CVE-2024-56633: 
Linux Kernel vulnerability analysis and mitigation

CVE-2024-56633 is a vulnerability discovered in the Linux kernel's TCP BPF implementation, specifically in the sk_mem_uncharge logic within tcp_bpf_sendmsg. The issue was identified and disclosed on December 27, 2024, affecting the memory accounting mechanism in the __SK_REDIRECT functionality (NVD).

The vulnerability stems from the sk memory accounting logic in __SK_REDIRECT pre-uncharging tosend bytes, which can be either msg->sg.size or a smaller value apply_bytes. Two specific issues were identified: 1) When actual sent bytes are smaller than tosend, some bytes need to be charged back, and 2) When tosend is set to apply_bytes and (ret < 0), the system may miss uncharging (msg->sg.size - apply_bytes) bytes. The issue manifests when running the selftest test_txmsg_redir_wait_sndmem with txmsg_apply, triggering a warning in the inet_sock_destruct function (Kernel Commit).

The vulnerability affects the memory management system of the Linux kernel's TCP BPF implementation, potentially leading to memory accounting inconsistencies. This could result in memory leaks or incorrect memory allocation tracking in the kernel (Debian Tracker).

The vulnerability was discovered during testing with the test_txmsg_redir_wait_sndmem selftest with txmsg_apply. It primarily affects systems using TCP BPF functionality and can be triggered through specific memory operations in the tcp_bpf_sendmsg function (Kernel Commit).

The issue has been fixed in multiple Linux kernel versions, including 5.10.234-1 for Debian 11 bullseye and 6.1.128-1~deb11u1 for the linux-6.1 package. The fix involves modifying the memory uncharging logic to delay the uncharging until after sent bytes are finalized, and using sk_msg_free instead of sk_msg_free_partial in certain cases (DLA-4075-1, DLA-4076-1).