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security: gracefully handle RX pool exhaustion to prevent socket app deadlock #77

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security: gracefully handle RX pool exhaustion to prevent socket app deadlock #77

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17 changes: 17 additions & 0 deletions src/core/tcp_in.c
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Original file line number Diff line number Diff line change
Expand Up @@ -992,6 +992,23 @@ tcp_process(struct tcp_pcb *pcb)
case CLOSE_WAIT:
/* FALLTHROUGH */
case ESTABLISHED:
#if LIMIT_PAYLOAD_THRESHOLD_ENABLE && (LWIP_SOCKET || LWIP_NETCONN)
/* Security: Gracefully handle RX pool exhaustion to prevent socket application
* deadlock. Drop payload data (but ACK it) when approaching pool limit, reserving
* 2 buffers for critical TCP control messages (SYN, FIN) which have minimal payload,
* are not queued to application layer, and are released within the same ethernet_input
* call. This ensures the stack can process connection state changes under memory
* pressure, preventing resource exhaustion attacks and application hangs.
* NOTE: Only active with Socket/Netconn API where queued payload can cause deadlock.
* NOTE: Assumes statistics are enabled (MEMP_STATS).
* NOTE: Assumes no custom buffer allocation. */
if (MEMP_STATS_GET(used, MEMP_PBUF_POOL) > MEMP_STATS_GET(max, MEMP_PBUF_POOL) - 2) {
if (tcplen > 0) {
pcb->rcv_nxt += tcplen;
}
return ERR_OK;
}
#endif
tcp_receive(pcb);
if (recv_flags & TF_GOT_FIN) { /* passive close */
tcp_ack_now(pcb);
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19 changes: 19 additions & 0 deletions src/include/lwip/opt.h
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Original file line number Diff line number Diff line change
Expand Up @@ -1455,6 +1455,25 @@
#endif
#endif

/**
* LIMIT_PAYLOAD_THRESHOLD_ENABLE: Graceful RX pool exhaustion handling to prevent
* socket application deadlock.
*
* Security: When enabled, prevents resource exhaustion attacks and application hangs
* by dropping incoming payload (while still ACKing it) when PBUF_POOL approaches
* exhaustion. This reserves 2 buffers for critical TCP control messages (SYN, FIN)
* which are released immediately within the same ethernet_input call, ensuring the
* stack can always process connection state changes even under memory pressure.
*
* Only effective with Socket/Netconn API where payload is queued and can cause
* deadlock if the application cannot read due to buffer starvation.
* Requires MEMP_STATS to be enabled for buffer usage tracking.
* Define to 0 to disable this feature.
*/
#if !defined LIMIT_PAYLOAD_THRESHOLD_ENABLE || defined __DOXYGEN__
#define LIMIT_PAYLOAD_THRESHOLD_ENABLE 0
#endif

/**
* TCP_LISTEN_BACKLOG: Enable the backlog option for tcp listen pcb.
*/
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