[RFC PATCH 1/3] newrole: constant time password comparison

["Christian Göttsche" <cgoettsche@seltendoof.de>]

From: Christian Göttsche <cgzones@googlemail.com>

Perform the password hash comparison in a time constant way to avoid
leaking the length of the identical prefix via a side-channel.
Since only hashes are compared leaking the total length is non critical.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
---
 policycoreutils/newrole/newrole.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/policycoreutils/newrole/newrole.c b/policycoreutils/newrole/newrole.c
index 5a1a1129..1e01d2ef 100644
--- a/policycoreutils/newrole/newrole.c
+++ b/policycoreutils/newrole/newrole.c
@@ -338,6 +338,24 @@ static void memzero(void *ptr, size_t size)
 	}
 }
 
+static int streq_constant(const char *userinput, const char *secret)
+{
+	const volatile char *x = userinput, *y = secret;
+	size_t i, u_len, s_len;
+	int ret = 0;
+
+	u_len = strlen(userinput);
+	s_len = strlen(secret);
+
+	if (u_len != s_len)
+		return 0;
+
+	for (i = 0; i < u_len; i++)
+		ret |= x[i] ^ y[i];
+
+	return ret == 0;
+}
+
 /* authenticate_via_shadow_passwd()
  *
  * in:     uname - the calling user's user name
@@ -383,7 +401,7 @@ static int authenticate_via_shadow_passwd(const char *uname)
 		return 0;
 	}
 
-	ret = !strcmp(encrypted_password_s, p_shadow_line->sp_pwdp);
+	ret = streq_constant(encrypted_password_s, p_shadow_line->sp_pwdp);
 	memzero(encrypted_password_s, strlen(encrypted_password_s));
 	return ret;
 }
-- 
2.43.0