CVE-2025-21710

CVE-2025-21710 updated by NVD

Modified

2026-08-01

Sources

nvd

Summary

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( 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( 39222]: ADVERTISING WIN 0, ACK_SEQ: 265600160 [5201 39222]: [__tcp_transmit_skb( 39222]: tcp_rcv_established( 39222]: tcp_v4_rcv( 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( 260040464 (0), unread 5559696, qlen 85, ofoq 0] returning 6104 bytes [5201 39222]: tcp_recvmsg_locked( 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---

References

• https://nvd.nist.gov/vuln/detail/CVE-2025-21710