Re: [PATCH v3] mm: hugetlb: fix UAF in hugetlb_handle_userfault

[Andrew Morton]

On Fri, 23 Sep 2022 12:21:13 +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.
> 
> [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+

Patch is against mm-unstable, which isn't appropriate for a backport. 
Could you please something against current -linus (which will be more
backportable), then I'll figure out the fallout on mm-unstable.

Thanks.