Re: [PATCH net] mptcp: Ensure RCU read lock is held when calling mptcp_sched_find()

[Eric Dumazet]

On Wed, Oct 30, 2024 at 3:02 PM Breno Leitao <leitao@xxxxxxxxxx> wrote:
>
> The mptcp_sched_find() function must be called with the RCU read lock
> held, as it accesses RCU-protected data structures. This requirement was
> not properly enforced in the mptcp_init_sock() function, leading to a
> RCU list traversal in a non-reader section error when
> CONFIG_PROVE_RCU_LIST is enabled.
>
>         net/mptcp/sched.c:44 RCU-list traversed in non-reader section!!
>
> Fix it by acquiring the RCU read lock before calling the
> mptcp_sched_find() function. This ensures that the function is invoked
> with the necessary RCU protection in place, as it accesses RCU-protected
> data structures.
>
> Additionally, the patch breaks down the mptcp_init_sched() call into
> smaller parts, with the RCU read lock only covering the specific call to
> mptcp_sched_find(). This helps minimize the critical section, reducing
> the time during which RCU grace periods are blocked.
>
> The mptcp_sched_list_lock is not held in this case, and it is not clear
> if it is necessary.
>
> Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
> Fixes: 1730b2b2c5a5 ("mptcp: add sched in mptcp_sock")
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>  net/mptcp/protocol.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> index 6d0e201c3eb2..8ece630f80d4 100644
> --- a/net/mptcp/protocol.c
> +++ b/net/mptcp/protocol.c
> @@ -2854,6 +2854,7 @@ static void mptcp_ca_reset(struct sock *sk)
>  static int mptcp_init_sock(struct sock *sk)
>  {
>         struct net *net = sock_net(sk);
> +       struct mptcp_sched_ops *sched;
>         int ret;
>
>         __mptcp_init_sock(sk);
> @@ -2864,8 +2865,10 @@ static int mptcp_init_sock(struct sock *sk)
>         if (unlikely(!net->mib.mptcp_statistics) && !mptcp_mib_alloc(net))
>                 return -ENOMEM;
>
> -       ret = mptcp_init_sched(mptcp_sk(sk),
> -                              mptcp_sched_find(mptcp_get_scheduler(net)));
> +       rcu_read_lock();
> +       sched = mptcp_sched_find(mptcp_get_scheduler(net));
> +       rcu_read_unlock();

You are silencing the warning, but a potential UAF remains.

sched could have been freed already, it is illegal to deref it.

> +       ret = mptcp_init_sched(mptcp_sk(sk), sched);

The rcu_read_unlock() should be moved after this point.

This means that mptcp_sched_ops ->init() functions are not allowed to sleep.

>         if (ret)
>                 return ret;
>
> --
> 2.43.5
>