CVE-2025-21710 — tcp: correct handling of extreme memory squeeze

Affected: Linux

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..…

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CVE-2025-21710 — tcp: correct handling of extreme memory squeeze