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@xxxxxxxxxxxx> > Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx> Looks sane, thanks. Acked-by: Vasily Kulikov <segoon@xxxxxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
