random: always mix cycle counter in add_latent_entropy()

stable inclusion
from stable-v4.19.274
commit e4935368448ce8097dada35163598e93567f1110
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6TIG1
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e4935368448ce8097dada35163598e93567f1110

--------------------------------

[ Upstream commit d7bf7f3b ]

add_latent_entropy() is called every time a process forks, in
kernel_clone(). This in turn calls add_device_randomness() using the
latent entropy global state. add_device_randomness() does two things:

   2) Mixes into the input pool the latent entropy argument passed; and
   1) Mixes in a cycle counter, a sort of measurement of when the event
      took place, the high precision bits of which are presumably
      difficult to predict.

(2) is impossible without CONFIG_GCC_PLUGIN_LATENT_ENTROPY=y. But (1) is
always possible. However, currently CONFIG_GCC_PLUGIN_LATENT_ENTROPY=n
disables both (1) and (2), instead of just (2).

This commit causes the CONFIG_GCC_PLUGIN_LATENT_ENTROPY=n case to still
do (1) by passing NULL (len 0) to add_device_randomness() when add_latent_
entropy() is called.

Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: PaX Team <pageexec@freemail.hu>
Cc: Emese Revfy <re.emese@gmail.com>
Fixes: 38addce8 ("gcc-plugins: Add latent_entropy plugin")
Signed-off-by: NJason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: NSasha Levin <sashal@kernel.org>

Conflicts:
	include/linux/random.h
Signed-off-by: NCui GaoSheng <cuigaosheng1@huawei.com>
Reviewed-by: Nyiyang <yiyang13@huawei.com>
Reviewed-by: Nguozihua <guozihua@huawei.com>
Reviewed-by: NWang Weiyang <wangweiyang2@huawei.com>
Reviewed-by: NXiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: NYongqiang Liu <liuyongqiang13@huawei.com>

include/linux/random.h

@@ -20,15 +20,15 @@ struct random_ready_callback {
 
 extern void add_device_randomness(const void *, unsigned int);
 
-#if defined(CONFIG_GCC_PLUGIN_LATENT_ENTROPY) && !defined(__CHECKER__)
 static inline void add_latent_entropy(void)
 {
+#if defined(CONFIG_GCC_PLUGIN_LATENT_ENTROPY) && !defined(__CHECKER__)
 	add_device_randomness((const void *)&latent_entropy,
 			      sizeof(latent_entropy));
-}
 #else
-static inline void add_latent_entropy(void) {}
+	add_device_randomness(NULL, 0);
 #endif
+}
 
 extern void add_input_randomness(unsigned int type, unsigned int code,
 				 unsigned int value) __latent_entropy;