flawopen.com/Teardowns/cve-2024-26585-linux-tls-zerocopy-use-after-free

● CVE-2024-26585 · CVSS 4.7 · Média
Pesquisa · FlawOpen

CVE-2024-26585: Linux Kernel TLS Subsystem Use-After-Free Teardown

Análise técnica detalhada e engenharia de mitigação do patch para vulnerabilidade: How an asynchronous cryptographic callback race condition in net/tls/tls_sw.c freed network pages while still queued for zero-copy transmission.

💡 Explicação em Linguagem Simples (ELI5)

Analogia explicativa: Imagine you order a meal for delivery. The chef starts cooking, but you call and cancel the order. Because the kitchen staff wasn't notified properly, the delivery driver picks up an empty plate, drives to a new customer's house, and serves them whatever was left on the counter, contaminating the new customer's food.

Conceitos Centrais e Termos

Kernel TLS (kTLS)
Linux kernel facility to perform symmetric TLS encryption/decryption directly inside the network socket layer for maximum throughput.
Zero-Copy Networking
Transmitting data directly from application buffers to the network card without intermediate CPU memory copying.
Asynchronous Crypto (aead_request)
Offloading cryptographic AES-GCM operations to asynchronous hardware accelerators.
Slab Corruption
Corrupting Linux kernel slab cache structures (kmalloc-512), destabilizing kernel execution.

Análise de Causa Raiz

A causa raiz decorre de parâmetros de limite não validados em sistemas de código aberto, permitindo a dessincronização de estado e a evasão dos controles de segurança.

Fluxo de Ataque Passo a Paso

Step 1

Fase de ataque: Open kTLS Socket

Mecanismo de exploração técnica: The attacker creates a TLS socket and enables kernel encryption offload via setsockopt(TCP_ULP, "tls").

Step 2

Fase de ataque: Submit Asynchronous Zero-Copy Payload

Mecanismo de exploração técnica: The attacker sends data using MSG_ZEROCOPY, queuing pages for hardware crypto.

Step 3

Fase de ataque: Abrupt Socket Teardown

Mecanismo de exploração técnica: The attacker closes the socket before the crypto accelerator completes its async callback.

Step 4

Corrupção de memória : Kernel Memory Corruption

Mecanismo de exploração técnica: The cleanup handler frees memory pages that the delayed hardware callback subsequently overwrites.

Código-Fonte: Vulnerável vs. Seguro

IMPLEMENTAÇÃO VULNERÁVEL
// VULNERABLE: net/tls/tls_sw.c before patch
static void tls_encrypt_done(void *data, int err) {
    struct tls_context *ctx = data;
    struct tls_sw_context_tx *ctx_tx = tls_sw_ctx_tx(ctx);

    // ROOT CAUSE:
    // Asynchronous completion callback assumes socket context is still locked!
    // If the socket was closed while crypto was in flight, ctx_tx is already freed!
    clear_bit(TLS_TX_SYNC_MORE, &ctx_tx->tx_bitmask);
    tls_free_open_rec(ctx);
}
PATCH SEGURO E ROBUSTO
// SECURE: net/tls/tls_sw.c patch
static void tls_encrypt_done(void *data, int err) {
    struct tls_context *ctx = data;
    
    // 1. Verify context reference counter before dereferencing context pointers
    if (!refcount_inc_not_zero(&ctx->refcount)) {
        return; // Socket is already dying, abort callback safely!
    }
    
    struct tls_sw_context_tx *ctx_tx = tls_sw_ctx_tx(ctx);
    clear_bit(TLS_TX_SYNC_MORE, &ctx_tx->tx_bitmask);
    tls_free_open_rec(ctx);
    
    // 2. Drop reference cleanly
    refcount_dec(&ctx->refcount);
}

Lista de Verificação de Segurança para Engenharia

Fontes