From: Andrew Vagin <avagin@openvz.org>
Date: Thu, 25 Oct 2012 01:13:57 +0000 (+1100)
Subject: pidns: limit the nesting depth of pid namespaces
X-Git-Tag: next-20121025~2^2~7
X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=2d78eda4ae89c8b18b1687202b4122b3d6ff5474;p=karo-tx-linux.git

pidns: limit the nesting depth of pid namespaces

'struct pid' is a "variable sized struct" - a header with an array of
upids at the end.

The size of the array depends on a level (depth) of pid namespaces.  Now a
level of pidns is not limited, so 'struct pid' can be more than one page.

Looks reasonable, that it should be less than a page.  MAX_PIS_NS_LEVEL is
not calculated from PAGE_SIZE, because in this case it depends on
architectures, config options and it will be reduced, if someone adds a
new fields in struct pid or struct upid.

I suggest to set MAX_PIS_NS_LEVEL = 32, because it saves ability to expand
"struct pid" and it's more than enough for all known for me use-cases.
When someone finds a reasonable use case, we can add a config option or a
sysctl parameter.

In addition it will reduce the 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 make a system inaccessible for a long time.  For example my
system becomes inaccessible for a few minutes with 4000 processes.

Signed-off-by: Andrew Vagin <avagin@openvz.org>
Acked-by: 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 Morton <akpm@linux-foundation.org>
---

diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index eb00be205811..dd82418c6b59 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -71,12 +71,18 @@ err_alloc:
 	return NULL;
 }
 
+/* MAX_PID_NS_LEVEL is needed for limiting size of 'struct pid' */
+#define MAX_PID_NS_LEVEL 32
+
 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;