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Date:   Sat, 30 May 2020 09:07:16 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Sargun Dhillon <sargun@sargun.me>, christian.brauner@ubuntu.com,
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        linux-kernel@vger.kernel.org, palmer@google.com, rsesek@google.com,
        tycho@tycho.ws, Matt Denton <mpdenton@google.com>
Subject: Re: [PATCH v2 2/3] seccomp: Introduce addfd ioctl to seccomp user
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On Sat, May 30, 2020 at 03:08:37PM +0100, Al Viro wrote:
> On Fri, May 29, 2020 at 07:43:10PM -0700, Kees Cook wrote:
> 
> > 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/

-- 
Kees Cook