Use explicit_bzero to clear GnuTLS keys

* admin/merge-gnulib (GNULIB_MODULES): Add explicit_bzero.
* lib/explicit_bzero.c, m4/explicit_bzero.m4: New files.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.
* src/gnutls.c (clear_storage): New function.
(gnutls_symmetric_aead): Use it instead of memset.

6 files changed, 110 insertions, 4 deletions

diff --git a/admin/merge-gnulib b/admin/merge-gnulib
index 85921ba1ba6..2b1a16a10ec 100755
--- a/admin/merge-gnulib
+++ b/admin/merge-gnulib
@@ -30,7 +30,7 @@ GNULIB_MODULES='
   careadlinkat close-stream
   count-leading-zeros count-one-bits count-trailing-zeros
   crypto/md5 crypto/sha1 crypto/sha256 crypto/sha512
-  diffseq dtoastr dtotimespec dup2 environ execinfo faccessat
+  diffseq dtoastr dtotimespec dup2 environ execinfo explicit_bzero faccessat
   fcntl fcntl-h fdatasync fdopendir
   filemode filevercmp flexmember fstatat fsync
   getloadavg getopt-gnu gettime gettimeofday gitlog-to-changelog
diff --git a/lib/explicit_bzero.c b/lib/explicit_bzero.c
new file mode 100644
index 00000000000..262c68f9cd6
--- /dev/null
+++ b/lib/explicit_bzero.c
@@ -0,0 +1,48 @@
+/* Erasure of sensitive data, generic implementation.
+   Copyright (C) 2016-2017 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 General Public
+   License as published by the Free Software Foundation; either
+   version 3 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
+   General Public License for more details.
+
+   You should have received a copy of the GNU General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+/* An assembler implementation of explicit_bzero can be created as an
+   assembler alias of an optimized bzero implementation.
+   Architecture-specific implementations also need to define
+   __explicit_bzero_chk.  */
+
+#if !_LIBC
+# include <config.h>
+#endif
+
+#include <string.h>
+
+/* glibc-internal users use __explicit_bzero_chk, and explicit_bzero
+   redirects to that.  */
+#undef explicit_bzero
+
+/* Set LEN bytes of S to 0.  The compiler will not delete a call to
+   this function, even if S is dead after the call.  */
+void
+explicit_bzero (void *s, size_t len)
+{
+#ifdef HAVE_EXPLICIT_MEMSET
+  explicit_memset (s, 0, len);
+#else
+  memset (s, '\0', len);
+# ifdef __GNUC__
+  /* Compiler barrier.  */
+  asm volatile ("" ::: "memory");
+# endif
+#endif
+}
diff --git a/lib/gnulib.mk.in b/lib/gnulib.mk.in
index ae5ae87a521..e20487b10b4 100644
--- a/lib/gnulib.mk.in
+++ b/lib/gnulib.mk.in
@@ -21,7 +21,7 @@
 # the same distribution terms as the rest of that program.
 #
 # Generated by gnulib-tool.
-# Reproduce by: gnulib-tool --import --lib=libgnu --source-base=lib --m4-base=m4 --doc-base=doc --tests-base=tests --aux-dir=build-aux --avoid=close --avoid=dup --avoid=fchdir --avoid=fstat --avoid=malloc-posix --avoid=msvc-inval --avoid=msvc-nothrow --avoid=open --avoid=openat-die --avoid=opendir --avoid=raise --avoid=save-cwd --avoid=select --avoid=setenv --avoid=sigprocmask --avoid=stat --avoid=stdarg --avoid=stdbool --avoid=threadlib --avoid=tzset --avoid=unsetenv --avoid=utime --avoid=utime-h --gnu-make --makefile-name=gnulib.mk.in --conditional-dependencies --no-libtool --macro-prefix=gl --no-vc-files alloca-opt binary-io byteswap c-ctype c-strcase careadlinkat close-stream count-leading-zeros count-one-bits count-trailing-zeros crypto/md5 crypto/sha1 crypto/sha256 crypto/sha512 diffseq dtoastr dtotimespec dup2 environ execinfo faccessat fcntl fcntl-h fdatasync fdopendir filemode filevercmp flexmember fstatat fsync getloadavg getopt-gnu gettime gettimeofday gitlog-to-changelog ignore-value intprops largefile lstat manywarnings memrchr minmax mkostemp mktime pipe2 pselect pthread_sigmask putenv qcopy-acl readlink readlinkat sig2str socklen stat-time std-gnu11 stdalign stddef stdio stpcpy strftime strtoimax symlink sys_stat sys_time time time_r time_rz timegm timer-time timespec-add timespec-sub unlocked-io update-copyright utimens vla warnings
+# Reproduce by: gnulib-tool --import --lib=libgnu --source-base=lib --m4-base=m4 --doc-base=doc --tests-base=tests --aux-dir=build-aux --avoid=close --avoid=dup --avoid=fchdir --avoid=fstat --avoid=malloc-posix --avoid=msvc-inval --avoid=msvc-nothrow --avoid=open --avoid=openat-die --avoid=opendir --avoid=raise --avoid=save-cwd --avoid=select --avoid=setenv --avoid=sigprocmask --avoid=stat --avoid=stdarg --avoid=stdbool --avoid=threadlib --avoid=tzset --avoid=unsetenv --avoid=utime --avoid=utime-h --gnu-make --makefile-name=gnulib.mk.in --conditional-dependencies --no-libtool --macro-prefix=gl --no-vc-files alloca-opt binary-io byteswap c-ctype c-strcase careadlinkat close-stream count-leading-zeros count-one-bits count-trailing-zeros crypto/md5 crypto/sha1 crypto/sha256 crypto/sha512 diffseq dtoastr dtotimespec dup2 environ execinfo explicit_bzero faccessat fcntl fcntl-h fdatasync fdopendir filemode filevercmp flexmember fstatat fsync getloadavg getopt-gnu gettime gettimeofday gitlog-to-changelog ignore-value intprops largefile lstat manywarnings memrchr minmax mkostemp mktime pipe2 pselect pthread_sigmask putenv qcopy-acl readlink readlinkat sig2str socklen stat-time std-gnu11 stdalign stddef stdio stpcpy strftime strtoimax symlink sys_stat sys_time time time_r time_rz timegm timer-time timespec-add timespec-sub unlocked-io update-copyright utimens vla warnings
 
 
 MOSTLYCLEANFILES += core *.stackdump
@@ -1358,6 +1358,17 @@ EXTRA_libgnu_a_SOURCES += execinfo.c
 endif
 ## end   gnulib module execinfo
 
+## begin gnulib module explicit_bzero
+ifeq (,$(OMIT_GNULIB_MODULE_explicit_bzero))
+
+
+EXTRA_DIST += explicit_bzero.c
+
+EXTRA_libgnu_a_SOURCES += explicit_bzero.c
+
+endif
+## end   gnulib module explicit_bzero
+
 ## begin gnulib module faccessat
 ifeq (,$(OMIT_GNULIB_MODULE_faccessat))
 
diff --git a/m4/explicit_bzero.m4 b/m4/explicit_bzero.m4
new file mode 100644
index 00000000000..f9dc678207a
--- /dev/null
+++ b/m4/explicit_bzero.m4
@@ -0,0 +1,22 @@
+dnl Copyright 2017 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+AC_DEFUN([gl_FUNC_EXPLICIT_BZERO],
+[
+  AC_REQUIRE([gl_HEADER_STRING_H_DEFAULTS])
+
+  dnl Persuade glibc <string.h> to declare explicit_bzero.
+  AC_REQUIRE([AC_USE_SYSTEM_EXTENSIONS])
+
+  AC_CHECK_FUNCS_ONCE([explicit_bzero])
+  if test $ac_cv_func_explicit_bzero = no; then
+    HAVE_EXPLICIT_BZERO=0
+  fi
+])
+
+AC_DEFUN([gl_PREREQ_EXPLICIT_BZERO],
+[
+  AC_CHECK_FUNCS([explicit_memset])
+])
diff --git a/m4/gnulib-comp.m4 b/m4/gnulib-comp.m4
index 107645df4fd..038d78aafea 100644
--- a/m4/gnulib-comp.m4
+++ b/m4/gnulib-comp.m4
@@ -72,6 +72,7 @@ AC_DEFUN([gl_EARLY],
   # Code from module errno:
   # Code from module euidaccess:
   # Code from module execinfo:
+  # Code from module explicit_bzero:
   # Code from module extensions:
   # Code from module extern-inline:
   # Code from module faccessat:
@@ -210,6 +211,12 @@ AC_DEFUN([gl_INIT],
   gl_UNISTD_MODULE_INDICATOR([environ])
   gl_HEADER_ERRNO_H
   gl_EXECINFO_H
+  gl_FUNC_EXPLICIT_BZERO
+  if test $HAVE_EXPLICIT_BZERO = 0; then
+    AC_LIBOBJ([explicit_bzero])
+    gl_PREREQ_EXPLICIT_BZERO
+  fi
+  gl_STRING_MODULE_INDICATOR([explicit_bzero])
   AC_REQUIRE([gl_EXTERN_INLINE])
   gl_FUNC_FACCESSAT
   if test $HAVE_FACCESSAT = 0; then
@@ -837,6 +844,7 @@ AC_DEFUN([gl_FILE_LIST], [
   lib/euidaccess.c
   lib/execinfo.c
   lib/execinfo.in.h
+  lib/explicit_bzero.c
   lib/faccessat.c
   lib/fcntl.c
   lib/fcntl.in.h
@@ -967,6 +975,7 @@ AC_DEFUN([gl_FILE_LIST], [
   m4/errno_h.m4
   m4/euidaccess.m4
   m4/execinfo.m4
+  m4/explicit_bzero.m4
   m4/extensions.m4
   m4/extern-inline.m4
   m4/faccessat.m4
diff --git a/src/gnutls.c b/src/gnutls.c
index e6f01a9cfe1..7d19f90fbb8 100644
--- a/src/gnutls.c
+++ b/src/gnutls.c
@@ -1883,6 +1883,22 @@ The alist key is the cipher name. */)
   return ciphers;
 }
 
+/* Zero out STORAGE (even if it will become inaccessible.  It has
+   STORAGE_LENGTH bytes.  The goal is to improve security a bit, in
+   case an Emacs module or some buggy part of Emacs attempts to
+   inspect STORAGE later to retrieve a secret.
+
+   Calls to this function document when storage containing a secret is
+   known to go out of scope.  This function is not guaranteed to erase
+   the secret, as copies of STORAGE may well be accessible elsewhere
+   on the machine.  */
+
+static void
+clear_storage (void *storage, ptrdiff_t storage_length)
+{
+  explicit_bzero (storage, storage_length);
+}
+
 static Lisp_Object
 gnutls_symmetric_aead (bool encrypting, gnutls_cipher_algorithm_t gca,
                        Lisp_Object cipher,
@@ -1949,7 +1965,7 @@ gnutls_symmetric_aead (bool encrypting, gnutls_cipher_algorithm_t gca,
 
   if (ret < GNUTLS_E_SUCCESS)
     {
-      memset (storage, 0, storage_length);
+      clear_storage (storage, storage_length);
       SAFE_FREE ();
       gnutls_aead_cipher_deinit (acipher);
       if (encrypting)
@@ -1963,7 +1979,7 @@ gnutls_symmetric_aead (bool encrypting, gnutls_cipher_algorithm_t gca,
   gnutls_aead_cipher_deinit (acipher);
 
   Lisp_Object output = make_unibyte_string (storage, storage_length);
-  memset (storage, 0, storage_length);
+  clear_storage (storage, storage_length);
   SAFE_FREE ();
   return list2 (output, actual_iv);
 #else