From f7a79879c0b2bef0dadd6caaaeeb0d26423e04e5 Mon Sep 17 00:00:00 2001
From: Siddhesh Poyarekar <siddhesh@sourceware.org>
Date: Thu, 13 Jan 2022 11:28:36 +0530
Subject: [PATCH] realpath: Set errno to ENAMETOOLONG for result larger than
 PATH_MAX [BZ #28770]

realpath returns an allocated string when the result exceeds PATH_MAX,
which is unexpected when its second argument is not NULL.  This results
in the second argument (resolved) being uninitialized and also results
in a memory leak since the caller expects resolved to be the same as the
returned value.

Return NULL and set errno to ENAMETOOLONG if the result exceeds
PATH_MAX.  This fixes [BZ #28770], which is CVE-2021-3998.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
(cherry picked from commit ee8d5e33adb284601c00c94687bc907e10aec9bb)
---
 NEWS                          |  4 +++
 stdlib/Makefile               |  1 +
 stdlib/canonicalize.c         | 12 +++++++--
 stdlib/tst-realpath-toolong.c | 49 +++++++++++++++++++++++++++++++++++
 4 files changed, 64 insertions(+), 2 deletions(-)
 create mode 100644 stdlib/tst-realpath-toolong.c

--- ./NEWS.orig	2021-02-01 11:15:33.000000000 -0600
+++ ./NEWS	2022-01-24 13:32:24.268678228 -0600
@@ -118,6 +118,10 @@
   CVE-2019-25013: A buffer overflow has been fixed in the iconv function when
   invoked with EUC-KR input containing invalid multibyte input sequences.
 
+  CVE-2021-3998: Passing a path longer than PATH_MAX to the realpath
+  function could result in a memory leak and potential access of
+  uninitialized memory.  Reported by Qualys.
+
 The following bugs are resolved with this release:
 
   [10635] libc: realpath portability patches
--- ./stdlib/canonicalize.c.orig	2021-02-01 11:15:33.000000000 -0600
+++ ./stdlib/canonicalize.c	2022-01-24 13:32:24.268678228 -0600
@@ -400,8 +400,16 @@
 
 error:
   *dest++ = '\0';
-  if (resolved != NULL && dest - rname <= get_path_max ())
-    rname = strcpy (resolved, rname);
+  if (resolved != NULL)
+    {
+      if (dest - rname <= get_path_max ())
+	rname = strcpy (resolved, rname);
+      else
+	{
+	  failed = true;
+	  __set_errno (ENAMETOOLONG);
+	}
+    }
 
 error_nomem:
   scratch_buffer_free (&extra_buffer);
--- ./stdlib/Makefile.orig	2022-01-24 13:32:24.268678228 -0600
+++ ./stdlib/Makefile	2022-01-24 13:32:59.968676254 -0600
@@ -86,7 +86,7 @@
 		   tst-makecontext-align test-bz22786 tst-strtod-nan-sign \
 		   tst-swapcontext1 tst-setcontext4 tst-setcontext5 \
 		   tst-setcontext6 tst-setcontext7 tst-setcontext8 \
-		   tst-setcontext9 tst-bz20544 tst-canon-bz26341
+		   tst-setcontext9 tst-bz20544 tst-canon-bz26341 tst-realpath-toolong
 
 tests-internal	:= tst-strtod1i tst-strtod3 tst-strtod4 tst-strtod5i \
 		   tst-tls-atexit tst-tls-atexit-nodelete
--- ./stdlib/tst-realpath-toolong.c.orig	2022-01-24 13:32:24.268678228 -0600
+++ ./stdlib/tst-realpath-toolong.c	2022-01-24 13:32:24.268678228 -0600
@@ -0,0 +1,49 @@
+/* Verify that realpath returns NULL with ENAMETOOLONG if the result exceeds
+   NAME_MAX.
+   Copyright The GNU Toolchain Authors.
+   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 <errno.h>
+#include <limits.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <support/check.h>
+#include <support/temp_file.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+
+#define BASENAME "tst-realpath-toolong."
+
+int
+do_test (void)
+{
+  char *base = support_create_and_chdir_toolong_temp_directory (BASENAME);
+
+  char buf[PATH_MAX + 1];
+  const char *res = realpath (".", buf);
+
+  /* canonicalize.c states that if the real path is >= PATH_MAX, then
+     realpath returns NULL and sets ENAMETOOLONG.  */
+  TEST_VERIFY (res == NULL);
+  TEST_VERIFY (errno == ENAMETOOLONG);
+
+  free (base);
+  return 0;
+}
+
+#include <support/test-driver.c>