma->vm_end - ALIGN_DOWN(vaddr, PAGE_SIZE)) >> PAGE_SHIFT);
>>  
>>  		if (pages || vmas)
>>  			record_subpages_vmas(mem_map_offset(page, pfn_offset),
>>
> 
> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
> 
Thanks!