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

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 21 Sep 2022 16:34:40 +0800 Liu Shixin <liushixin2@huawei.com> 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] 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.
> 
> @@ -5508,17 +5507,12 @@ static inline vm_fault_t hugetlb_handle_userfault(struct vm_area_struct *vma,
>  
>  	/*
>  	 * vma_lock and hugetlb_fault_mutex must be
> -	 * dropped before handling userfault.  Reacquire
> -	 * after handling fault to make calling code simpler.
> +	 * dropped before handling userfault.
>  	 */
>  	hugetlb_vma_unlock_read(vma);
>  	hash = hugetlb_fault_mutex_hash(mapping, idx);
>  	mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> -	ret = handle_userfault(&vmf, reason);
> -	mutex_lock(&hugetlb_fault_mutex_table[hash]);
> -	hugetlb_vma_lock_read(vma);
> -
> -	return ret;
> +	return handle_userfault(&vmf, reason);
>  }

Current code is rather different from this.  So if the bug still exists
in current code, please verify this and redo the patch appropriately?

And hang on to this version to help with the -stable backporting.

Thanks.