linux-kernel - Re: [PATCH v5 3/6] fs: Enable to enforce noexec mounts or file exec through O

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Message-ID: <CAEjxPJ4R_juwvRbKiCg5OGuhAi1ZuVytK4fKCDT_kT6VKc8iRg@mail.gmail.com>
Date:   Thu, 14 May 2020 12:10:31 -0400
From:   Stephen Smalley <stephen.smalley.work@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Mickaël Salaün <mic@...ikod.net>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Heimes <christian@...hon.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Deven Bowers <deven.desai@...ux.microsoft.com>,
        Eric Chiang <ericchiang@...gle.com>,
        Florian Weimer <fweimer@...hat.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>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        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>,
        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: [PATCH v5 3/6] fs: Enable to enforce noexec mounts or file exec
 through O_MAYEXEC

On Thu, May 14, 2020 at 11:45 AM Kees Cook <keescook@...omium.org> wrote:
>
> On Thu, May 14, 2020 at 08:22:01AM -0400, Stephen Smalley wrote:
> > On Wed, May 13, 2020 at 11:05 PM Kees Cook <keescook@...omium.org> wrote:
> > >
> > > On Wed, May 13, 2020 at 04:27:39PM -0700, Kees Cook wrote:
> > > > Like, couldn't just the entire thing just be:
> > > >
> > > > diff --git a/fs/namei.c b/fs/namei.c
> > > > index a320371899cf..0ab18e19f5da 100644
> > > > --- a/fs/namei.c
> > > > +++ b/fs/namei.c
> > > > @@ -2849,6 +2849,13 @@ static int may_open(const struct path *path, int acc_mode, int flag)
> > > >               break;
> > > >       }
> > > >
> > > > +     if (unlikely(mask & MAY_OPENEXEC)) {
> > > > +             if (sysctl_omayexec_enforce & OMAYEXEC_ENFORCE_MOUNT &&
> > > > +                 path_noexec(path))
> > > > +                     return -EACCES;
> > > > +             if (sysctl_omayexec_enforce & OMAYEXEC_ENFORCE_FILE)
> > > > +                     acc_mode |= MAY_EXEC;
> > > > +     }
> > > >       error = inode_permission(inode, MAY_OPEN | acc_mode);
> > > >       if (error)
> > > >               return error;
> > > >
> > >
> > > FYI, I've confirmed this now. Effectively with patch 2 dropped, patch 3
> > > reduced to this plus the Kconfig and sysctl changes, the self tests
> > > pass.
> > >
> > > I think this makes things much cleaner and correct.
> >
> > I think that covers inode-based security modules but not path-based
> > ones (they don't implement the inode_permission hook).  For those, I
> > would tentatively guess that we need to make sure FMODE_EXEC is set on
> > the open file and then they need to check for that in their file_open
> > hooks.
>
> I kept confusing myself about what order things happened in, so I made
> these handy notes about the call graph:
>
> openat2(dfd, char * filename, open_how)
>     do_filp_open(dfd, filename, open_flags)
>         path_openat(nameidata, open_flags, flags)
>             do_open(nameidata, file, open_flags)
>                 may_open(path, acc_mode, open_flag)
>                     inode_permission(inode, MAY_OPEN | acc_mode)
>                         security_inode_permission(inode, acc_mode)
>                 vfs_open(path, file)
>                     do_dentry_open(file, path->dentry->d_inode, open)
>                         if (unlikely(f->f_flags & FMODE_EXEC && !S_ISREG(inode->i_mode))) ...
>                         security_file_open(f)
>                         open()
>
> So, it looks like adding FMODE_EXEC into f_flags in do_open() is needed in
> addition to injecting MAY_EXEC into acc_mode in do_open()? Hmmm

Just do both in build_open_flags() and be done with it? Looks like he
was already setting FMODE_EXEC in patch 1 so we just need to teach
AppArmor/TOMOYO to check for it and perform file execute checking in
that case if !current->in_execve?