[PATCH v2] utsname: contribute changes to RNG

["Jason A. Donenfeld" <Jason@zx2c4.com>]

On some small machines with little entropy, a quasi-unique hostname is
sometimes a relevant factor. I've seen, for example, 8 character
alpha-numeric serial numbers. In addition, the time at which the hostname
is set is usually a decent measurement of how long early boot took. So,
call add_device_randomness() on new hostnames, which feeds its arguments
to the RNG in addition to a fresh cycle counter.

Low cost hooks like this never hurt and can only ever help, and since
this costs basically nothing for an operation that is never a fast path,
this is an overall easy win.

Cc: Kees Cook <keescook@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 kernel/sys.c            | 2 ++
 kernel/utsname_sysctl.c | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/kernel/sys.c b/kernel/sys.c
index b911fa6d81ab..b4b40ccf0949 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -25,6 +25,7 @@
 #include <linux/times.h>
 #include <linux/posix-timers.h>
 #include <linux/security.h>
+#include <linux/random.h>
 #include <linux/suspend.h>
 #include <linux/tty.h>
 #include <linux/signal.h>
@@ -1366,6 +1367,7 @@ SYSCALL_DEFINE2(sethostname, char __user *, name, int, len)
 	if (!copy_from_user(tmp, name, len)) {
 		struct new_utsname *u;
 
+		add_device_randomness(tmp, len);
 		down_write(&uts_sem);
 		u = utsname();
 		memcpy(u->nodename, tmp, len);
diff --git a/kernel/utsname_sysctl.c b/kernel/utsname_sysctl.c
index 4ca61d49885b..de16bcf14b03 100644
--- a/kernel/utsname_sysctl.c
+++ b/kernel/utsname_sysctl.c
@@ -8,6 +8,7 @@
 #include <linux/export.h>
 #include <linux/uts.h>
 #include <linux/utsname.h>
+#include <linux/random.h>
 #include <linux/sysctl.h>
 #include <linux/wait.h>
 #include <linux/rwsem.h>
@@ -57,6 +58,7 @@ static int proc_do_uts_string(struct ctl_table *table, int write,
 		 * theoretically be incorrect if there are two parallel writes
 		 * at non-zero offsets to the same sysctl.
 		 */
+		add_device_randomness(tmp_data, sizeof(tmp_data));
 		down_write(&uts_sem);
 		memcpy(get_uts(table), tmp_data, sizeof(tmp_data));
 		up_write(&uts_sem);
-- 
2.37.3