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Date:   Thu, 14 May 2020 08:45:13 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Stephen Smalley <stephen.smalley.work@gmail.com>
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Subject: Re: [PATCH v5 3/6] fs: Enable to enforce noexec mounts or file exec
 through O_MAYEXEC
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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@chromium.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

-- 
Kees Cook