linux-kernel - Re: [PATCH v2 00/15] Make the user mode driver code a better citizen

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]

Date:   Thu, 2 Jul 2020 22:40:09 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
        Greg Kroah-Hartman <greg@...ah.com>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>, bpf <bpf@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Gary Lin <GLin@...e.com>, Bruno Meneguele <bmeneg@...hat.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 00/15] Make the user mode driver code a better citizen

On 2020/07/02 22:08, Eric W. Biederman wrote:
> Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp> writes:
> 
>> On 2020/06/30 21:29, Eric W. Biederman wrote:
>>> Hmm.  The wake up happens just of tgid->wait_pidfd happens just before
>>> release_task is called so there is a race.  As it is possible to wake
>>> up and then go back to sleep before pid_has_task becomes false.
>>
>> What is the reason we want to wait until pid_has_task() becomes false?
>>
>> - wait_event(tgid->wait_pidfd, !pid_has_task(tgid, PIDTYPE_TGID));
>> + while (!wait_event_timeout(tgid->wait_pidfd, !pid_has_task(tgid, PIDTYPE_TGID), 1));
> 
> So that it is safe to call bpfilter_umh_cleanup.  The previous code
> performed the wait by having a callback in do_exit.

But bpfilter_umh_cleanup() does only

	fput(info->pipe_to_umh);
	fput(info->pipe_from_umh);
	put_pid(info->tgid);
	info->tgid = NULL;

which is (I think) already safe regardless of the usermode process because
bpfilter_umh_cleanup() merely closes one side of two pipes used between
two processes and forgets about the usermode process.

> 
> It might be possible to call bpf_umh_cleanup early but I have not done
> that analysis.
> 
> To perform the test correctly what I have right now is:

Waiting for the termination of a SIGKILLed usermode process is not
such simple. If a usermode process was killed by the OOM killer, it
might take minutes for the killed process to reach do_exit() due to
invisible memory allocation dependency chain. Since the OOM killer
kicks the OOM reaper, and the OOM reaper forgets about the killed
process after one second if mmap_sem could not be held (in order to
avoid OOM deadlock), the OOM situation will be eventually solved; but
there is no guarantee that the killed process can reach do_exit()
in a short period.