tcp: fix tcp_init_transfer() to not reset icsk_ca_initialized

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

tcp: fix tcp_init_transfer() to not reset icsk_ca_initialized

This commit fixes a bug (found by syzkaller) that could cause spurious

double-initializations for congestion control modules, which could cause

memory leaks or other problems for congestion control modules (like CDG)

that allocate memory in their init functions.

The buggy scenario constructed by syzkaller was something like:

(1) create a TCP socket

(2) initiate a TFO connect via sendto()

(3) while socket is in TCP_SYN_SENT, call setsockopt(TCP_CONGESTION),

which calls:

tcp_set_congestion_control() ->

tcp_reinit_congestion_control() ->

tcp_init_congestion_control()

(4) receive ACK, connection is established, call tcp_init_transfer(),

set icsk_ca_initialized=0 (without first calling cc->release()),

call tcp_init_congestion_control() again.

Note that in this sequence tcp_init_congestion_control() is called

twice without a cc->release() call in between. Thus, for CC modules

that allocate memory in their init() function, e.g, CDG, a memory leak

may occur. The syzkaller tool managed to find a reproducer that

triggered such a leak in CDG.

The bug was introduced when that commit 8919a9b31eb4 ("tcp: Only init

congestion control if not initialized already")

introduced icsk_ca_initialized and set icsk_ca_initialized to 0 in

tcp_init_transfer(), missing the possibility for a sequence like the

one above, where a process could call setsockopt(TCP_CONGESTION) in

state TCP_SYN_SENT (i.e. after the connect() or TFO open sendmsg()),

which would call tcp_init_congestion_control(). It did not intend to

reset any initialization that the user had already explicitly made;

it just missed the possibility of that particular sequence (which

syzkaller managed to find).