Re: [PATCH v2] mm: hugetlb: fix UAF in hugetlb_handle_userfault

[Mike Kravetz]

On 09/22/22 22:10, Liu Shixin wrote:
> The vma_lock and hugetlb_fault_mutex are dropped before handling
> userfault and reacquire them again after handle_userfault(), but
> reacquire the vma_lock could lead to UAF[1,2] due to the following
> race,
> 
> hugetlb_fault
>   hugetlb_no_page
>     /*unlock vma_lock */
>     hugetlb_handle_userfault
>       handle_userfault
>         /* unlock mm->mmap_lock*/
>                                            vm_mmap_pgoff
>                                              do_mmap
>                                                mmap_region
>                                                  munmap_vma_range
>                                                    /* clean old vma */
>         /* lock vma_lock again  <--- UAF */
>     /* unlock vma_lock */
> 
> Since the vma_lock will unlock immediately after hugetlb_handle_userfault(),
> let's drop the unneeded lock and unlock in hugetlb_handle_userfault() to fix
> the issue.
> 
> [1] https://lore.kernel.org/linux-mm/000000000000d5e00a05e834962e@xxxxxxxxxx/
> [2] https://lore.kernel.org/linux-mm/20220921014457.1668-1-liuzixian4@xxxxxxxxxx/
> Reported-by: syzbot+193f9cee8638750b23cf@xxxxxxxxxxxxxxxxxxxxxxxxx
> Reported-by: Liu Zixian <liuzixian4@xxxxxxxxxx>
> Fixes: 1a1aad8a9b7b ("userfaultfd: hugetlbfs: add userfaultfd hugetlb hook")
> CC: stable@xxxxxxxxxxxxxxx # 4.14+
> Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>
> Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
> ---
>  mm/hugetlb.c | 30 +++++++++++-------------------
>  1 file changed, 11 insertions(+), 19 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
<snip>
> @@ -5792,11 +5786,9 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>  
>  	entry = huge_ptep_get(ptep);
>  	/* PTE markers should be handled the same way as none pte */
> -	if (huge_pte_none_mostly(entry)) {
> -		ret = hugetlb_no_page(mm, vma, mapping, idx, address, ptep,
> +	if (huge_pte_none_mostly(entry))

As previously mentioned, I think we want a comment here saying that
hugetlb_no_page will release the locks previously taken in this routine.
Otherwise, readers of this routine may think code is returning without
releasing the locks.  Releasing locks in another routine as is done here
is usually discouraged practice.  However, I think it is acceptable in
this case.  Hence, the need for a comment.

> +		return hugetlb_no_page(mm, vma, mapping, idx, address, ptep,
>  				      entry, flags);
> -		goto out_mutex;
> -	}
>  
>  	ret = 0;
>  
> -- 
> 2.25.1

With comment added, you can add:

Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
-- 
Mike Kravetz