Re: [PATCH] pidns: limit a size of pid to one page

[Oleg Nesterov <oleg@redhat.com>]

On 10/10, Andrew Vagin wrote:
>
> A size of pid depends on a level of pidns and now a level of pidns
> is not limited, so it can be more than one page.
>
> Looks reasonable, that it should be limited to a page size.  On x86_64
> it will allow to create 125 nested pid namespaces.  I don't know a
> usecase for which, it will be not enough. When someone finds a
> reasonable use case, we can add a config option or a sysctl parameter.

I think this makes sense.

Acked-by: Oleg Nesterov <oleg@redhat.com>

> In addition it will reduce effect of another problem, when we have many
> nested namespaces and the oldest one starts dying.  zap_pid_ns_processe
> will be called for each namespace and find_vpid will be called for each
> process in a namespace. find_vpid will be called minimum max_level^2 / 2
> times. The reason of that is that when we found a bit in pidmap, we
> can't determine this pidns is top for this process or it isn't.
>
> vpid is a heavy operation, so a fork bomb, which create many nested
> namespace, can do a system inaccessible for a long time.
>
> For example my system becomes inaccessible for a few minutes with
> 4000 processes.
>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Cyrill Gorcunov <gorcunov@openvz.org>
> Cc: "Eric W. Biederman" <ebiederm@xmission.com>
> Cc: Pavel Emelyanov <xemul@parallels.com>
> Signed-off-by: Andrew Vagin <avagin@openvz.org>
> ---
>  kernel/pid_namespace.c |    6 ++++++
>  1 files changed, 6 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
> index b051fa6..bc822e6 100644
> --- a/kernel/pid_namespace.c
> +++ b/kernel/pid_namespace.c
> @@ -70,12 +70,18 @@ err_alloc:
>  	return NULL;
>  }
>
> +/* Limit a size of pid to one page */
> +#define MAX_PID_NS_LEVEL ((PAGE_SIZE - sizeof(struct pid)) / sizeof(struct upid))
> +
>  static struct pid_namespace *create_pid_namespace(struct pid_namespace *parent_pid_ns)
>  {
>  	struct pid_namespace *ns;
>  	unsigned int level = parent_pid_ns->level + 1;
>  	int i, err = -ENOMEM;
>
> +	if (level > MAX_PID_NS_LEVEL)
> +		goto out;
> +
>  	ns = kmem_cache_zalloc(pid_ns_cachep, GFP_KERNEL);
>  	if (ns == NULL)
>  		goto out;
> --
> 1.7.1
>