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

Mimi Zohar
2020-08-11T14:31:38Z

On Tue, 2020-08-11 at 15:02 +0100, Matthew Wilcox wrote:
> 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.

Being able to define IMA appraise and measure file open
(func=FILE_CHECK) policy rules to prevent the interpreter from
executing unsigned files would be really nice. Otherwise, the file
would be measured and appraised multiple times, once on file open and
again at the point of this new syscall.

Mimi