test: Fix explicit_bzero() test on the Hurd

On the Hurd a small read(3) might end up (indirectly) copying the data
on the stack, which we will end up finding even when we have cleared
the buffer.

To avoid these side effects, we add a new function, that we force not
to be inlined, so that we can reuse the same stack space, that will
blank any possible stack side effects. This should be portable
regardless of stack growing up or down.

Diagnosis-by: Samuel Thibault <sthibault@debian.org>

diff --git a/COPYING b/COPYING
index 67223d4e41dc81ef16ec97e4d574c7e6eedd57fa..cf43edd6f7dc0523ce716b74777855467628011a 100644 (file)
--- a/COPYING
+++ b/COPYING
@@ -369,7 +369,7 @@ Copyright:
  Copyright © 2014 Theo de Raadt <deraadt@openbsd.org>
  Copyright © 2014 Google Inc.
  Copyright © 2015 Michael Felt <aixtools@gmail.com>
- Copyright © 2015 Guillem Jover <guillem@hadrons.org>
+ Copyright © 2015, 2022 Guillem Jover <guillem@hadrons.org>
 License: ISC
  Permission to use, copy, modify, and distribute this software for any
  purpose with or without fee is hereby granted, provided that the above

diff --git a/test/explicit_bzero.c b/test/explicit_bzero.c
index 74993c2d742c0a6139adf008b70c3f710b4f5d28..bee29de72230b4c66cf731d782a9366ec31d84ea 100644 (file)
--- a/test/explicit_bzero.c
+++ b/test/explicit_bzero.c
@@ -1,6 +1,7 @@
 /*     $OpenBSD: explicit_bzero.c,v 1.7 2021/03/27 11:17:58 bcook Exp $        */
 /*
  * Copyright (c) 2014 Google Inc.
+ * Copyright (c) 2022 Guillem Jover <guillem@hadrons.org>
  *
  * Permission to use, copy, modify, and distribute this software for any
  * purpose with or without fee is hereby granted, provided that the above
@@ -123,6 +124,18 @@ populate_secret(char *buf, ssize_t len)
        ASSERT_EQ(0, close(fds[0]));
 }
 
+static void __attribute__((__noinline__))
+blank_stack_side_effects(char *buf, size_t len)
+{
+       char scratch[SECRETBYTES * 4];
+
+       /* If the read(3) in populate_secret() wrote into the stack, as it
+        * might happen on the Hurd for small data, then we might incorrectly
+        * detect the wrong secret on the stack. */
+       memset(scratch, 0xFF, sizeof(scratch));
+       ASSERT_EQ(NULL, memmem(scratch, sizeof(scratch), buf, len));
+}
+
 static int
 count_secrets(const char *buf)
 {
@@ -143,6 +156,7 @@ test_without_bzero(void)
        char *res;
        assert_on_stack();
        populate_secret(buf, sizeof(buf));
+       blank_stack_side_effects(buf, sizeof(buf));
        res = memmem(altstack, ALTSTACK_SIZE, buf, sizeof(buf));
        ASSERT_NE(NULL, res);
        return (res);
@@ -155,6 +169,7 @@ test_with_bzero(void)
        char *res;
        assert_on_stack();
        populate_secret(buf, sizeof(buf));
+       blank_stack_side_effects(buf, sizeof(buf));
        res = memmem(altstack, ALTSTACK_SIZE, buf, sizeof(buf));
        ASSERT_NE(NULL, res);
        explicit_bzero(buf, sizeof(buf));