linux-kernel - Re: How about just O_EXEC? (was Re: [PATCH v5 3/6] fs: Enable to enforce noexec mounts or file exec through O

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Message-ID: <87r1vluuli.fsf@oldenburg2.str.redhat.com>
Date:   Fri, 15 May 2020 16:43:37 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Mickaël Salaün <mic@...ikod.net>,
        Al Viro <viro@...iv.linux.org.uk>,
        Aleksa Sarai <cyphar@...har.com>,
        Andy Lutomirski <luto@...nel.org>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Christian Heimes <christian@...hon.org>,
        Deven Bowers <deven.desai@...ux.microsoft.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        John Johansen <john.johansen@...onical.com>,
        Kentaro Takeda <takedakn@...data.co.jp>,
        "Lev R. Oshvang ." <levonshe@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
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Subject: Re: How about just O_EXEC? (was Re: [PATCH v5 3/6] fs: Enable to enforce noexec mounts or file exec through O_MAYEXEC)

* Kees Cook:

> On Fri, May 15, 2020 at 10:43:34AM +0200, Florian Weimer wrote:
>> * Kees Cook:
>> 
>> > Maybe I've missed some earlier discussion that ruled this out, but I
>> > couldn't find it: let's just add O_EXEC and be done with it. It actually
>> > makes the execve() path more like openat2() and is much cleaner after
>> > a little refactoring. Here are the results, though I haven't emailed it
>> > yet since I still want to do some more testing:
>> > https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/log/?h=kspp/o_exec/v1
>> 
>> I think POSIX specifies O_EXEC in such a way that it does not confer
>> read permissions.  This seems incompatible with what we are trying to
>> achieve here.
>
> I was trying to retain this behavior, since we already make this
> distinction between execve() and uselib() with the MAY_* flags:
>
> execve():
>         struct open_flags open_exec_flags = {
>                 .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
>                 .acc_mode = MAY_EXEC,
>
> uselib():
>         static const struct open_flags uselib_flags = {
>                 .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
>                 .acc_mode = MAY_READ | MAY_EXEC,
>
> I tried to retain this in my proposal, in the O_EXEC does not imply
> MAY_READ:

That doesn't quite parse for me, sorry.

The point is that the script interpreter actually needs to *read* those
files in order to execute them.

Thanks,
Florian