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

From: Mike Kravetz
Date: Wed Sep 21 2022 - 13:48:40 EST


On 09/21/22 16:34, 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] 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.

Thank you very much!

When I saw this report, the obvious fix was to do something like what you have
done below. That looks fine with a few minor comments.

One question I have not yet answered is, "Does this same issue apply to
follow_hugetlb_page()?". I believe it does. follow_hugetlb_page calls
hugetlb_fault which could result in the fault being processed by userfaultfd.
If we experience the race above, then the associated vma could no longer be
valid when returning from hugetlb_fault. follow_hugetlb_page and callers
have a flag (locked) to deal with dropping mmap lock. However, I am not sure
if it is handled correctly WRT userfaultfd. I think this needs to be answered
before fixing. And, if the follow_hugetlb_page code needs to be fixed it
should be done at the same time.

> [1] https://lore.kernel.org/linux-mm/20220921014457.1668-1-liuzixian4@xxxxxxxxxx/
> Reported-by: Liu Zixian <liuzixian4@xxxxxxxxxx>

Perhaps reported by should be,
Reported-by: syzbot+193f9cee8638750b23cf@xxxxxxxxxxxxxxxxxxxxxxxxx
https://lore.kernel.org/linux-mm/000000000000d5e00a05e834962e@xxxxxxxxxx/

Should also add,
Fixes: 1a1aad8a9b7b ("userfaultfd: hugetlbfs: add userfaultfd hugetlb hook")

as well as,
Cc: <stable@xxxxxxxxxxxxxxx>

> 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
> index 9b8526d27c29..5a5d466692cf 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
...
> @@ -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))

We should add a big comment noting that hugetlb_no_page will drop vma lock
and hugetl fault mutex. This will make it easier for people reading the code
and immediately thinking we are returning without dropping the locks.

--
Mike Kravetz

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