In the Linux kernel, the following vulnerability has been resolved:

tcp: Fix shift-out-of-bounds in dctcp_update_alpha().

In dctcp_update_alpha(), we use a module parameter dctcp_shift_g as follows:

alpha -= min_not_zero(alpha, alpha >> dctcp_shift_g); ... delivered_ce <<= (10 - dctcp_shift_g);

It seems syzkaller started fuzzing module parameters and triggered shift-out-of-bounds [0] by setting 100 to dctcp_shift_g:

memcpy((void*)0x20000080, "/sys/module/tcp_dctcp/parameters/dctcp_shift_g\000", 47); res = syscall(__NR_openat, /fd=/0xffffffffffffff9cul, /file=/0x20000080ul, /flags=/2ul, /mode=/0ul); memcpy((void*)0x20000000, "100\000", 4); syscall(__NR_write, /fd=/r[0], /val=/0x20000000ul, /len=/4ul);

Let's limit the max value of dctcp_shift_g by param_set_uint_minmax().

With this patch:

echo 10 > /sys/module/tcp_dctcp/parameters/dctcp_shift_g

cat /sys/module/tcp_dctcp/parameters/dctcp_shift_g

10

echo 11 > /sys/module/tcp_dctcp/parameters/dctcp_shift_g

-bash: echo: write error: Invalid argument

[0]: UBSAN: shift-out-of-bounds in net/ipv4/tcp_dctcp.c:143:12 shift exponent 100 is too large for 32-bit type 'u32' (aka 'unsigned int') CPU: 0 PID: 8083 Comm: syz-executor345 Not tainted 6.9.0-05151-g1b294a1f3561 #2 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014 Call Trace: __dump_stack lib/dump_stack.c:88 [inline] dump_stack_lvl+0x201/0x300 lib/dump_stack.c:114 ubsan_epilogue lib/ubsan.c:231 [inline] __ubsan_handle_shift_out_of_bounds+0x346/0x3a0 lib/ubsan.c:468 dctcp_update_alpha+0x540/0x570 net/ipv4/tcp_dctcp.c:143 tcp_in_ack_event net/ipv4/tcp_input.c:3802 [inline] tcp_ack+0x17b1/0x3bc0 net/ipv4/tcp_input.c:3948 tcp_rcv_state_process+0x57a/0x2290 net/ipv4/tcp_input.c:6711 tcp_v4_do_rcv+0x764/0xc40 net/ipv4/tcp_ipv4.c:1937 sk_backlog_rcv include/net/sock.h:1106 [inline] __release_sock+0x20f/0x350 net/core/sock.c:2983 release_sock+0x61/0x1f0 net/core/sock.c:3549 mptcp_subflow_shutdown+0x3d0/0x620 net/mptcp/protocol.c:2907 mptcp_check_send_data_fin+0x225/0x410 net/mptcp/protocol.c:2976 __mptcp_close+0x238/0xad0 net/mptcp/protocol.c:3072 mptcp_close+0x2a/0x1a0 net/mptcp/protocol.c:3127 inet_release+0x190/0x1f0 net/ipv4/af_inet.c:437 __sock_release net/socket.c:659 [inline] sock_close+0xc0/0x240 net/socket.c:1421 __fput+0x41b/0x890 fs/file_table.c:422 task_work_run+0x23b/0x300 kernel/task_work.c:180 exit_task_work include/linux/task_work.h:38 [inline] do_exit+0x9c8/0x2540 kernel/exit.c:878 do_group_exit+0x201/0x2b0 kernel/exit.c:1027 __do_sys_exit_group kernel/exit.c:1038 [inline] __se_sys_exit_group kernel/exit.c:1036 [inline] __x64_sys_exit_group+0x3f/0x40 kernel/exit.c:1036 do_syscall_x64 arch/x86/entry/common.c:52 [inline] do_syscall_64+0xe4/0x240 arch/x86/entry/common.c:83 entry_SYSCALL_64_after_hwframe+0x67/0x6f RIP: 0033:0x7f6c2b5005b6 Code: Unable to access opcode bytes at 0x7f6c2b50058c. RSP: 002b:00007ffe883eb948 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7 RAX: ffffffffffffffda RBX: 00007f6c2b5862f0 RCX: 00007f6c2b5005b6 RDX: 0000000000000001 RSI: 000000000000003c RDI: 0000000000000001 RBP: 0000000000000001 R08: 00000000000000e7 R09: ffffffffffffffc0 R10: 0000000000000006 R11: 0000000000000246 R12: 00007f6c2b5862f0 R13: 0000000000000001 R14: 0000000000000000 R15: 0000000000000001