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Date:   Fri, 23 Sep 2022 17:05:54 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Liu Shixin <liushixin2@huawei.com>
Cc:     Liu Zixian <liuzixian4@huawei.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Sidhartha Kumar <sidhartha.kumar@oracle.com>,
        John Hubbard <jhubbard@nvidia.com>,
        "David Hildenbrand" <david@redhat.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v3] mm: hugetlb: fix UAF in hugetlb_handle_userfault
Message-Id: <20220923170554.7046bb023f8cf582b5909b77@linux-foundation.org>
In-Reply-To: <20220923042113.137273-1-liushixin2@huawei.com>
References: <20220923042113.137273-1-liushixin2@huawei.com>
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On Fri, 23 Sep 2022 12:21:13 +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[1,2] 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@google.com/
> [2] https://lore.kernel.org/linux-mm/20220921014457.1668-1-liuzixian4@huawei.com/
> Reported-by: syzbot+193f9cee8638750b23cf@syzkaller.appspotmail.com
> Reported-by: Liu Zixian <liuzixian4@huawei.com>
> Fixes: 1a1aad8a9b7b ("userfaultfd: hugetlbfs: add userfaultfd hugetlb hook")
> CC: stable@vger.kernel.org # 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.