CVE-2021-47242: 
Linux Kernel vulnerability analysis and mitigation

In the Linux kernel, CVE-2021-47242 addresses a soft lockup vulnerability in the MPTCP (Multipath TCP) subsystem, specifically in the subflow_error_report() function. The vulnerability was discovered and reported by Maxim Galaganov. The issue affects the Linux kernel's MPTCP implementation and was fixed in June 2021 (Kernel Commit).

The vulnerability occurs when subflow_error_report() attempts to acquire the mptcp_data_lock while being called from different contexts. The calling function mptcp_subflow_data_available() can be invoked from three different contexts: plain ssk socket lock, ssk socket lock + mptcp_data_lock, or ssk socket lock + mptcp_data_lock + msk socket lock. When called from contexts that already hold the mptcp_data_lock, attempting to acquire it again causes a soft lockup condition (Kernel Commit).

The vulnerability can result in a CPU soft lockup condition, where a CPU becomes unresponsive for an extended period (reported as 22 seconds in the original bug report). This can affect system performance and stability (Kernel Commit).

The vulnerability requires specific conditions in the MPTCP subsystem to trigger and appears to be primarily a reliability issue rather than a security concern. There are no reports of this vulnerability being exploited in the wild (NVD).

The issue was fixed by moving the error reporting call to outer functions where the held locks list is known and only the needed locks can be acquired. The fix involves restructuring the code to handle error reporting at appropriate points in the call chain (Kernel Commit).