CVE-2021-47590: 
Linux Kernel vulnerability analysis and mitigation

CVE-2021-47590 affects the Linux kernel's MPTCP (Multipath TCP) implementation. The vulnerability was discovered in the __mptcp_push_pending() function where a deadlock condition can occur when mptcp_flush_join_list() is called with a subflow socket lock held. This issue affects Linux kernel versions from 5.13 through 5.15.11 (NVD).

The vulnerability occurs when __mptcp_push_pending() calls mptcp_flush_join_list() while holding the subflow socket lock. If this call reaches mptcp_sockopt_sync_all(), then __mptcp_sockopt_sync() attempts to lock the subflow socket for itself, resulting in a deadlock condition. The issue has a CVSS v3.1 Base Score of 5.5 (MEDIUM) with the vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H (NVD).

The vulnerability can result in a system deadlock, potentially causing denial of service conditions in affected systems. This is particularly evident when the ss-server process enters a blocked state (D state) during execution (Kernel Patch).

The vulnerability requires local access and low privileges to exploit. It can be triggered through normal MPTCP operations when specific socket operations are performed (NVD).

The issue has been fixed by replacing mptcp_flush_join_list() with __mptcp_flush_join_list() inside __mptcp_push_pending(). This change defers the sockopt sync to the workqueue, preventing the deadlock condition. Users should upgrade to kernel versions containing the fix (Kernel Patch).