Introduce variation in location of junked bytes; ok tb@

1 files changed, 8 insertions, 3 deletions

diff --git a/src/lib/libc/stdlib/malloc.c b/src/lib/libc/stdlib/malloc.c
index 2ac4b73ec0..0df1fe3e4a 100644
--- a/src/lib/libc/stdlib/malloc.c
+++ b/src/lib/libc/stdlib/malloc.c
@@ -1,4 +1,4 @@
-/*      $OpenBSD: malloc.c,v 1.279 2023/04/01 18:47:51 otto Exp $       */
+/*      $OpenBSD: malloc.c,v 1.280 2023/04/05 06:25:38 otto Exp $       */
 /*
  * Copyright (c) 2008, 2010, 2011, 2016 Otto Moerbeek <otto@drijf.net>
  * Copyright (c) 2012 Matthew Dempsky <matthew@openbsd.org>
@@ -221,6 +221,7 @@ struct malloc_readonly {
         u_int   chunk_canaries;         /* use canaries after chunks? */
         int     internal_funcs;         /* use better recallocarray/freezero? */
         u_int   def_maxcache;           /* free pages we cache */
+        u_int   junk_loc;               /* variation in location of junk */
         size_t  malloc_guard;           /* use guard pages after allocations? */
 #ifdef MALLOC_STATS
         int     malloc_stats;           /* dump statistics at end */
@@ -493,6 +494,7 @@ omalloc_init(void)
 
         while ((mopts.malloc_canary = arc4random()) == 0)
                 ;
+        mopts.junk_loc = arc4random();
         if (mopts.chunk_canaries)
                 do {
                         mopts.chunk_canaries = arc4random();
@@ -676,7 +678,9 @@ junk_free(int junk, void *p, size_t sz)
                 if (step == 0)
                         step = 1;
         }
-        for (i = 0; i < sz; i += step)
+        /* Do not always put the free junk bytes in the same spot.
+           There is modulo bias here, but we ignore that. */
+        for (i = mopts.junk_loc % step; i < sz; i += step)
                 lp[i] = SOME_FREEJUNK_ULL;
 }
 
@@ -696,7 +700,8 @@ validate_junk(struct dir_info *pool, void *p, size_t sz)
                 if (step == 0)
                         step = 1;
         }
-        for (i = 0; i < sz; i += step) {
+        /* see junk_free */
+        for (i = mopts.junk_loc % step; i < sz; i += step) {
                 if (lp[i] != SOME_FREEJUNK_ULL)
                         wrterror(pool, "write after free %p", p);
         }