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SubjectRe: [PATCH review 1/6] userns: Avoid recursion in put_user_ns
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
> 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 <>
> Signed-off-by: "Eric W. Biederman" <>

Looks sane, thanks.

Acked-by: Vasily Kulikov <>

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 - bringing security into open computing environments

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