IB/mlx5: Fix initializing CQ fragments buffer

The function init_cq_frag_buf() can be called to initialize the current CQ
fragments buffer cq->buf, or the temporary cq->resize_buf that is filled
during CQ resize operation.

However, the offending commit started to use function get_cqe() for
getting the CQEs, the issue with this change is that get_cqe() always
returns CQEs from cq->buf, which leads us to initialize the wrong buffer,
and in case of enlarging the CQ we try to access elements beyond the size
of the current cq->buf and eventually hit a kernel panic.

 [exception RIP: init_cq_frag_buf+103]
  [ffff9f799ddcbcd8] mlx5_ib_resize_cq at ffffffffc0835d60 [mlx5_ib]
  [ffff9f799ddcbdb0] ib_resize_cq at ffffffffc05270df [ib_core]
  [ffff9f799ddcbdc0] llt_rdma_setup_qp at ffffffffc0a6a712 [llt]
  [ffff9f799ddcbe10] llt_rdma_cc_event_action at ffffffffc0a6b411 [llt]
  [ffff9f799ddcbe98] llt_rdma_client_conn_thread at ffffffffc0a6bb75 [llt]
  [ffff9f799ddcbec8] kthread at ffffffffa66c5da1
  [ffff9f799ddcbf50] ret_from_fork_nospec_begin at ffffffffa6d95ddd

Fix it by getting the needed CQE by calling mlx5_frag_buf_get_wqe() that
takes the correct source buffer as a parameter.

Fixes: 388ca8be00 ("IB/mlx5: Implement fragmented completion queue (CQ)")
Link: https://lore.kernel.org/r/90a0e8c924093cfa50a482880ad7e7edb73dc19a.1623309971.git.leonro@nvidia.com
Signed-off-by: Alaa Hleihel <alaa@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/hw/mlx5/cq.c

@@ -849,15 +849,14 @@ static void destroy_cq_user(struct mlx5_ib_cq *cq, struct ib_udata *udata)
 	ib_umem_release(cq->buf.umem);
 }
 
-static void init_cq_frag_buf(struct mlx5_ib_cq *cq,
-			     struct mlx5_ib_cq_buf *buf)
+static void init_cq_frag_buf(struct mlx5_ib_cq_buf *buf)
 {
 	int i;
 	void *cqe;
 	struct mlx5_cqe64 *cqe64;
 
 	for (i = 0; i < buf->nent; i++) {
-		cqe = get_cqe(cq, i);
+		cqe = mlx5_frag_buf_get_wqe(&buf->fbc, i);
 		cqe64 = buf->cqe_size == 64 ? cqe : cqe + 64;
 		cqe64->op_own = MLX5_CQE_INVALID << 4;
 	}
@@ -883,7 +882,7 @@ static int create_cq_kernel(struct mlx5_ib_dev *dev, struct mlx5_ib_cq *cq,
 	if (err)
 		goto err_db;
 
-	init_cq_frag_buf(cq, &cq->buf);
+	init_cq_frag_buf(&cq->buf);
 
 	*inlen = MLX5_ST_SZ_BYTES(create_cq_in) +
 		 MLX5_FLD_SZ_BYTES(create_cq_in, pas[0]) *
@@ -1184,7 +1183,7 @@ static int resize_kernel(struct mlx5_ib_dev *dev, struct mlx5_ib_cq *cq,
 	if (err)
 		goto ex;
 
-	init_cq_frag_buf(cq, cq->resize_buf);
+	init_cq_frag_buf(cq->resize_buf);
 
 	return 0;