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

● CVE-2024-26585 · CVSS 4.7 · Media
Investigación · FlawOpen

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

Análisis técnico del código fuente y mitigaciones de ingeniería 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.

💡 Explicación en Lenguaje Sencillo (ELI5)

Analogía práctica: 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.

Conceptos Clave y Términos

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.

Análisis de Causa Raíz

La causa raíz se debe a parámetros de límite no validados en sistemas de código abierto, lo que permite la desincronización de estado y la elusión de controles de seguridad.

Flujo de Ataque Paso a Paso

Step 1

Fase de ataque: Open kTLS Socket

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

Step 3

Fase de ataque: Abrupt Socket Teardown

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

Step 4

Corrupción de memoria : Kernel Memory Corruption

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

Código Fuente: Vulnerable vs. Seguro

IMPLEMENTACIÓN VULNERABLE
// 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);
}
PARCHE SEGURO Y 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 Verificación de Seguridad para Ingeniería

Fuentes