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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 9 Mar 2018 10:53:45 -0800
Message-ID: <CA+55aFx7w1s7=5HzeczPkkksaU4oVsWQf+_c=aHS0O7i_9g+Kg@mail.gmail.com>
Subject: Re: [PATCH net-next] modules: allow modprobe load regular elf binaries
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Alexei Starovoitov <ast@fb.com>, Kees Cook <keescook@chromium.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Djalal Harouni <tixxdz@gmail.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        "David S. Miller" <davem@davemloft.net>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Greg KH <gregkh@linuxfoundation.org>,
        "Luis R. Rodriguez" <mcgrof@kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kernel-team <kernel-team@fb.com>,
        Linux API <linux-api@vger.kernel.org>
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On Fri, Mar 9, 2018 at 10:48 AM, Andy Lutomirski <luto@amacapital.net> wrot=
e:
>> On Mar 9, 2018, at 10:17 AM, Linus Torvalds <torvalds@linux-foundation.o=
rg> wrote:
>>
>> Hmm. I wish we had an "execute blob" model, but we really don't, and
>> it would be hard/impossible to do without pinning the pages in memory.
>>
>
> Why so hard?  We can already execute a struct file for execveat, and Alex=
ei already has this working for umh.
> Surely we can make an immutable (as in even root can=E2=80=99t write it) =
kernel-internal tmpfs file, execveat it, then unlink it.

And what do you think that does? It pins the memory for the whole
time. As a *copy* of the original file.

Anyway, see my other suggestion that makes this all irrelevant. Just
wait synchronously (until the exit), and just use deny_write_access().

The "synchronous wait" means that you don't have the semantic change
(and really., it's *required* anyway for the whole mutual exclusion
against another thread racing to load the same module), and the
deny_write_access() means that we don't neeed to make another copy.

                Linus