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:
<TASK>
__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
</TASK>