Re: [PATCH v4] mm: hugetlb: fix UAF in hugetlb_handle_userfault

[Andrew Morton]

On Sat, 24 Sep 2022 11:49:05 +0800 Liu Shixin <liushixin2@xxxxxxxxxx> 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.
> 

Thanks.  Turns out that porting all the pending material on top of this
change was not a confidence-inspiring activity.  So I ended up merging
your v3.  Please work with Greg on the backporting when he gets on to
it?  Hopefully that will merely involve sending him this v4.