[PATCH v12 1/9] security: Introduce ENOFILEOPS return value for IOCTL hooks

[Paul Moore]

On Mon, Mar 25, 2024 at 9:40 AM Günther Noack <gnoack at google.com> wrote:
>
> If security_file_ioctl or security_file_ioctl_compat return
> ENOFILEOPS, the IOCTL logic in fs/ioctl.c will permit the given IOCTL
> command, but only as long as the IOCTL command is implemented directly
> in fs/ioctl.c and does not use the f_ops->unhandled_ioctl or
> f_ops->compat_ioctl operations, which are defined by the given file.
>
> The possible return values for security_file_ioctl and
> security_file_ioctl_compat are now:
>
>  * 0 - to permit the IOCTL
>  * ENOFILEOPS - to permit the IOCTL, but forbid it if it needs to fall
>    back to the file implementation.
>  * any other error - to forbid the IOCTL and return that error

At this point I think this thread has resolved itself, but I wanted to
add a quick comment for those who may stumble across this in the
future ... I want to discourage magic return values in the LSM hooks
as much as possible; they have caused issues in the past and I suspect
they will continue to do so in the future (although now that we have
proper function header comments the risk may be slightly lower).  If
there is absolutely no way around it, then that's okay, but if
possible I would prefer we stick with the 0:allowed, !0:rejected model
for the LSM hook return values.

> This is an alternative to the previously discussed approaches [1] and [2],
> and implements the proposal from [3].
>
> Cc: Christian Brauner <brauner at kernel.org>
> Cc: Paul Moore <paul at paul-moore.com>
> Cc: Mickaël Salaün <mic at digikod.net>
> Cc: linux-fsdevel at vger.kernel.org
> Link: https://lore.kernel.org/r/20240309075320.160128-2-gnoack@google.com [1]
> Link: https://lore.kernel.org/r/20240322151002.3653639-2-gnoack@google.com/ [2]
> Link: https://lore.kernel.org/r/32b1164e-9d5f-40c0-9a4e-001b2c9b822f@app.fastmail.com/ [3]
> Suggested-by: Arnd Bergmann <arnd at arndb.de>
> Signed-off-by: Günther Noack <gnoack at google.com>
> ---
>  fs/ioctl.c               | 25 ++++++++++++++++++++-----
>  include/linux/security.h |  6 ++++++
>  security/security.c      | 10 ++++++++--
>  3 files changed, 34 insertions(+), 7 deletions(-)

-- 
paul-moore.com