arc4random: Replace memset(3) with explicit_bzero(3)

[Fabio Scotoni]

As far as I can tell, all of the calls to memset(3) in
lib/libc/crypt/arc4random.c are intended to wipe memory to avoid having
the randomly generated data in memory twice, so it would seem good
practice to use explicit_bzero(3) to avoid this being optimized out.

Index: lib/libc/crypt/arc4random.c
===================================================================
RCS file: /cvs/src/lib/libc/crypt/arc4random.c,v
retrieving revision 1.55
diff -u -p -r1.55 arc4random.c
--- lib/libc/crypt/arc4random.c 24 Mar 2019 17:56:54 -0000      1.55
+++ lib/libc/crypt/arc4random.c 19 Dec 2019 12:51:23 -0000
@@ -98,7 +98,7 @@ _rs_stir(void)

        /* invalidate rs_buf */
        rs->rs_have = 0;
-       memset(rsx->rs_buf, 0, sizeof(rsx->rs_buf));
+       explicit_bzero(rsx->rs_buf, sizeof(rsx->rs_buf));

        rs->rs_count = 1600000;
 }
@@ -119,7 +119,7 @@ static inline void
 _rs_rekey(u_char *dat, size_t datlen)
 {
 #ifndef KEYSTREAM_ONLY
-       memset(rsx->rs_buf, 0, sizeof(rsx->rs_buf));
+       explicit_bzero(rsx->rs_buf, sizeof(rsx->rs_buf));
 #endif
        /* fill rs_buf with the keystream */
        chacha_encrypt_bytes(&rsx->rs_chacha, rsx->rs_buf,
@@ -134,7 +134,7 @@ _rs_rekey(u_char *dat, size_t datlen)
        }
        /* immediately reinit for backtracking resistance */
        _rs_init(rsx->rs_buf, KEYSZ + IVSZ);
-       memset(rsx->rs_buf, 0, KEYSZ + IVSZ);
+       explicit_bzero(rsx->rs_buf, KEYSZ + IVSZ);
        rs->rs_have = sizeof(rsx->rs_buf) - KEYSZ - IVSZ;
 }

@@ -152,7 +152,7 @@ _rs_random_buf(void *_buf, size_t n)
                        keystream = rsx->rs_buf + sizeof(rsx->rs_buf)
                            - rs->rs_have;
                        memcpy(buf, keystream, m);
-                       memset(keystream, 0, m);
+                       explicit_bzero(keystream, m);
                        buf += m;
                        n -= m;
                        rs->rs_have -= m;
@@ -172,7 +172,7 @@ _rs_random_u32(uint32_t *val)
                _rs_rekey(NULL, 0);
        keystream = rsx->rs_buf + sizeof(rsx->rs_buf) - rs->rs_have;
        memcpy(val, keystream, sizeof(*val));
-       memset(keystream, 0, sizeof(*val));
+       explicit_bzero(keystream, sizeof(*val));
        rs->rs_have -= sizeof(*val);
 }