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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Date:   Fri, 15 May 2020 08:50:16 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Florian Weimer <fweimer@...hat.com>
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>,
        Eric Chiang <ericchiang@...gle.com>,
        James Morris <jmorris@...ei.org>, Jan Kara <jack@...e.cz>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
        Matthew Garrett <mjg59@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Mickaël Salaün <mickael.salaun@....gouv.fr>,
        Philippe Trébuchet 
        <philippe.trebuchet@....gouv.fr>,
        Scott Shell <scottsh@...rosoft.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Shuah Khan <shuah@...nel.org>,
        Steve Dower <steve.dower@...hon.org>,
        Steve Grubb <sgrubb@...hat.com>,
        Thibaut Sautereau <thibaut.sautereau@....gouv.fr>,
        Vincent Strubel <vincent.strubel@....gouv.fr>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        kernel-hardening@...ts.openwall.com, linux-api@...r.kernel.org,
        linux-integrity@...r.kernel.org,
        LSM List <linux-security-module@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>
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)

On Fri, May 15, 2020 at 04:43:37PM +0200, Florian Weimer wrote:
> * 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.

I think I misunderstood what you meant (Mickaël got me sorted out
now). If O_EXEC is already meant to be "EXEC and _not_ READ nor WRITE",
then yes, this new flag can't be O_EXEC. I was reading the glibc
documentation (which treats it as a permission bit flag, not POSIX,
which treats it as a complete mode description).

-- 
Kees Cook