Add timingsafe_bcmp(), for constant-time memory comparison

timingsafe_bcmp() should be used instead of memcmp() or a naive
for-loop, when comparing passwords or secret tokens, to avoid leaking
information about the secret token by timing. This commit just
introduces the function but does not change any existing code to use
it yet.

This has been initially applied as of 09be391126 in v18 and newer
versions, and will be used in all the stable branches for an upcoming
fix.

Co-authored-by: Jelte Fennema-Nio <github-tech@jeltef.nl>
Discussion: https://www.postgresql.org/message-id/7b86da3b-9356-4e50-aa1b-56570825e234@iki.fi
Security: CVE-2026-6478
Backpatch-through: 14

configure

@@ -16974,6 +16974,16 @@ fi
 cat >>confdefs.h <<_ACEOF
 #define HAVE_DECL_STRNLEN $ac_have_decl
 _ACEOF
+ac_fn_c_check_decl "$LINENO" "timingsafe_bcmp" "ac_cv_have_decl_timingsafe_bcmp" "$ac_includes_default"
+if test "x$ac_cv_have_decl_timingsafe_bcmp" = xyes; then :
+  ac_have_decl=1
+else
+  ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_TIMINGSAFE_BCMP $ac_have_decl
+_ACEOF
 
 
 # We can't use AC_REPLACE_FUNCS to replace these functions, because it
@@ -17326,6 +17336,19 @@ esac
 
 fi
 
+ac_fn_c_check_func "$LINENO" "timingsafe_bcmp" "ac_cv_func_timingsafe_bcmp"
+if test "x$ac_cv_func_timingsafe_bcmp" = xyes; then :
+  $as_echo "#define HAVE_TIMINGSAFE_BCMP 1" >>confdefs.h
+
+else
+  case " $LIBOBJS " in
+  *" timingsafe_bcmp.$ac_objext "* ) ;;
+  *) LIBOBJS="$LIBOBJS timingsafe_bcmp.$ac_objext"
+ ;;
+esac
+
+fi
+
 
 
 if test "$enable_thread_safety" = yes; then

configure.ac

@@ -1917,7 +1917,7 @@ AC_CHECK_DECLS(posix_fadvise, [], [], [#include <fcntl.h>])
 ]) # fi
 
 AC_CHECK_DECLS(fdatasync, [], [], [#include <unistd.h>])
-AC_CHECK_DECLS([strlcat, strlcpy, strnlen])
+AC_CHECK_DECLS([strlcat, strlcpy, strnlen, timingsafe_bcmp])
 
 # We can't use AC_REPLACE_FUNCS to replace these functions, because it
 # won't handle deployment target restrictions on macOS
@@ -1967,6 +1967,7 @@ AC_REPLACE_FUNCS(m4_normalize([
 	strlcpy
 	strnlen
 	strtof
+	timingsafe_bcmp
 ]))
 
 if test "$enable_thread_safety" = yes; then

src/include/pg_config.h.in

@@ -178,6 +178,10 @@
    don't. */
 #undef HAVE_DECL_STRTOULL
 
+/* Define to 1 if you have the declaration of `timingsafe_bcmp', and to 0 if
+   you don't. */
+#undef HAVE_DECL_TIMINGSAFE_BCMP
+
 /* Define to 1 if you have the `dlopen' function. */
 #undef HAVE_DLOPEN
 
@@ -669,6 +673,9 @@
 /* Define to 1 if you have the <termios.h> header file. */
 #undef HAVE_TERMIOS_H
 
+/* Define to 1 if you have the `timingsafe_bcmp' function. */
+#undef HAVE_TIMINGSAFE_BCMP
+
 /* Define to 1 if your compiler understands `typeof' or something similar. */
 #undef HAVE_TYPEOF
 

src/include/port.h

@@ -519,6 +519,10 @@ extern bool pg_get_user_name(uid_t user_id, char *buffer, size_t buflen);
 extern bool pg_get_user_home_dir(uid_t user_id, char *buffer, size_t buflen);
 #endif
 
+#if !HAVE_DECL_TIMINGSAFE_BCMP
+extern int	timingsafe_bcmp(const void *b1, const void *b2, size_t len);
+#endif
+
 extern void pg_qsort(void *base, size_t nel, size_t elsize,
 					 int (*cmp) (const void *, const void *));
 extern int	pg_qsort_strcmp(const void *a, const void *b);

src/port/timingsafe_bcmp.c

@@ -0,0 +1,43 @@
+/*
+ * src/port/timingsafe_bcmp.c
+ *
+ *	$OpenBSD: timingsafe_bcmp.c,v 1.3 2015/08/31 02:53:57 guenther Exp $
+ */
+
+/*
+ * Copyright (c) 2010 Damien Miller.  All rights reserved.
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include "c.h"
+
+#ifdef USE_SSL
+#include <openssl/crypto.h>
+#endif
+
+int
+timingsafe_bcmp(const void *b1, const void *b2, size_t n)
+{
+#ifdef USE_SSL
+	return CRYPTO_memcmp(b1, b2, n);
+#else
+	const unsigned char *p1 = b1,
+			   *p2 = b2;
+	int			ret = 0;
+
+	for (; n > 0; n--)
+		ret |= *p1++ ^ *p2++;
+	return (ret != 0);
+#endif
+}

src/tools/msvc/Mkvcbuild.pm

@@ -106,7 +106,7 @@ sub mkvcbuild
 	  pread.c preadv.c pwrite.c pwritev.c pg_bitutils.c
 	  pg_strong_random.c pgcheckdir.c pgmkdirp.c pgsleep.c pgstrcasecmp.c
 	  pqsignal.c mkdtemp.c qsort.c qsort_arg.c bsearch_arg.c quotes.c system.c
-	  strerror.c tar.c
+	  strerror.c tar.c timingsafe_bcmp.c
 	  win32common.c win32env.c win32error.c win32fseek.c win32ntdll.c
 	  win32security.c win32setlocale.c win32stat.c);
 

src/tools/msvc/Solution.pm

@@ -253,6 +253,7 @@ sub GenerateFiles
 		HAVE_DECL_STRNLEN                           => 1,
 		HAVE_DECL_STRTOLL                           => 1,
 		HAVE_DECL_STRTOULL                          => 1,
+		HAVE_DECL_TIMINGSAFE_BCMP                   => 0,
 		HAVE_DLOPEN                                 => undef,
 		HAVE_EDITLINE_HISTORY_H                     => undef,
 		HAVE_EDITLINE_READLINE_H                    => undef,
@@ -415,6 +416,7 @@ sub GenerateFiles
 		HAVE_SYS_UIO_H                           => undef,
 		HAVE_SYS_UN_H                            => undef,
 		HAVE_TERMIOS_H                           => undef,
+		HAVE_TIMINGSAFE_BCMP                     => undef,
 		HAVE_TYPEOF                              => undef,
 		HAVE_UCRED_H                             => undef,
 		HAVE_UINT64                              => undef,