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Date:   Tue, 14 Apr 2020 21:44:00 +0200
From:   Daniel Borkmann <>
To:     Andrii Nakryiko <>,
        Jann Horn <>
Cc:     Andrii Nakryiko <>, bpf <>,
        Network Development <>,
        Alexei Starovoitov <>,
        Kernel Team <>
Subject: Re: [PATCH v2 bpf 1/2] bpf: prevent re-mmap()'ing BPF map as writable
 for initially r/o mapping

On 4/14/20 8:28 PM, Andrii Nakryiko wrote:
> On Tue, Apr 14, 2020 at 9:58 AM Jann Horn <> wrote:
>> On Fri, Apr 10, 2020 at 10:26 PM Andrii Nakryiko <> wrote:
>>> VM_MAYWRITE flag during initial memory mapping determines if already mmap()'ed
>>> pages can be later remapped as writable ones through mprotect() call. To
>>> prevent user application to rewrite contents of memory-mapped as read-only and
>>> subsequently frozen BPF map, remove VM_MAYWRITE flag completely on initially
>>> read-only mapping.
>>> Alternatively, we could treat any memory-mapping on unfrozen map as writable
>>> and bump writecnt instead. But there is little legitimate reason to map
>>> BPF map as read-only and then re-mmap() it as writable through mprotect(),
>>> instead of just mmap()'ing it as read/write from the very beginning.
>>> Also, at the suggestion of Jann Horn, drop unnecessary refcounting in mmap
>>> operations. We can just rely on VMA holding reference to BPF map's file
>>> properly.
>>> Fixes: fc9702273e2e ("bpf: Add mmap() support for BPF_MAP_TYPE_ARRAY")
>>> Reported-by: Jann Horn <>
>>> Signed-off-by: Andrii Nakryiko <>
>> Reviewed-by: Jann Horn <>
>> (in the sense that I think this patch is good and correct, but does
>> not fix the entire problem in the bigger picture)
> I agree, we'll continue discussion on the other thread, but this
> should be applied as a bug fix anyways.

Applied, thanks!

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