Re: [PATCH v2 2/3] seccomp: Introduce addfd ioctl to seccomp user notifier

On Mon, Jun 01, 2020 at 12:02:10PM -0700, Sargun Dhillon wrote:

On Sat, May 30, 2020 at 9:07 AM Kees Cook [off-list ref] wrote:

quoted

On Sat, May 30, 2020 at 03:08:37PM +0100, Al Viro wrote:

quoted

On Fri, May 29, 2020 at 07:43:10PM -0700, Kees Cook wrote:

quoted

Can anyone clarify the expected failure mode from SCM_RIGHTS? Can we
move the put_user() after instead? I think cleanup would just be:
replace_fd(fd, NULL, 0)

Bollocks.

Repeat after me: descriptor tables can be shared.  There is no
"cleanup" after you've put something there.

Right -- this is what I was trying to ask about, and why I didn't like
the idea of just leaving the fd in the table on failure. But yeah, there
is a race if the process is about to fork or something.

So the choice here is how to handle the put_user() failure:

- add the put_user() address to the new helper, as I suggest in [1].
  (exactly duplicates current behavior)
- just leave the fd in place (not current behavior: dumps a fd into
  the process without "agreed" notification).
- do a double put_user (once before and once after), also in [1].
  (sort of a best-effort combo of the above two. and SCM_RIGHTS is
  hardly fast-pth).

-Kees

[1] https://lore.kernel.org/linux-api/202005282345.573B917@keescook/ (local)

--
Kees Cook

I'm going to suggest we stick to the approach of doing[1]:
1. Allocate FD
2. put_user
3. "Receive" and install file into FD

That is the only way to preserve the current behaviour in which userspace
is notified about *every* FD that is received via SCM_RIGHTS. The
scm_detach_fds code as it reads today does effectively what is above,
in that the fd is not installed until *after* the put user. Therefore
if put_user
gets an EFAULT or ENOMEM, it falls through to the MSG_CTRUNC bit.

The approach suggested[2] has a "change" in behaviour, in that (all in
file_receive):
1. Allocate FD
2. Receive file
3. put_user

Based on what Al Viro said, I don't think we can simply add step #4,
being "just" uninstall the FD.

[1]: https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg2179418.html
[2]: https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg2179453.html

Agreed. Thanks!

-- 
Kees Cook