Re: [PATCH] pidns: remove recursion from free_pid_ns() v5

[ebiederm@xmission.com (Eric W. Biederman)]

Andrew Morton <akpm@linux-foundation.org> writes:

> On Thu, 11 Oct 2012 00:42:56 +0400
> Cyrill Gorcunov <gorcunov@openvz.org> wrote:
>
>> The free_pid_ns function done in recursion fashion:
>> 
>> free_pid_ns(parent)
>>   put_pid_ns(parent)
>>     kref_put(&ns->kref, free_pid_ns);
>>       free_pid_ns
>> 
>> thus if there was a huge nesting of namespaces the userspace
>> may trigger avalanche calling of free_pid_ns leading to
>> kernel stack exhausting and a panic eventually.
>> 
>> This patch turns the recursion into iterative loop.
>> 
>> v5 (from oleg@):
>>  - Drop @ret variable
>>  - Make put_pid_ns non-inline since it grows in size,
>>    in turn make free_pid_ns static
>
> OK, let's try that.  I'll sit on this until -rc2 to give it a bit of
> time to cook.
>
> A -stable backport might be needed.  What capabilities does userspace
> need to be able to trigger the kernel stack overflow?

CAP_SYS_ADMIN is required to create a new pid namespace today.

With a little luck the user namespace bits that allow unprivelged
creation of pid namespaces will be ready for 3.8.

Eric