linux-kernel - Re: [PATCH 4.9 022/310] pidns: disable pid allocation if pid_ns_prepare_proc() is failed in alloc

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Date:   Wed, 11 Apr 2018 14:27:19 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Andrei Vagin <avagin@...tuozzo.com>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Michael Kerrisk <mtk.manpages@...glemail.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Oleg Nesterov <oleg@...hat.com>,
        Paul Moore <paul@...l-moore.com>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...nel.org>,
        Serge Hallyn <serge@...lyn.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sasha Levin <alexander.levin@...rosoft.com>
Subject: Re: [PATCH 4.9 022/310] pidns: disable pid allocation if pid_ns_prepare_proc() is failed in alloc_pid()

Greg Kroah-Hartman <gregkh@...uxfoundation.org> writes:

> 4.9-stable review patch.  If anyone has any objections, please let me
> know.

No objections but if you are grabbing that one please check if you
have it's follow on fix.

c0ee554906c3 ("pid: Handle failure to allocate the first pid in a pid namespace")

There were a few cases not handled but the fix below, that made a more
comprehensive fix desirable.

Eric

> ------------------
>
> From: Kirill Tkhai <ktkhai@...tuozzo.com>
>
>
> [ Upstream commit 8896c23d2ef803f1883fea73117a435925c2b4c4 ]
>
> alloc_pidmap() advances pid_namespace::last_pid.  When first pid
> allocation fails, then next created process will have pid 2 and
> pid_ns_prepare_proc() won't be called.  So, pid_namespace::proc_mnt will
> never be initialized (not to mention that there won't be a child
> reaper).
>
> I saw crash stack of such case on kernel 3.10:
>
>     BUG: unable to handle kernel NULL pointer dereference at (null)
>     IP: proc_flush_task+0x8f/0x1b0
>     Call Trace:
>         release_task+0x3f/0x490
>         wait_consider_task.part.10+0x7ff/0xb00
>         do_wait+0x11f/0x280
>         SyS_wait4+0x7d/0x110
>
> We may fix this by restore of last_pid in 0 or by prohibiting of futher
> allocations.  Since there was a similar issue in Oleg Nesterov's commit
> 314a8ad0f18a ("pidns: fix free_pid() to handle the first fork failure").
> and it was fixed via prohibiting allocation, let's follow this way, and
> do the same.
>
> Link: http://lkml.kernel.org/r/149201021004.4863.6762095011554287922.stgit@localhost.localdomain
> Signed-off-by: Kirill Tkhai <ktkhai@...tuozzo.com>
> Acked-by: Cyrill Gorcunov <gorcunov@...nvz.org>
> Cc: Andrei Vagin <avagin@...tuozzo.com>
> Cc: Andreas Gruenbacher <agruenba@...hat.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Michael Kerrisk <mtk.manpages@...glemail.com>
> Cc: Al Viro <viro@...iv.linux.org.uk>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Paul Moore <paul@...l-moore.com>
> Cc: Eric Biederman <ebiederm@...ssion.com>
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Serge Hallyn <serge@...lyn.com>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> ---
>  kernel/pid.c |    4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -322,8 +322,10 @@ struct pid *alloc_pid(struct pid_namespa
>  	}
>  
>  	if (unlikely(is_child_reaper(pid))) {
> -		if (pid_ns_prepare_proc(ns))
> +		if (pid_ns_prepare_proc(ns)) {
> +			disable_pid_allocation(ns);
>  			goto out_free;
> +		}
>  	}
>  
>  	get_pid_ns(ns);