auth: Use consttime_memequal(3) to compare hashes

This stops any attacker from trying to infer secrets from latency.

Thanks to Maxime Villard <max@m00nbsd.net>

3 files changed, 50 insertions, 1 deletions

diff --git a/auth.c b/auth.c
index 45fe9aa5..accba805 100644
--- a/auth.c
+++ b/auth.c
@@ -336,7 +336,7 @@ gottoken:
 	}
 
 	free(mm);
-	if (memcmp(d, &hmac, dlen)) {
+	if (consttime_memequal(d, &hmac, dlen)) {
 		errno = EPERM;
 		return NULL;
 	}
diff --git a/compat/consttime_memequal.h b/compat/consttime_memequal.h
new file mode 100644
index 00000000..98306484
--- /dev/null
+++ b/compat/consttime_memequal.h
@@ -0,0 +1,28 @@
+/*
+ * Written by Matthias Drochner <drochner@NetBSD.org>.
+ * Public domain.
+ */
+
+#ifndef CONSTTIME_MEMEQUAL_H
+#define CONSTTIME_MEMEQUAL_H
+inline static int
+consttime_memequal(const void *b1, const void *b2, size_t len)
+{
+	const unsigned char *c1 = b1, *c2 = b2;
+	unsigned int res = 0;
+
+	while (len--)
+		res |= *c1++ ^ *c2++;
+
+	/*
+	 * Map 0 to 1 and [1, 256) to 0 using only constant-time
+	 * arithmetic.
+	 *
+	 * This is not simply `!res' because although many CPUs support
+	 * branchless conditional moves and many compilers will take
+	 * advantage of them, certain compilers generate branches on
+	 * certain CPUs for `!res'.
+	 */
+	return (1 & ((res - 1) >> 8));
+}
+#endif /* CONSTTIME_MEMEQUAL_H */
diff --git a/configure b/configure
index 3a6304b8..44848918 100755
--- a/configure
+++ b/configure
@@ -820,6 +820,27 @@ if [ "$STRTOI" = no ]; then
 	echo "#include		\"compat/strtoi.h\"" >>$CONFIG_H
 fi
 
+if [ -z "$CONSTTIME_MEMEQUAL" ]; then
+	printf "Testing for consttime_memequal ... "
+	cat <<EOF >_consttime_memequal.c
+#include <string.h>
+int main(void) {
+	return consttime_memequal("deadbeef", "deadbeef", 8);
+}
+EOF
+	if $XCC _consttime_memequal.c -o _consttime_memequal 2>&3; then
+		CONSTTIME_MEMEQUAL=yes
+	else
+		CONSTTIME_MEMEQUAL=no
+	fi
+	echo "$CONSTTIME_MEMEQUAL"
+	rm -f _consttime_memequal.c _consttime_memequal
+fi
+if [ "$CONSTTIME_MEMEQUAL" = no ]; then
+	echo "#include			\"compat/consttime_memequal.h\"" \
+	    >>$CONFIG_H
+fi
+
 if [ -z "$DPRINTF" ]; then
 	printf "Testing for dprintf ... "
 	cat <<EOF >_dprintf.c