Re: [PATCH 4.19 016/133] cgroup: fix cgroup_sk_alloc() for sk_clone_lock()

[Yang Yingliang]

Hi,

On 2020/7/20 23:36, Greg Kroah-Hartman wrote:
> From: Cong Wang <xiyou.wangcong@xxxxxxxxx>
>
> [ Upstream commit ad0f75e5f57ccbceec13274e1e242f2b5a6397ed ]
>
> When we clone a socket in sk_clone_lock(), its sk_cgrp_data is
> copied, so the cgroup refcnt must be taken too. And, unlike the
> sk_alloc() path, sock_update_netprioidx() is not called here.
> Therefore, it is safe and necessary to grab the cgroup refcnt
> even when cgroup_sk_alloc is disabled.
>
> sk_clone_lock() is in BH context anyway, the in_interrupt()
> would terminate this function if called there. And for sk_alloc()
> skcd->val is always zero. So it's safe to factor out the code
> to make it more readable.
>
> The global variable 'cgroup_sk_alloc_disabled' is used to determine
> whether to take these reference counts. It is impossible to make
> the reference counting correct unless we save this bit of information
> in skcd->val. So, add a new bit there to record whether the socket
> has already taken the reference counts. This obviously relies on
> kmalloc() to align cgroup pointers to at least 4 bytes,
> ARCH_KMALLOC_MINALIGN is certainly larger than that.
>
> This bug seems to be introduced since the beginning, commit
> d979a39d7242 ("cgroup: duplicate cgroup reference when cloning sockets")
> tried to fix it but not compeletely. It seems not easy to trigger until
> the recent commit 090e28b229af
> ("netprio_cgroup: Fix unlimited memory leak of v2 cgroups") was merged.
>
> Fixes: bd1060a1d671 ("sock, cgroup: add sock->sk_cgroup")
> Reported-by: Cameron Berkenpas <cam@xxxxxxxxxxx>
> Reported-by: Peter Geis <pgwipeout@xxxxxxxxx>
> Reported-by: Lu Fengqi <lufq.fnst@xxxxxxxxxxxxxx>
> Reported-by: Daniël Sonck <dsonck92@xxxxxxxxx>
> Reported-by: Zhang Qiang <qiang.zhang@xxxxxxxxxxxxx>
> Tested-by: Cameron Berkenpas <cam@xxxxxxxxxxx>
> Tested-by: Peter Geis <pgwipeout@xxxxxxxxx>
> Tested-by: Thomas Lamprecht <t.lamprecht@xxxxxxxxxxx>
> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> Cc: Zefan Li <lizefan@xxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Roman Gushchin <guro@xxxxxx>
> Signed-off-by: Cong Wang <xiyou.wangcong@xxxxxxxxx>
> Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> ---
[...]
>   
> +void cgroup_sk_clone(struct sock_cgroup_data *skcd)
> +{
> +	/* Socket clone path */
> +	if (skcd->val) {

Compare to mainline patch, it's missing *if (skcd->no_refcnt)* check here.

Is it a mistake here ?

Thanks,

Yang

> +		/*
> +		 * We might be cloning a socket which is left in an empty
> +		 * cgroup and the cgroup might have already been rmdir'd.
> +		 * Don't use cgroup_get_live().
> +		 */
> +		cgroup_get(sock_cgroup_ptr(skcd));
> +	}
> +}
> +
>   void cgroup_sk_free(struct sock_cgroup_data *skcd)
>   {
> +	if (skcd->no_refcnt)
> +		return;
> +
>   	cgroup_put(sock_cgroup_ptr(skcd));
>   }
>   
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -1694,7 +1694,7 @@ struct sock *sk_clone_lock(const struct
>   		/* sk->sk_memcg will be populated at accept() time */
>   		newsk->sk_memcg = NULL;
>   
> -		cgroup_sk_alloc(&newsk->sk_cgrp_data);
> +		cgroup_sk_clone(&newsk->sk_cgrp_data);
>   
>   		rcu_read_lock();
>   		filter = rcu_dereference(sk->sk_filter);
>
>
> .