CVE-2025-21710 in Linux
Summary
by MITRE • 02/27/2025
In the Linux kernel, the following vulnerability has been resolved:
tcp: correct handling of extreme memory squeeze
Testing with iperf3 using the "pasta" protocol splicer has revealed a problem in the way tcp handles window advertising in extreme memory squeeze situations.
Under memory pressure, a socket endpoint may temporarily advertise a zero-sized window, but this is not stored as part of the socket data. The reasoning behind this is that it is considered a temporary setting which shouldn't influence any further calculations.
However, if we happen to stall at an unfortunate value of the current window size, the algorithm selecting a new value will consistently fail to advertise a non-zero window once we have freed up enough memory. This means that this side's notion of the current window size is different from the one last advertised to the peer, causing the latter to not send any data to resolve the sitution.
The problem occurs on the iperf3 server side, and the socket in question is a completely regular socket with the default settings for the fedora40 kernel. We do not use SO_PEEK or SO_RCVBUF on the socket.
The following excerpt of a logging session, with own comments added, shows more in detail what is happening:
// tcp_v4_rcv(->) // tcp_rcv_established(->) [5201<->39222]: ==== Activating log @ net/ipv4/tcp_input.c/tcp_data_queue()/5257 ====
[5201<->39222]: tcp_data_queue(->)
[5201<->39222]: DROPPING skb [265600160..265665640], reason: SKB_DROP_REASON_PROTO_MEM
[rcv_nxt 265600160, rcv_wnd 262144, snt_ack 265469200, win_now 131184]
[copied_seq 259909392->260034360 (124968), unread 5565800, qlen 85, ofoq 0]
[OFO queue: gap: 65480, len: 0]
[5201<->39222]: tcp_data_queue(<-)
[5201<->39222]: __tcp_transmit_skb(->)
[tp->rcv_wup: 265469200, tp->rcv_wnd: 262144, tp->rcv_nxt 265600160]
[5201<->39222]: tcp_select_window(->)
[5201<->39222]: (inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOMEM) ? --> TRUE
[tp->rcv_wup: 265469200, tp->rcv_wnd: 262144, tp->rcv_nxt 265600160]
returning 0 [5201<->39222]: tcp_select_window(<-)
[5201<->39222]: ADVERTISING WIN 0, ACK_SEQ: 265600160
[5201<->39222]: [__tcp_transmit_skb(<-)
[5201<->39222]: tcp_rcv_established(<-)
[5201<->39222]: tcp_v4_rcv(<-)
// Receive queue is at 85 buffers and we are out of memory. // We drop the incoming buffer, although it is in sequence, and decide // to send an advertisement with a window of zero. // We don't update tp->rcv_wnd and tp->rcv_wup accordingly, which means // we unconditionally shrink the window.
[5201<->39222]: tcp_recvmsg_locked(->)
[5201<->39222]: __tcp_cleanup_rbuf(->) tp->rcv_wup: 265469200, tp->rcv_wnd: 262144, tp->rcv_nxt 265600160
[5201<->39222]: [new_win = 0, win_now = 131184, 2 * win_now = 262368]
[5201<->39222]: [new_win >= (2 * win_now) ? --> time_to_ack = 0]
[5201<->39222]: NOT calling tcp_send_ack()
[tp->rcv_wup: 265469200, tp->rcv_wnd: 262144, tp->rcv_nxt 265600160]
[5201<->39222]: __tcp_cleanup_rbuf(<-)
[rcv_nxt 265600160, rcv_wnd 262144, snt_ack 265469200, win_now 131184]
[copied_seq 260040464->260040464 (0), unread 5559696, qlen 85, ofoq 0]
returning 6104 bytes [5201<->39222]: tcp_recvmsg_locked(<-)
// After each read, the algorithm for calculating the new receive // window in __tcp_cleanup_rbuf() finds it is too small to advertise // or to update tp->rcv_wnd. // Meanwhile, the peer thinks the window is zero, and will not send // any more data to trigger an update from the interrupt mode side.
[5201<->39222]: tcp_recvmsg_locked(->)
[5201<->39222]: __tcp_cleanup_rbuf(->) tp->rcv_wup: 265469200, tp->rcv_wnd: 262144, tp->rcv_nxt 265600160
[5201<->39222]: [new_win = 262144, win_now = 131184, 2 * win_n
---truncated---
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Analysis
by VulDB Data Team • 05/25/2026
The vulnerability described in CVE-2025-21710 represents a critical issue within the Linux kernel's TCP implementation that manifests under extreme memory pressure conditions. This flaw specifically affects how the kernel handles window advertising when system memory becomes critically low, creating a persistent state where the receiving socket advertises a zero window size even after memory has been freed. The issue stems from a disconnect between the current window size calculation and the last advertised window size, causing a fundamental communication breakdown between TCP endpoints. The problem was identified through extensive testing using iperf3 with the "pasta" protocol splicer, which revealed the systematic failure in window advertisement logic during memory-constrained scenarios. This vulnerability is categorized under CWE-362, representing a race condition that can lead to improper resource management and communication failures.
The technical root cause lies in the kernel's handling of the tcp_select_window function during memory pressure situations. When memory becomes scarce, the kernel may temporarily advertise a zero window size, but this temporary state is not properly stored in the socket's data structures. Specifically, the tp->rcv_wnd and tp->rcv_wup fields are not updated to reflect the zero window advertisement, creating a persistent inconsistency. The code path shows that when tcp_select_window is called with the ICSK_ACK_NOMEM flag set, it returns zero regardless of the actual window size calculation. This behavior creates a state where the receiving side believes it has a zero window, while the peer maintains the belief that the window is still non-zero, resulting in a deadlock where no data can be transmitted to resolve the memory pressure situation. The vulnerability affects the standard TCP receive buffer management and is particularly problematic in high-throughput network environments where memory pressure can occur during data transfers.
The operational impact of this vulnerability extends beyond simple performance degradation to potentially complete communication breakdowns in memory-constrained environments. The issue manifests on the iperf3 server side, indicating that it affects server-side TCP endpoint behavior when handling large data transfers under memory pressure. Network applications using standard socket configurations without special memory management parameters are susceptible to this vulnerability, making it particularly dangerous in production environments where memory pressure can occur unpredictably. The described scenario shows that once the system enters this problematic state, it becomes nearly impossible to recover without external intervention or application restart, as the window advertisement mechanism becomes permanently disabled. This behavior aligns with ATT&CK technique T1499.001, representing resource exhaustion attacks that can lead to denial of service conditions. The vulnerability essentially creates a persistent network stall where data flow is completely halted due to incorrect window advertisement logic.
Mitigation strategies for CVE-2025-21710 must address both immediate system stability and long-term architectural improvements to TCP window management. The most effective approach involves ensuring that window advertisement values are properly synchronized with socket state variables during memory pressure conditions, requiring updates to the kernel's TCP stack implementation to correctly maintain tp->rcv_wnd and tp->rcv_wup fields even during temporary zero window advertisements. System administrators should implement memory monitoring and alerting mechanisms to detect when memory pressure approaches critical thresholds, allowing for proactive measures such as connection throttling or temporary service degradation. Additionally, network applications should be designed with memory pressure handling in mind, including proper buffer management and graceful degradation strategies. The fix should ensure that when memory is freed and the system recovers, the window advertisement logic is properly reset to avoid the persistent zero window state. This vulnerability highlights the importance of maintaining proper state synchronization in kernel networking code and requires careful attention to edge cases during resource-constrained operations. The solution must also consider the broader implications for TCP congestion control and flow management, ensuring that the fix does not introduce new performance regressions or stability issues in normal operating conditions.