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

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Date:   Fri, 9 Mar 2018 10:53:45 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Alexei Starovoitov <ast@...com>, Kees Cook <keescook@...omium.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Djalal Harouni <tixxdz@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        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 Fri, Mar 9, 2018 at 10:48 AM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Mar 9, 2018, at 10:17 AM, Linus Torvalds <torvalds@...ux-foundation.org> 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 Alexei already has this working for umh.
> Surely we can make an immutable (as in even root can’t 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