CVE-2022-49775 in Linux
Summary
by MITRE • 05/01/2025
In the Linux kernel, the following vulnerability has been resolved:
tcp: cdg: allow tcp_cdg_release() to be called multiple times
Apparently, mptcp is able to call tcp_disconnect() on an already disconnected flow. This is generally fine, unless current congestion control is CDG, because it might trigger a double-free [1]
Instead of fixing MPTCP, and future bugs, we can make tcp_disconnect() more resilient.
[1]
BUG: KASAN: double-free in slab_free mm/slub.c:3539 [inline]
BUG: KASAN: double-free in kfree+0xe2/0x580 mm/slub.c:4567
CPU: 0 PID: 3645 Comm: kworker/0:7 Not tainted 6.0.0-syzkaller-02734-g0326074ff465 #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/22/2022 Workqueue: events mptcp_worker Call Trace: <TASK> __dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106 print_address_description mm/kasan/report.c:317 [inline]
print_report.cold+0x2ba/0x719 mm/kasan/report.c:433 kasan_report_invalid_free+0x81/0x190 mm/kasan/report.c:462 ____kasan_slab_free+0x18b/0x1c0 mm/kasan/common.c:356 kasan_slab_free include/linux/kasan.h:200 [inline]
slab_free_hook mm/slub.c:1759 [inline]
slab_free_freelist_hook+0x8b/0x1c0 mm/slub.c:1785 slab_free mm/slub.c:3539 [inline]
kfree+0xe2/0x580 mm/slub.c:4567 tcp_disconnect+0x980/0x1e20 net/ipv4/tcp.c:3145 __mptcp_close_ssk+0x5ca/0x7e0 net/mptcp/protocol.c:2327 mptcp_do_fastclose net/mptcp/protocol.c:2592 [inline]
mptcp_worker+0x78c/0xff0 net/mptcp/protocol.c:2627 process_one_work+0x991/0x1610 kernel/workqueue.c:2289 worker_thread+0x665/0x1080 kernel/workqueue.c:2436 kthread+0x2e4/0x3a0 kernel/kthread.c:376 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:306 </TASK>
Allocated by task 3671: kasan_save_stack+0x1e/0x40 mm/kasan/common.c:38 kasan_set_track mm/kasan/common.c:45 [inline]
set_alloc_info mm/kasan/common.c:437 [inline]
____kasan_kmalloc mm/kasan/common.c:516 [inline]
____kasan_kmalloc mm/kasan/common.c:475 [inline]
__kasan_kmalloc+0xa9/0xd0 mm/kasan/common.c:525 kmalloc_array include/linux/slab.h:640 [inline]
kcalloc include/linux/slab.h:671 [inline]
tcp_cdg_init+0x10d/0x170 net/ipv4/tcp_cdg.c:380 tcp_init_congestion_control+0xab/0x550 net/ipv4/tcp_cong.c:193 tcp_reinit_congestion_control net/ipv4/tcp_cong.c:217 [inline]
tcp_set_congestion_control+0x96c/0xaa0 net/ipv4/tcp_cong.c:391 do_tcp_setsockopt+0x505/0x2320 net/ipv4/tcp.c:3513 tcp_setsockopt+0xd4/0x100 net/ipv4/tcp.c:3801 mptcp_setsockopt+0x35f/0x2570 net/mptcp/sockopt.c:844 __sys_setsockopt+0x2d6/0x690 net/socket.c:2252 __do_sys_setsockopt net/socket.c:2263 [inline]
__se_sys_setsockopt net/socket.c:2260 [inline]
__x64_sys_setsockopt+0xba/0x150 net/socket.c:2260 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80 entry_SYSCALL_64_after_hwframe+0x63/0xcd
Freed by task 16: kasan_save_stack+0x1e/0x40 mm/kasan/common.c:38 kasan_set_track+0x21/0x30 mm/kasan/common.c:45 kasan_set_free_info+0x20/0x30 mm/kasan/generic.c:370 ____kasan_slab_free mm/kasan/common.c:367 [inline]
____kasan_slab_free+0x166/0x1c0 mm/kasan/common.c:329 kasan_slab_free include/linux/kasan.h:200 [inline]
slab_free_hook mm/slub.c:1759 [inline]
slab_free_freelist_hook+0x8b/0x1c0 mm/slub.c:1785 slab_free mm/slub.c:3539 [inline]
kfree+0xe2/0x580 mm/slub.c:4567 tcp_cleanup_congestion_control+0x70/0x120 net/ipv4/tcp_cong.c:226 tcp_v4_destroy_sock+0xdd/0x750 net/ipv4/tcp_ipv4.c:2254 tcp_v6_destroy_sock+0x11/0x20 net/ipv6/tcp_ipv6.c:1969 inet_csk_destroy_sock+0x196/0x440 net/ipv4/inet_connection_sock.c:1157 tcp_done+0x23b/0x340 net/ipv4/tcp.c:4649 tcp_rcv_state_process+0x40e7/0x4990 net/ipv4/tcp_input.c:6624 tcp_v6_do_rcv+0x3fc/0x13c0 net/ipv6/tcp_ipv6.c:1525 tcp_v6_rcv+0x2e8e/0x3830 net/ipv6/tcp_ipv6.c:1759 ip6_protocol_deliver_rcu+0x2db/0x1950 net/ipv6/ip6_input.c:439 ip6_input_finish+0x14c/0x2c0 net/ipv6/ip6_input.c:484 NF_HOOK include/linux/netfilter.h:302 [inline]
NF_HOOK include/linux/netfilter.h:296 [inline]
ip6_input+0x9c/0xd ---truncated---
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Analysis
by VulDB Data Team • 11/07/2025
The vulnerability described in CVE-2022-49775 resides within the Linux kernel's implementation of the Multipath TCP (MPTCP) protocol, specifically in how congestion control is managed during socket disconnection events. This flaw manifests when MPTCP attempts to call tcp_disconnect() on a flow that has already been disconnected, which normally would be harmless. However, under certain conditions involving the CDG (Congestion Detection and Growth) congestion control algorithm, this sequence can result in a double-free error. The issue stems from a lack of idempotency in the tcp_disconnect() function, which fails to properly handle multiple invocations on the same congestion control structure. The double-free occurs because the tcp_cleanup_congestion_control function is called twice during the socket destruction process, leading to a memory corruption scenario that can be exploited for denial of service or potentially arbitrary code execution.
The technical root cause of this vulnerability can be traced to the improper handling of congestion control state management within the TCP subsystem. When a socket transitions through various states during connection termination, the CDG congestion control module maintains references to allocated memory structures that are freed during the first tcp_disconnect() call. A subsequent call to tcp_disconnect() on the same structure attempts to free memory that has already been deallocated, resulting in a double-free condition. This pattern is classified under CWE-415 as "Double Free" and represents a classic memory safety issue that can be exploited by malicious actors to corrupt kernel memory. The KASAN (Kernel Address Sanitizer) reports clearly indicate the double-free occurring in the slab_free function within the SLUB memory allocator, confirming the memory corruption pattern. The call trace demonstrates how the issue propagates through the MPTCP worker thread, where __mptcp_close_ssk() leads to tcp_disconnect(), eventually triggering the memory corruption through tcp_cleanup_congestion_control.
The operational impact of CVE-2022-49775 extends beyond simple denial of service to potentially compromise system stability and security. An attacker could exploit this vulnerability by establishing MPTCP connections and manipulating the socket state to trigger multiple disconnection events on the same congestion control structure. This scenario could lead to system crashes, making the affected system unavailable to legitimate users, or in more severe cases, could allow for privilege escalation or memory corruption that might be leveraged for further exploitation. The vulnerability is particularly concerning in network-intensive environments where MPTCP is actively used, as it could be triggered through normal network operations. From an ATT&CK perspective, this vulnerability aligns with techniques involving privilege escalation and denial of service, specifically mapping to T1499.004 (Unnecessary Administrative Access) and T1499.001 (Network Denial of Service) within the MITRE ATT&CK framework for enterprise environments.
The recommended mitigation for CVE-2022-49775 involves applying the official kernel patch that modifies the tcp_disconnect() function to make it idempotent, ensuring that multiple calls to the function do not result in double-free conditions. The patch implements a check to prevent redundant cleanup operations when congestion control structures have already been freed. System administrators should prioritize updating their Linux kernel versions to include this fix, particularly in production environments where MPTCP is enabled. Additionally, monitoring for potential exploitation attempts through kernel logs and system behavior analysis can provide early warning of attempted exploitation. Organizations should also consider disabling MPTCP if it is not essential for their network operations, as a temporary workaround to avoid exposure to this vulnerability. The fix demonstrates the importance of defensive programming practices in kernel code, specifically ensuring that cleanup functions are resilient to multiple invocations and maintain proper state tracking to prevent memory safety issues that can compromise system integrity and availability.