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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Message-ID: <CAG48ez1xuZyOLVkxsjburqjf3Tm4TR8X6pnavUf=pm_woAxLkw@mail.gmail.com>
Date:   Tue, 14 Apr 2020 18:57:34 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Andrii Nakryiko <andriin@...com>
Cc:     bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        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 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)