stdlib: random_r: fix unaligned access in initstate and initstate_r [BZ #30584]

The initstate{,_r} interfaces are documented in BSD as needing an aligned
array of 32-bit values, but neither POSIX nor glibc's own documentation
require it to be aligned. glibc's documentation says it "should" be a power
of 2, but not must.

Use memcpy to read and write to `state` to handle such an unaligned
argument.

Co-authored-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Reviewed-by: Florian Weimer <fweimer@redhat.com>

stdlib/Makefile

@@ -300,6 +300,7 @@ tests := \
   tst-rand48-2 \
   tst-random \
   tst-random2 \
+  tst-random-bz30584 \
   tst-realpath \
   tst-realpath-toolong \
   tst-secure-getenv \

stdlib/random_r.c

@@ -55,6 +55,7 @@
 #include <limits.h>
 #include <stddef.h>
 #include <stdlib.h>
+#include <string.h>
 
 
 /* An improved random number generation package.  In addition to the standard
@@ -146,7 +147,21 @@ static const struct random_poly_info random_poly_info =
   { DEG_0, DEG_1, DEG_2, DEG_3, DEG_4 }
 };
 
+static inline int32_t
+read_state (int32_t *b, int idx)
+{
+  int32_t r;
+  memcpy (&r, (char *) b + idx * 4, sizeof (int32_t));
+  return r;
+}
 
+static inline void
+write_state (int32_t *b, int idx, int32_t v)
+{
+  /* Use literal 4 to avoid conversion to an unsigned type and pointer
+     wraparound.  */
+  memcpy ((char *) b + idx * 4, &v, 4);
+}
 
 
 /* Initialize the random number generator based on the given seed.  If the
@@ -177,7 +192,7 @@ __srandom_r (unsigned int seed, struct random_data *buf)
   /* We must make sure the seed is not 0.  Take arbitrarily 1 in this case.  */
   if (seed == 0)
     seed = 1;
-  state[0] = seed;
+  write_state (state, 0, seed);
   if (type == TYPE_0)
     goto done;
 
@@ -194,7 +209,7 @@ __srandom_r (unsigned int seed, struct random_data *buf)
       word = 16807 * lo - 2836 * hi;
       if (word < 0)
 	word += 2147483647;
-      *++dst = word;
+      write_state (++dst, 0, word);
     }
 
   buf->fptr = &state[buf->rand_sep];
@@ -238,9 +253,10 @@ __initstate_r (unsigned int seed, char *arg_state, size_t n,
     {
       int old_type = buf->rand_type;
       if (old_type == TYPE_0)
-	old_state[-1] = TYPE_0;
+	write_state (old_state, -1, TYPE_0);
       else
-	old_state[-1] = (MAX_TYPES * (buf->rptr - old_state)) + old_type;
+	write_state (old_state, -1, (MAX_TYPES * (buf->rptr - old_state))
+				    + old_type);
     }
 
   int type;
@@ -270,9 +286,9 @@ __initstate_r (unsigned int seed, char *arg_state, size_t n,
 
   __srandom_r (seed, buf);
 
-  state[-1] = TYPE_0;
+  write_state (state, -1, TYPE_0);
   if (type != TYPE_0)
-    state[-1] = (buf->rptr - state) * MAX_TYPES + type;
+    write_state (state, -1, (buf->rptr - state) * MAX_TYPES + type);
 
   return 0;
 
@@ -307,9 +323,10 @@ __setstate_r (char *arg_state, struct random_data *buf)
   old_type = buf->rand_type;
   old_state = buf->state;
   if (old_type == TYPE_0)
-    old_state[-1] = TYPE_0;
+    write_state (old_state, -1, TYPE_0);
   else
-    old_state[-1] = (MAX_TYPES * (buf->rptr - old_state)) + old_type;
+    write_state (old_state, -1, (MAX_TYPES * (buf->rptr - old_state))
+				+ old_type);
 
   type = new_state[-1] % MAX_TYPES;
   if (type < TYPE_0 || type > TYPE_4)
@@ -361,8 +378,9 @@ __random_r (struct random_data *buf, int32_t *result)
 
   if (buf->rand_type == TYPE_0)
     {
-      int32_t val = ((state[0] * 1103515245U) + 12345U) & 0x7fffffff;
-      state[0] = val;
+      int32_t val = ((read_state(state, 0) * 1103515245U) + 12345U)
+		     & 0x7fffffff;
+      write_state (state, 0, val);
       *result = val;
     }
   else
@@ -372,7 +390,9 @@ __random_r (struct random_data *buf, int32_t *result)
       int32_t *end_ptr = buf->end_ptr;
       uint32_t val;
 
-      val = *fptr += (uint32_t) *rptr;
+      val = read_state (rptr, 0);
+      int32_t t = read_state (fptr, 0);
+      write_state (fptr, 0, t + val);
       /* Chucking least random bit.  */
       *result = val >> 1;
       ++fptr;

stdlib/tst-random-bz30584.c

@@ -0,0 +1,38 @@
+/* Test program for initstate(), initstate_r() for BZ #30584.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public License as
+   published by the Free Software Foundation; either version 2.1 of the
+   License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; see the file COPYING.LIB.  If
+   not, see <https://www.gnu.org/licenses/>.  */
+
+#include <stdlib.h>
+#include <time.h>
+
+static int
+do_test (void)
+{
+  struct random_data rand_state = { .state = NULL };
+  _Alignas (double) char buf[128 + sizeof (int32_t)];
+
+  /* Test initstate_r with an unaligned `state` array.  */
+  initstate_r (time (NULL), buf + 1, sizeof buf, &rand_state);
+
+  /* Ditto initstate.  */
+  initstate (time (NULL), buf + 1, sizeof buf);
+
+  return 0;
+}
+
+#define TEST_FUNCTION do_test ()
+#include "../test-skeleton.c"