mptcp: Correctly set DATA_FIN timeout when number of retransmits is large

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

mptcp: Correctly set DATA_FIN timeout when number of retransmits is large

Syzkaller with UBSAN uncovered a scenario where a large number of

DATA_FIN retransmits caused a shift-out-of-bounds in the DATA_FIN

timeout calculation:

================================================================================

UBSAN: shift-out-of-bounds in net/mptcp/protocol.c:470:29

shift exponent 32 is too large for 32-bit type 'unsigned int'

CPU: 1 PID: 13059 Comm: kworker/1:0 Not tainted 5.17.0-rc2-00630-g5fbf21c90c60 #1

Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014

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

ubsan_epilogue+0xb/0x5a lib/ubsan.c:151

__ubsan_handle_shift_out_of_bounds.cold+0xb2/0x20e lib/ubsan.c:330

mptcp_set_datafin_timeout net/mptcp/protocol.c:470 [inline]

__mptcp_retrans.cold+0x72/0x77 net/mptcp/protocol.c:2445

mptcp_worker+0x58a/0xa70 net/mptcp/protocol.c:2528

process_one_work+0x9df/0x16d0 kernel/workqueue.c:2307

worker_thread+0x95/0xe10 kernel/workqueue.c:2454

kthread+0x2f4/0x3b0 kernel/kthread.c:377

ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

</TASK>

================================================================================

This change limits the maximum timeout by limiting the size of the

shift, which keeps all intermediate values in-bounds.