CVE-2021-47261

In the Linux kernel, the following vulnerability has been resolved:

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.