CVE-2021-47001: 
Linux Kernel vulnerability analysis and mitigation

CVE-2021-47001 is a vulnerability in the Linux kernel's xprtrdma module that was discovered and fixed in 2024. The issue involves incorrect ordering of cwnd (congestion window) updates in the RDMA transport layer, where after a reconnect, the reply handler opens the cwnd before enough Receive Work Requests (WRs) can be posted to receive their replies (CVE Details).

The vulnerability occurs in the Linux kernel's RDMA transport implementation where a race condition exists between the reply handler opening the congestion window and posting receive requests. After a reconnect, the reply handler opens the cwnd (enabling more RPC Calls to be sent) before rpcrdmapostrecvs() can post enough Receive WRs to receive their replies. This causes an RNR (Receiver Not Ready) condition and the new connection is lost immediately. The issue is particularly noticeable when KASAN and disconnect injection are enabled, as it slows down rpcrdmarepcreate() enough to allow the send side to post multiple RPC Calls before the Receive completion handler can invoke ibpostrecv() (Kernel Patch).

When exploited, this vulnerability results in immediate connection loss after reconnection attempts due to the RNR (Receiver Not Ready) condition. This can affect system stability and availability of RDMA-based network communications (Red Hat).

The vulnerability has been fixed in various Linux kernel versions. Ubuntu has released patches for affected versions including focal (5.4.0-204.224) and other distributions. Fixed versions are available for multiple kernel variants including linux-aws, linux-azure, and linux-gcp (Ubuntu Security).