[PATCH] pidns: limit the nesting depth of pid namespaces

[Andrew Vagin]

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

A 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 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.

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
Signed-off-by: Andrew Vagin <avagin@xxxxxxxxxx>
---
 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..598bfb3 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -70,12 +70,18 @@ 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;
-- 
1.7.1

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