[PATCH v2] RDMA/siw: Clear association under lock if siw_qp_modify fails in siw_accept

[Guoqing Jiang <guoqing.jiang@linux.dev>]

We need to clear cep before release state_lock as siw_qp_llp_close and
siw_qp_modify->siw_qp_llp_close did.

Otherwise if siw_qp_modify() fails in siw_accept(), the QP's state_lock
is released before the error path cleanup. A concurrent ibv_modify_qp()
transitioning the QP to ERROR can race in this window:

  siw_accept()                       ibv_modify_qp(ERROR)
  ----------------------             ----------------------
  siw_qp_modify() fails
  up_write(&qp->state_lock)
                                     down_write(&qp->state_lock)
                                     nextstate_from_idle():
				     if (qp->cep)
                                       siw_cep_put(qp->cep) <- frees cep
                                       qp->cep = NULL
  goto error
    cep->qp = NULL                   <- UAF

Clear qp->cep and drop the association reference taken by siw_cep_get(),
all under the write lock held from the initial down_write(&qp->state_lock).
Thread B therefore sees qp->cep == NULL, skips its own put, and cannot free
the cep before siw_accept() is done with it.

Reported-by: Shuangpeng Bai <shuangpeng.kernel@gmail.com>
Link: https://lore.kernel.org/linux-rdma/d6fbe475-a5c2-f975-99b0-a0bd6b6d10e8@linux.dev/T/#m5876c1ff2de8686a9a1173b8f1aa0ff5363a785c
Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
---
V2: remove redundant code per Bernard's review

 drivers/infiniband/sw/siw/siw_cm.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/infiniband/sw/siw/siw_cm.c b/drivers/infiniband/sw/siw/siw_cm.c
index 0245b25e7271..ed49818793dd 100644
--- a/drivers/infiniband/sw/siw/siw_cm.c
+++ b/drivers/infiniband/sw/siw/siw_cm.c
@@ -1719,9 +1719,12 @@ int siw_accept(struct iw_cm_id *id, struct iw_cm_conn_param *params)
 			   SIW_QP_ATTR_STATE | SIW_QP_ATTR_LLP_HANDLE |
 				   SIW_QP_ATTR_ORD | SIW_QP_ATTR_IRD |
 				   SIW_QP_ATTR_MPA);
+	if (rv) {
+		qp->cep = NULL;
+		siw_cep_put(cep);
+		goto error_unlock;
+	}
 	up_write(&qp->state_lock);
-	if (rv)
-		goto error;
 
 	siw_dbg_cep(cep, "[QP %u]: send mpa reply, %d byte pdata\n",
 		    qp_id(qp), params->private_data_len);
-- 
2.35.3