CVE-2020-25704: 
Linux Kernel vulnerability analysis and mitigation

A memory leak vulnerability (CVE-2020-25704) was discovered in the Linux kernel performance monitoring subsystem, specifically in the PERF_EVENT_IOC_SET_FILTER functionality. The vulnerability was discovered by kiyin(尹亮) and disclosed on November 9, 2020. This flaw affects Linux kernel versions up to and including 5.9, as well as various Linux distributions and products that use the affected kernel versions (Kernel Git, NVD).

The vulnerability stems from a memory leak in the perf_event_parse_addr_filter function where allocated memory for the 'filename' variable is not properly freed in certain code paths. The issue occurs when using PERF_EVENT_IOC_SET_FILTER, where memory is allocated using match_strdup but not freed in the failure branch. The vulnerability has been assigned a CVSS v3.1 base score of 5.5 (Medium) with vector CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H (NVD, OSS Security).

The vulnerability allows a local user with appropriate permissions to cause a denial of service through resource exhaustion (memory leak). Testing demonstrated that it could exhaust 16GB of memory in approximately 40 seconds on affected systems. The length of the filename parameter is not limited, which can be exploited to accelerate the memory exhaustion (OSS Security).

The vulnerability requires local access and appropriate permissions to access performance events. Some Linux distributions, such as Debian, mitigate this issue by default as their kernel configuration does not allow unprivileged users to access performance events (Debian LTS).

The vulnerability was fixed in Linux kernel 5.10-rc3 with commit 7bdb157cdebbf95a1cd94ed2e01b338714075d00. The fix ensures proper memory deallocation by cleaning up the code flow and ensuring 'filename' is always freed in both reallocation and return paths. Various Linux distributions have backported the fix to their supported kernel versions (Kernel Git).