Re: [PATCH review 1/6] userns: Avoid recursion in put_user_ns

[Vasily Kulikov <segoon@openwall.com>]

On Fri, Jan 25, 2013 at 18:19 -0800, Eric W. Biederman wrote:
> 
> When freeing a deeply nested user namespace free_user_ns calls
> put_user_ns on it's parent which may in turn call free_user_ns again.
> When -fno-optimize-sibling-calls is passed to gcc one stack frame per
> user namespace is left on the stack, potentially overflowing the
> kernel stack.  CONFIG_FRAME_POINTER forces -fno-optimize-sibling-calls
> so we can't count on gcc to optimize this code.
> 
> Remove struct kref and use a plain atomic_t.  Making the code more
> flexible and easier to comprehend.  Make the loop in free_user_ns
> explict to guarantee that the stack does not overflow with
> CONFIG_FRAME_POINTER enabled.
> 
> I have tested this fix with a simple program that uses unshare to
> create a deeply nested user namespace structure and then calls exit.
> With 1000 nesteuser namespaces before this change running my test
> program causes the kernel to die a horrible death.  With 10,000,000
> nested user namespaces after this change my test program runs to
> completion and causes no harm.
> 
> Pointed-out-by: Vasily Kulikov <segoon@openwall.com>
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

Looks sane, thanks.

Acked-by: Vasily Kulikov <segoon@openwall.com>

The second bug I've noted in the same email (OOM) looks like should be
"fixed" by using memcg to limit kernel memory.  So, I'm fine with this
side of user_ns :)

-- 
Vasily Kulikov
http://www.openwall.com - bringing security into open computing environments