CVE-2021-47593: 
Linux Kernel vulnerability analysis and mitigation

In the Linux kernel, a vulnerability (CVE-2021-47593) was discovered in the MPTCP (Multipath TCP) implementation. The issue affects Linux kernel versions from 5.6 up to (excluding) 5.10.88, from 5.11 up to (excluding) 5.15.11, and 5.16 release candidates (rc1-rc5). The vulnerability was assigned a CVSS v3.1 base score of 5.5 (Medium) (NVD).

The vulnerability stems from incorrect handling of the 'kern' flag in fallback sockets. The MPTCP ULP (Upper Layer Protocol) extension relies on sk->sk_sock_kern being set correctly to prevent setsockopt(fd, IPPROTO_TCP, TCP_ULP, "mptcp", 6) from working on plain TCP sockets. However, when a fallback occurs, accept() can return a plain TCP socket that is incorrectly tagged as 'kernel', allowing setsockopt to work when it shouldn't. This results in a NULL pointer dereference in the subflow extension due to a NULL ctx->conn MPTCP socket (Kernel Patch).

When exploited, this vulnerability causes a kernel crash due to a NULL pointer dereference in the subflow_data_ready function. This can lead to a denial of service condition on affected systems (NVD).

The vulnerability can be triggered through the setsockopt system call on fallback sockets in MPTCP connections. It requires local access to the system and low privileges to exploit (NVD).

The vulnerability has been fixed in Linux kernel versions 5.10.88, 5.15.11, and later releases. The fix involves properly clearing the 'kern' flag from fallback sockets in the MPTCP implementation (Kernel Patch). Users are advised to upgrade to the patched kernel versions.