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Date:   Sat, 20 Mar 2021 09:32:37 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Bui Quang Minh <minhquangbui99@gmail.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Peter Xu <peterx@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Axel Rasmussen <axelrasmussen@google.com>
Subject: Re: [PATCH] userfaultfd: Write protect when virtual memory range
 has no page table entry
Message-Id: <20210320093237.c369eba59a0e5f452109c4ef@linux-foundation.org>
In-Reply-To: <20210319152428.52683-1-minhquangbui99@gmail.com>
References: <20210319152428.52683-1-minhquangbui99@gmail.com>
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On Fri, 19 Mar 2021 22:24:28 +0700 Bui Quang Minh <minhquangbui99@gmail.com> wrote:

> userfaultfd_writeprotect() use change_protection() to clear write bit in
> page table entries (pte/pmd). So, later write to this virtual address
> range causes a page fault, which is then handled by userspace program.
> However, change_protection() has no effect when there is no page table
> entries associated with that virtual memory range (a newly mapped memory
> range). As a result, later access to that memory range causes allocating a
> page table entry with write bit still set (due to VM_WRITE flag in
> vma->vm_flags).
> 
> Add checks for VM_UFFD_WP in vma->vm_flags when allocating new page table
> entry in missing page table entry page fault path.

This sounds like a pretty significant bug?

Would it be possible to add a test to
tools/testing/selftests/vm/userfaultfd.c to check for this?  It should
fail without your patch and succeed with it.

Thanks.