netdev - Re: [PATCH v2 bpf 1/2] bpf: prevent re-mmap()'ing BPF map as writable for initially r/o mapping

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Date:   Tue, 14 Apr 2020 11:28:20 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v2 bpf 1/2] bpf: prevent re-mmap()'ing BPF map as writable
 for initially r/o mapping

On Tue, Apr 14, 2020 at 9:58 AM Jann Horn <jannh@...gle.com> wrote:
>
> On Fri, Apr 10, 2020 at 10:26 PM Andrii Nakryiko <andriin@...com> 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 <jannh@...gle.com>
> > Signed-off-by: Andrii Nakryiko <andriin@...com>
>
> Reviewed-by: Jann Horn <jannh@...gle.com>
>
> (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.