io: nftw/ftw: Fix stack overflow with large nopenfd [BZ #26353]

The nopenfd value is used as argument for the internal buffer on
ftw_statup, which is allocated with alloca and might trigger
a stack overflow for large values.  This patch replaces the memory
allocation to use malloc instead.

Checked on x86_64-linux-gnu.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

io/Makefile

@@ -68,7 +68,8 @@ tests		:= test-utime test-stat test-stat2 test-lfs tst-getcwd \
 		   tst-posix_fallocate tst-posix_fallocate64 \
 		   tst-fts tst-fts-lfs tst-open-tmpfile \
 		   tst-copy_file_range tst-getcwd-abspath tst-lockf \
-		   tst-ftw-lnk tst-file_change_detection tst-lchmod
+		   tst-ftw-lnk tst-file_change_detection tst-lchmod \
+		   tst-ftw-bz26353
 
 # Likewise for statx, but we do not need static linking here.
 tests-internal += tst-statx

io/ftw.c

@@ -646,15 +646,17 @@ ftw_startup (const char *dir, int is_nftw, void *func, int descriptors,
 
   data.maxdir = descriptors < 1 ? 1 : descriptors;
   data.actdir = 0;
-  data.dirstreams = (struct dir_data **) alloca (data.maxdir
-						 * sizeof (struct dir_data *));
-  memset (data.dirstreams, '\0', data.maxdir * sizeof (struct dir_data *));
-
   /* PATH_MAX is always defined when we get here.  */
   data.dirbufsize = MAX (2 * strlen (dir), PATH_MAX);
-  data.dirbuf = (char *) malloc (data.dirbufsize);
+  data.dirstreams = malloc (data.maxdir * sizeof (struct dir_data *)
-  if (data.dirbuf == NULL)
+                            + data.dirbufsize);
+  if (data.dirstreams == NULL)
     return -1;
+
+  memset (data.dirstreams, '\0', data.maxdir * sizeof (struct dir_data *));
+
+  data.dirbuf = (char *) data.dirstreams
+                + data.maxdir * sizeof (struct dir_data *);
   cp = __stpcpy (data.dirbuf, dir);
   /* Strip trailing slashes.  */
   while (cp > data.dirbuf + 1 && cp[-1] == '/')
@@ -803,7 +805,7 @@ ftw_startup (const char *dir, int is_nftw, void *func, int descriptors,
  out_fail:
   save_err = errno;
   __tdestroy (data.known_objects, free);
-  free (data.dirbuf);
+  free (data.dirstreams);
   __set_errno (save_err);
 
   return result;

io/tst-ftw-bz26353.c

@@ -0,0 +1,70 @@
+/* test ftw bz26353: Check whether stack overflow occurs when the value
+   of the nopenfd parameter is too large.
+
+   Copyright (C) 2020 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; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <ftw.h>
+#include <errno.h>
+#include <sys/resource.h>
+
+#include <support/temp_file.h>
+#include <support/capture_subprocess.h>
+#include <support/check.h>
+
+static int
+my_func (const char *file, const struct stat *sb, int flag)
+{
+  return 0;
+}
+
+static int
+get_large_nopenfd (void)
+{
+  struct rlimit r;
+  TEST_COMPARE (getrlimit (RLIMIT_STACK, &r), 0);
+  if (r.rlim_cur == RLIM_INFINITY)
+    {
+      r.rlim_cur = 8 * 1024 * 1024;
+      TEST_COMPARE (setrlimit (RLIMIT_STACK, &r), 0);
+    }
+  return (int) r.rlim_cur;
+}
+
+static void
+do_ftw (void *unused)
+{
+  char *tempdir = support_create_temp_directory ("tst-bz26353");
+  int large_nopenfd = get_large_nopenfd ();
+  TEST_COMPARE (ftw (tempdir, my_func, large_nopenfd), 0);
+  free (tempdir);
+}
+
+/* Check whether stack overflow occurs.  */
+static int
+do_test (void)
+{
+  struct support_capture_subprocess result;
+  result = support_capture_subprocess (do_ftw, NULL);
+  support_capture_subprocess_check (&result, "bz26353", 0, sc_allow_none);
+  support_capture_subprocess_free (&result);
+  return 0;
+}
+
+#include <support/test-driver.c>