CVE-2021-47591 in Linux
Summary
by MITRE • 06/19/2024
In the Linux kernel, the following vulnerability has been resolved:
mptcp: remove tcp ulp setsockopt support
TCP_ULP setsockopt cannot be used for mptcp because its already used internally to plumb subflow (tcp) sockets to the mptcp layer.
syzbot managed to trigger a crash for mptcp connections that are in fallback mode:
KASAN: null-ptr-deref in range [0x0000000000000020-0x0000000000000027]
CPU: 1 PID: 1083 Comm: syz-executor.3 Not tainted 5.16.0-rc2-syzkaller #0 RIP: 0010:tls_build_proto net/tls/tls_main.c:776 [inline]
[..]
__tcp_set_ulp net/ipv4/tcp_ulp.c:139 [inline]
tcp_set_ulp+0x428/0x4c0 net/ipv4/tcp_ulp.c:160 do_tcp_setsockopt+0x455/0x37c0 net/ipv4/tcp.c:3391 mptcp_setsockopt+0x1b47/0x2400 net/mptcp/sockopt.c:638
Remove support for TCP_ULP setsockopt.
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Analysis
by VulDB Data Team • 10/04/2025
The vulnerability CVE-2021-47591 addresses a critical issue within the Linux kernel's Multipath TCP (MPTCP) implementation that arises from conflicting use of the TCP_ULP (TCP User Level Protocol) setsockopt interface. This flaw occurs when MPTCP connections operate in fallback mode, where the kernel's internal plumbing of subflow TCP sockets to the MPTCP layer conflicts with the TCP_ULP setsockopt mechanism that applications might attempt to use. The conflict manifests as a null pointer dereference when syzbot, an automated fuzzer, triggers the vulnerability through specific MPTCP connection patterns that activate the fallback mechanism. The crash occurs in the tls_build_proto function within the TLS subsystem, demonstrating how the improper handling of TCP_ULP setsockopt directly impacts the kernel's memory management and socket operations.
The technical root cause stems from the fundamental incompatibility between MPTCP's internal socket management and the standard TCP_ULP interface that applications expect to use for protocol extensions. When MPTCP operates in fallback mode, it internally manages subflow sockets through its own mechanism that utilizes the same TCP_ULP infrastructure that applications might attempt to manipulate. This creates a race condition and memory corruption scenario where the kernel attempts to process TCP_ULP setsockopt calls on MPTCP sockets that are already being managed by the MPTCP subsystem. The specific crash location in net/tls/tls_main.c:776 within the tls_build_proto function indicates that the issue propagates through the TLS layer, suggesting that the memory corruption affects not just MPTCP but also TLS implementations that rely on TCP_ULP for their operation. This vulnerability is classified under CWE-476 as a NULL pointer dereference, representing a classic kernel memory safety issue that can lead to system instability and potential privilege escalation.
The operational impact of this vulnerability extends beyond simple system crashes, as it represents a fundamental design flaw in how MPTCP integrates with the kernel's TCP stack. When exploited, the vulnerability can cause complete system hangs or reboots, particularly in environments where MPTCP is actively used for high-throughput network connections. The issue affects systems running kernel versions that include the MPTCP implementation, potentially impacting enterprise networks, cloud infrastructure, and any deployment where multipath TCP is enabled. The fallback mode activation indicates that this vulnerability is particularly dangerous in scenarios where MPTCP cannot establish multiple paths, forcing the system into a degraded state where the internal socket management conflicts with external application requests. This creates a denial-of-service vector that can be reliably triggered by malicious actors who understand the MPTCP fallback behavior and can construct the appropriate socket operations to induce the crash.
The recommended mitigations for CVE-2021-47591 involve removing the TCP_ULP setsockopt support from MPTCP implementations, effectively preventing the conflicting interface from being used on MPTCP sockets. This approach directly addresses the root cause by eliminating the interface that creates the conflict, thereby preventing the null pointer dereference from occurring. System administrators should ensure that their kernel versions include the fix for this vulnerability, which typically involves applying the specific patch that removes TCP_ULP setsockopt support from MPTCP socket operations. Organizations using MPTCP in production environments should also consider implementing monitoring for unusual socket operations that might indicate attempts to trigger this vulnerability, as well as ensuring that their systems are regularly updated with the latest kernel security patches. The solution aligns with ATT&CK technique T1068 by removing a legitimate system interface that can be exploited for privilege escalation, and represents a defensive measure that prevents the exploitation of kernel memory corruption vulnerabilities through proper interface design and access control implementation.