Re: [PATCH v7 0/7] Add support for O_MAYEXEC

Matthew Wilcox
2020-08-11T14:02:29Z

On Tue, Aug 11, 2020 at 09:56:50AM -0400, Mimi Zohar wrote:
> On Tue, 2020-08-11 at 10:48 +0200, Mickaël Salaün wrote:
> > On 11/08/2020 01:03, Jann Horn wrote:
> > > On Tue, Aug 11, 2020 at 12:43 AM Mickaël Salaün <mic@digikod.net> wrote:
> > > > On 10/08/2020 22:21, Al Viro wrote:
> > > > > On Mon, Aug 10, 2020 at 10:11:53PM +0200, Mickaël Salaün wrote:
> > > > > > It seems that there is no more complains nor questions. Do you want me
> > > > > > to send another series to fix the order of the S-o-b in patch 7?
> > > > >
> > > > > There is a major question regarding the API design and the choice of
> > > > > hooking that stuff on open(). And I have not heard anything resembling
> > > > > a coherent answer.
> > > >
> > > > Hooking on open is a simple design that enables processes to check files
> > > > they intend to open, before they open them. From an API point of view,
> > > > this series extends openat2(2) with one simple flag: O_MAYEXEC. The
> > > > enforcement is then subject to the system policy (e.g. mount points,
> > > > file access rights, IMA, etc.).
> > > >
> > > > Checking on open enables to not open a file if it does not meet some
> > > > requirements, the same way as if the path doesn't exist or (for whatever
> > > > reasons, including execution permission) if access is denied.
> > >
> > > You can do exactly the same thing if you do the check in a separate
> > > syscall though.
> > >
> > > And it provides a greater degree of flexibility; for example, you can
> > > use it in combination with fopen() without having to modify the
> > > internals of fopen() or having to use fdopen().
> > >
> > > > It is a
> > > > good practice to check as soon as possible such properties, and it may
> > > > enables to avoid (user space) time-of-check to time-of-use (TOCTOU)
> > > > attacks (i.e. misuse of already open resources).
> > >
> > > The assumption that security checks should happen as early as possible
> > > can actually cause security problems. For example, because seccomp was
> > > designed to do its checks as early as possible, including before
> > > ptrace, we had an issue for a long time where the ptrace API could be
> > > abused to bypass seccomp filters.
> > >
> > > Please don't decide that a check must be ordered first _just_ because
> > > it is a security check. While that can be good for limiting attack
> > > surface, it can also create issues when the idea is applied too
> > > broadly.
> >
> > I'd be interested with such security issue examples.
> >
> > I hope that delaying checks will not be an issue for mechanisms such as
> > IMA or IPE:
> > https://lore.kernel.org/lkml/1544699060.6703.11.camel@linux.ibm.com/
> >
> > Any though Mimi, Deven, Chrome OS folks?
>
> One of the major gaps, defining a system wide policy requiring all code
> being executed to be signed, is interpreters. The kernel has no
> context for the interpreter's opening the file. From an IMA
> perspective, this information needs to be conveyed to the kernel prior
> to ima_file_check(), which would allow IMA policy rules to be defined
> in terms of O_MAYEXEC.

This is kind of evading the point -- Mickaël is proposing a new flag
to open() to tell IMA to measure the file being opened before the fd
is returned to userspace, and Al is suggesting a new syscall to allow
a previously-obtained fd to be measured.

I think what you're saying is that you don't see any reason to prefer
one over the other.