[RFC PATCH v1 1/5] fs: Add support for an O_MAYEXEC flag on sys_open()

[Andy Lutomirski]

On Wed, Dec 12, 2018 at 6:43 AM Jan Kara <jack at suse.cz> wrote:
>
> On Wed 12-12-18 09:17:08, Mickaël Salaün wrote:
> > When the O_MAYEXEC flag is passed, sys_open() may be subject to
> > additional restrictions depending on a security policy implemented by an
> > LSM through the inode_permission hook.
> >
> > The underlying idea is to be able to restrict scripts interpretation
> > according to a policy defined by the system administrator.  For this to
> > be possible, script interpreters must use the O_MAYEXEC flag
> > appropriately.  To be fully effective, these interpreters also need to
> > handle the other ways to execute code (for which the kernel can't help):
> > command line parameters (e.g., option -e for Perl), module loading
> > (e.g., option -m for Python), stdin, file sourcing, environment
> > variables, configuration files...  According to the threat model, it may
> > be acceptable to allow some script interpreters (e.g. Bash) to interpret
> > commands from stdin, may it be a TTY or a pipe, because it may not be
> > enough to (directly) perform syscalls.
> >
> > A simple security policy implementation is available in a following
> > patch for Yama.
> >
> > This is an updated subset of the patch initially written by Vincent
> > Strubel for CLIP OS:
> > https://github.com/clipos-archive/src_platform_clip-patches/blob/f5cb330d6b684752e403b4e41b39f7004d88e561/1901_open_mayexec.patch
> > This patch has been used for more than 10 years with customized script
> > interpreters.  Some examples can be found here:
> > https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC
> >
> > Signed-off-by: Mickaël Salaün <mic at digikod.net>
> > Signed-off-by: Thibaut Sautereau <thibaut.sautereau at ssi.gouv.fr>
> > Signed-off-by: Vincent Strubel <vincent.strubel at ssi.gouv.fr>
> > Reviewed-by: Philippe Trébuchet <philippe.trebuchet at ssi.gouv.fr>
> > Cc: Al Viro <viro at zeniv.linux.org.uk>
> > Cc: Kees Cook <keescook at chromium.org>
> > Cc: Mickaël Salaün <mickael.salaun at ssi.gouv.fr>
>
> ...
>
> > diff --git a/fs/open.c b/fs/open.c
> > index 0285ce7dbd51..75479b79a58f 100644
> > --- a/fs/open.c
> > +++ b/fs/open.c
> > @@ -974,6 +974,10 @@ static inline int build_open_flags(int flags, umode_t mode, struct open_flags *o
> >       if (flags & O_APPEND)
> >               acc_mode |= MAY_APPEND;
> >
> > +     /* Check execution permissions on open. */
> > +     if (flags & O_MAYEXEC)
> > +             acc_mode |= MAY_OPENEXEC;
> > +
> >       op->acc_mode = acc_mode;
> >
> >       op->intent = flags & O_PATH ? 0 : LOOKUP_OPEN;
>
> I don't feel experienced enough in security to tell whether we want this
> functionality or not. But if we do this, shouldn't we also set FMODE_EXEC
> on the resulting struct file? That way also security_file_open() can be
> used to arbitrate such executable opens and in particular
> fanotify permission event FAN_OPEN_EXEC will get properly generated which I
> guess is desirable (support for it is sitting in my tree waiting for the
> merge window) - adding some audit people involved in FAN_OPEN_EXEC to
> CC. Just an idea...
>

I would really like to land this patch.  I'm fiddling with making
bpffs handle permissions intelligently, and the lack of a way to say
"hey, I want to open this bpf program so that I can run it" is
annoying.

--Andy