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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <202005300834.6419E818A7@keescook>
Date:   Sat, 30 May 2020 09:07:16 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Sargun Dhillon <sargun@...gun.me>, christian.brauner@...ntu.com,
        containers@...ts.linux-foundation.org, cyphar@...har.com,
        jannh@...gle.com, jeffv@...gle.com, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org, palmer@...gle.com, rsesek@...gle.com,
        tycho@...ho.ws, Matt Denton <mpdenton@...gle.com>
Subject: Re: [PATCH v2 2/3] seccomp: Introduce addfd ioctl to seccomp user
 notifier

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