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

[Breno Leitao]

Hello Eric,

On Wed, Oct 30, 2024 at 03:18:14PM +0100, Eric Dumazet wrote:
> 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.

Thanks.  I got the impression that the scheduler list was append-only,
and the entries were never freed.