tcp: Specify cmsgbuf is user pointer for receive zerocopy.

A prior change (1f466e1) introduces separate handling for
->msg_control depending on whether the pointer is a kernel or user
pointer. However, while tcp receive zerocopy is using this field, it
is not properly annotating that the buffer in this case is a user
pointer. This can cause faults when the improper mechanism is used
within put_cmsg().

This patch simply annotates tcp receive zerocopy's use as explicitly
being a user pointer.

Fixes: 7eeba17 ("tcp: Add receive timestamp support for receive zerocopy.")
Signed-off-by: Arjun Roy <[email protected]>
Acked-by: Soheil Hassas Yeganeh <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: arjunroy

net/ipv4/tcp.c

@@ -2039,6 +2039,7 @@ static void tcp_zc_finalize_rx_tstamp(struct sock *sk,
 		(__kernel_size_t)zc->msg_controllen;
 	cmsg_dummy.msg_flags = in_compat_syscall()
 		? MSG_CMSG_COMPAT : 0;
+	cmsg_dummy.msg_control_is_user = true;
 	zc->msg_flags = 0;
 	if (zc->msg_control == msg_control_addr &&
 	    zc->msg_controllen == cmsg_dummy.msg_controllen) {