Re: [PATCH bpf-next v1] bpf, sockmap: Fix psock incorrectly pointing to sk

Jiayuan Chen
2025-05-29T02:42:36Z

May 29, 2025 at 07:46, "John Fastabend" <john.fastabend@gmail.com> wrote:

>
> On 2025-05-24 00:22:19, Jiayuan Chen wrote:
>
> >
> > We observed an issue from the latest selftest: sockmap_redir where
> >
> > sk_psock(psock->sk) != psock in the backlog. The root cause is the special
> >
> > behavior in sockmap_redir - it frequently performs map_update() and
> >
> > map_delete() on the same socket. During map_update(), we create a new
> >
> > psock and during map_delete(), we eventually free the psock via rcu_work
> >
> > in sk_psock_drop(). However, pending workqueues might still exist and not
> >
> > be processed yet. If users immediately perform another map_update(), a new
> >
> > psock will be allocated for the same sk, resulting in two psocks pointing
> >
> > to the same sk.
> >
> >
> >
> > When the pending workqueue is later triggered, it uses the old psock to
> >
> > access sk for I/O operations, which is incorrect.
> >
> >
> >
> > Timing Diagram:
> >
> >
> >
> > cpu0 cpu1
> >
> >
> >
> > map_update(sk):
> >
> > sk->psock = psock1
> >
> > psock1->sk = sk
> >
> > map_delete(sk):
> >
> > rcu_work_free(psock1)
> >
> >
> >
> > map_update(sk):
> >
> > sk->psock = psock2
> >
> > psock2->sk = sk
> >
> > workqueue:
> >
> > wakeup with psock1, but the sk of psock1
> >
> > doesn't belong to psock1
> >
> > rcu_handler:
> >
> > clean psock1
> >
> > free(psock1)
> >
> >
> >
> > Previously, we used reference counting to address the concurrency issue
> >
> > between backlog and sock_map_close(). This logic remains necessary as it
> >
> > prevents the sk from being freed while processing the backlog. But this
> >
> > patch prevents pending backlogs from using a psock after it has been
> >
> > freed.
> >
>
> Nit, its not that psock would be freed because we do have the
>
> cancel_delayed_work_sync() before the kfree(psock). But this
>
> is not a good state with two psocks referenceing the same sk.
>
> >
> > Note: We cannot call cancel_delayed_work_sync() in map_delete() since this
> >
> > might be invoked in BPF context by BPF helper, and the function may sleep.
> >
> >
> >
> > Fixes: 604326b41a6f ("bpf, sockmap: convert to generic sk_msg interface")
> >
> > Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> >
> >
> >
> > ---
> >
> > Thanks to Michal Luczaj for providing the sockmap_redir test case, which
> >
> > indeed covers almost all sockmap forwarding paths.
> >
> > ---
> >
> > include/linux/skmsg.h | 1 +
> >
> > net/core/skmsg.c | 5 ++++-
> >
> > 2 files changed, 5 insertions(+), 1 deletion(-)
> >
> >
> >
> > diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
> >
> > index 0b9095a281b8..b17221eef2f4 100644
> >
> > --- a/include/linux/skmsg.h
> >
> > +++ b/include/linux/skmsg.h
> >
> > @@ -67,6 +67,7 @@ struct sk_psock_progs {
> >
> > enum sk_psock_state_bits {
> >
> > SK_PSOCK_TX_ENABLED,
> >
> > SK_PSOCK_RX_STRP_ENABLED,
> >
> > + SK_PSOCK_DROPPED,
> >
> > };
> >
> >
> >
> > struct sk_psock_link {
> >
> > diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> >
> > index 34c51eb1a14f..bd58a693ce9a 100644
> >
> > --- a/net/core/skmsg.c
> >
> > +++ b/net/core/skmsg.c
> >
> > @@ -656,6 +656,9 @@ static void sk_psock_backlog(struct work_struct *work)
> >
> > bool ingress;
> >
> > int ret;
> >
> >
> >
> > + if (sk_psock_test_state(psock, SK_PSOCK_DROPPED))
> >
> > + return;
> >
>
> Could we use the SK_PSOCK_TX_ENABLED bit here? Its already used to
>
> ensure we wont requeue work after the psock has started being
>
> removed. Seems like we don't need two flags? wdyt?
>
> >
> > +
> >
> > /* Increment the psock refcnt to synchronize with close(fd) path in
> >
> > * sock_map_close(), ensuring we wait for backlog thread completion
> >
> > * before sk_socket freed. If refcnt increment fails, it indicates
> >
> > @@ -867,7 +870,7 @@ void sk_psock_drop(struct sock *sk, struct sk_psock *psock)
> >
> > write_unlock_bh(&sk->sk_callback_lock);
> >
> >
> >
> > sk_psock_stop(psock);
> >
>
> Can we add this to sk_psock_stop where we have the TX_ENABLED bit
>
> cleared.

Thanks, I just add SK_PSOCK_TX_ENABLED checking at the start of sk_psock_backlog().
Every works fine, and truly no more flag needed !

diff --git a/net/core/skmsg.c b/net/core/skmsg.c
index 34c51eb1a14f..83c78379932e 100644
--- a/net/core/skmsg.c
+++ b/net/core/skmsg.c
@@ -656,6 +656,13 @@ static void sk_psock_backlog(struct work_struct *work)
bool ingress;
int ret;

+ /* If sk is quickly removed from the map and then added back, the old
+ * psock should not be scheduled, because there are now two psocks
+ * pointing to the same sk.
+ */
+ if (!sk_psock_test_state(psock, SK_PSOCK_TX_ENABLED))
+ return;
+
/* Increment the psock refcnt to synchronize with close(fd) path in
* sock_map_close(), ensuring we wait for backlog thread completion
* before sk_socket freed. If refcnt increment fails, it indicates

> >
> > -
> >
> > + sk_psock_set_state(psock, SK_PSOCK_DROPPED);
> >
> > INIT_RCU_WORK(&psock->rwork, sk_psock_destroy);
> >
> > queue_rcu_work(system_wq, &psock->rwork);
> >
> > }
> >
> > --
> >
> > 2.47.1
> >
>