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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
Date: Sat, 20 Mar 2021 09:32:37 -0700	[thread overview]
Message-ID: <20210320093237.c369eba59a0e5f452109c4ef@linux-foundation.org> (raw)
In-Reply-To: <20210319152428.52683-1-minhquangbui99@gmail.com>

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.

  reply	other threads:[~2021-03-20 16:33 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-19 15:24 [PATCH] userfaultfd: Write protect when virtual memory range has no page table entry Bui Quang Minh
2021-03-20 16:32 ` Andrew Morton [this message]
2021-03-22 10:14 ` Michal Hocko
2021-03-22 10:23   ` Mike Rapoport
2021-03-22 13:00   ` Mike Rapoport
2021-03-22 13:27     ` Peter Xu
2021-03-22 13:49     ` Michal Hocko
2021-03-31 14:49       ` Michal Hocko
2021-04-01  0:24         ` Andrew Morton
2021-03-23  2:48     ` Bui Quang Minh
2021-03-23 15:12       ` Peter Xu

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