netdev - Re: [PATCH net-next] modules: allow modprobe load regular elf binaries

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Message-ID: <77cdc9f5-b51c-a18d-5422-763cc4e76279@fb.com>
Date:   Fri, 9 Mar 2018 10:58:06 -0800
From:   Alexei Starovoitov <ast@...com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>
CC:     David Miller <davem@...emloft.net>,
        Andy Lutomirski <luto@...capital.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Djalal Harouni <tixxdz@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Daniel Borkmann <daniel@...earbox.net>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH net-next] modules: allow modprobe load regular elf
 binaries

On 3/9/18 10:50 AM, Linus Torvalds wrote:
> On Fri, Mar 9, 2018 at 10:43 AM, Kees Cook <keescook@...omium.org> wrote:
>>
>> Module loading (via kernel_read_file()) already uses
>> deny_write_access(), and so does do_open_execat(). As long as module
>> loading doesn't call allow_write_access() before the execve() has
>> started in the new implementation, I think we'd be covered here.
>
> No. kernel_read_file() only does it *during* the read.
>
> So there's a huge big honking gap between the two.
>
> Also, the second part of my suggestion was to be entirely synchronous
> with the whole execution of the process, and do it within the "we do
> mutual exclusion fo rmodules with the same name" logic.
>
> Note that Andrei's patch uses UMH_WAIT_EXEC. That's basically
> "vfork+exec" - it only waits for the exec to have started, it doesn't
> wait for the whole thing.

It's not waiting for the whole thing, because once bpfilter starts it
stays running/sleeping because it's stateful. It needs normal
malloc-ed memory to keep the state of iptable->bpf translation that
it will use later during subsequent translation calls.
Theoretically it can use bpf maps pinned in kernel memory to keep
this state, but then it's non-swappable. It's better to keep bpfilter
state in its own user memory.