Re: [PATCH mptcp-net v3] mptcp: pm: userspace: unify entry free path via RCU callback

[Mat Martineau <martineau@kernel.org>]

On Wed, 1 Jul 2026, Geliang Tang wrote:

> From: Geliang Tang <tanggeliang@kylinos.cn>
>
> In mptcp_pm_nl_remove_doit(), sk_omem_alloc is decremented immediately
> but the memory is freed later via kfree_rcu(). This allows a CAP_NET_ADMIN
> user to bypass the socket memory quota and exhaust kernel memory by
> accumulating RCU callbacks.
>
> Fix by using call_rcu() with a custom callback that uses sock_kfree_s()
> to free the entry and decrement sk_omem_alloc atomically. To ensure the
> socket remains valid until the callback runs, take a reference with
> sock_hold() when storing the socket pointer in the entry, and release it
> with sock_put() in the callback.
>
> Convert the synchronous freeing paths in free_local_addr_list() and
> delete_local_addr() to use the same RCU callback, ensuring the socket
> reference is properly released.
>
> Additionally, mptcp_userspace_pm_append_new_local_addr() now checks
> SOCK_DEAD under the spinlock before allocating. A SYN+JOIN handler
> holding an msk reference from mptcp_token_get_sock() could otherwise
> race with __mptcp_destroy_sock() - sock_orphan() sets SOCK_DEAD and
> then mptcp_userspace_pm_release() clears the list, so a new entry
> allocated after that point would never be freed and its sock_hold()
> would leak the msk permanently.
>
> Fixes: 13b4ece33cf9 ("mptcp: pm: Defer freeing of MPTCP userspace path manager entries")
> Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
> ---
> v3:
> - checking sock_flag(sk, SOCK_DEAD)) before holding the reference.
> - update the subject.
>
> v2:
> - call mptcp_userspace_pm_free_entry in free_local_addr_list and
>   delete_local_addr.
> - Link: https://patchwork.kernel.org/project/mptcp/patch/df199842d10185a73084c79aee9cdc91888adb6a.1782799160.git.tanggeliang@kylinos.cn/
>
> v1:
> - Link: https://patchwork.kernel.org/project/mptcp/patch/9b443bafa57f40a51eb6a43f088ff37d71b39973.1782528088.git.tanggeliang@kylinos.cn/
>
> This patch addresses the pre-existing issue Sashiko mentioned in
> https://sashiko.dev/#/patchset/cover.1782457962.git.tanggeliang@kylinos.cn.
> ---
> net/mptcp/pm_userspace.c | 33 ++++++++++++++++++++++++---------
> net/mptcp/protocol.h     |  2 ++
> 2 files changed, 26 insertions(+), 9 deletions(-)
>
> diff --git a/net/mptcp/pm_userspace.c b/net/mptcp/pm_userspace.c
> index ad6ba658e5a5..c024c5cd5da1 100644
> --- a/net/mptcp/pm_userspace.c
> +++ b/net/mptcp/pm_userspace.c
> @@ -12,10 +12,19 @@
> 	list_for_each_entry(__entry,						\
> 			    &((__msk)->pm.userspace_pm_local_addr_list), list)
>
> +static void mptcp_userspace_pm_free_entry(struct rcu_head *head)
> +{
> +	struct mptcp_pm_addr_entry *entry =
> +		container_of(head, struct mptcp_pm_addr_entry, rcu);
> +	struct sock *sk = entry->sk;
> +
> +	sock_kfree_s(sk, entry, sizeof(*entry));
> +	sock_put(sk);
> +}
> +
> void mptcp_userspace_pm_free_local_addr_list(struct mptcp_sock *msk)
> {
> 	struct mptcp_pm_addr_entry *entry, *tmp;
> -	struct sock *sk = (struct sock *)msk;
> 	LIST_HEAD(free_list);
>
> 	spin_lock_bh(&msk->pm.lock);
> @@ -23,7 +32,7 @@ void mptcp_userspace_pm_free_local_addr_list(struct mptcp_sock *msk)
> 	spin_unlock_bh(&msk->pm.lock);
>
> 	list_for_each_entry_safe(entry, tmp, &free_list, list) {
> -		sock_kfree_s(sk, entry, sizeof(*entry));
> +		call_rcu(&entry->rcu, mptcp_userspace_pm_free_entry);
> 	}
> }

Hi Geliang -

The only code path that leads here is when the msk is being destroyed. It 
makes more sense to keep the existing synchronous code here.

>
> @@ -54,6 +63,15 @@ static int mptcp_userspace_pm_append_new_local_addr(struct mptcp_sock *msk,
> 	bitmap_zero(id_bitmap, MPTCP_PM_MAX_ADDR_ID + 1);
>
> 	spin_lock_bh(&msk->pm.lock);
> +	/* sock_orphan() has been called and mptcp_userspace_pm_release()
> +	 * has cleared userspace_pm_local_addr_list. Any entry we allocate
> +	 * here would never be freed via the list, leaking the sock_hold().
> +	 */
> +	if (sock_flag(sk, SOCK_DEAD)) {
> +		ret = -EINVAL;
> +		goto append_err;
> +	}
> +

This can be checked before the spinlock and bitmap_zero(), which allows a 
direct return instead of using the goto.

> 	mptcp_for_each_userspace_pm_addr(msk, e) {
> 		addr_match = mptcp_addresses_equal(&e->addr, &entry->addr, true);
> 		if (addr_match && entry->addr.id == 0 && needs_id)
> @@ -73,6 +91,8 @@ static int mptcp_userspace_pm_append_new_local_addr(struct mptcp_sock *msk,
> 			ret = -ENOMEM;
> 			goto append_err;
> 		}
> +		sock_hold(sk);
> +		e->sk = sk;

Better to set these immediately before using call_rcu(), it's not obvious 
where the matching sock_put() is. If taking this approach, a helper 
function could set these and then invoke call_rcu().

>
> 		if (!e->addr.id && needs_id)
> 			e->addr.id = find_next_zero_bit(id_bitmap,
> @@ -98,7 +118,6 @@ static int mptcp_userspace_pm_append_new_local_addr(struct mptcp_sock *msk,
> static int mptcp_userspace_pm_delete_local_addr(struct mptcp_sock *msk,
> 						struct mptcp_pm_addr_entry *addr)
> {
> -	struct sock *sk = (struct sock *)msk;
> 	struct mptcp_pm_addr_entry *entry;
>
> 	entry = mptcp_userspace_pm_lookup_addr(msk, &addr->addr);
> @@ -109,7 +128,7 @@ static int mptcp_userspace_pm_delete_local_addr(struct mptcp_sock *msk,
> 	 * be used multiple times (e.g. fullmesh mode).
> 	 */
> 	list_del_rcu(&entry->list);
> -	sock_kfree_s(sk, entry, sizeof(*entry));
> +	call_rcu(&entry->rcu, mptcp_userspace_pm_free_entry);
> 	msk->pm.local_addr_used--;
> 	return 0;
> }
> @@ -337,11 +356,7 @@ int mptcp_pm_nl_remove_doit(struct sk_buff *skb, struct genl_info *info)
>
> 	release_sock(sk);
>
> -	kfree_rcu_mightsleep(match);

The function name here reminded me that this is a sleepable context. 
Instead of adding all the new async code and increasing the size of 
mptcp_pm_addr_entry, another option is to insert a synchronize_rcu() here. 
However, that would delay completion of this netlink call and block other 
userspace PM operations for the RCU grace period, so maybe the async 
technique is worth it.

- Mat

> -	/* Adjust sk_omem_alloc like sock_kfree_s() does, to match
> -	 * with allocation of this memory by sock_kmemdup()
> -	 */
> -	atomic_sub(sizeof(*match), &sk->sk_omem_alloc);
> +	call_rcu(&match->rcu, mptcp_userspace_pm_free_entry);
>
> 	err = 0;
> out:
> diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
> index da40c6f3705f..250736eae0be 100644
> --- a/net/mptcp/protocol.h
> +++ b/net/mptcp/protocol.h
> @@ -257,6 +257,8 @@ struct mptcp_pm_addr_entry {
> 	u32			flags;
> 	int			ifindex;
> 	struct socket		*lsk;
> +	struct sock		*sk;
> +	struct rcu_head		rcu;
> };
>
> struct mptcp_data_frag {
> -- 
> 2.53.0
>
>
>