diff -up Linux-PAM-1.5.1/modules/pam_unix/passverify.c.fail1 Linux-PAM-1.5.1/modules/pam_unix/passverify.c
--- Linux-PAM-1.5.1/modules/pam_unix/passverify.c.fail1	2024-11-04 11:42:51.962791265 +0100
+++ Linux-PAM-1.5.1/modules/pam_unix/passverify.c	2024-11-04 11:45:18.246218579 +0100
@@ -239,17 +239,21 @@ PAMH_ARG_DECL(int get_account_info,
 			return PAM_UNIX_RUN_HELPER;
 #endif
 		} else if (is_pwd_shadowed(*pwd)) {
+#ifdef HELPER_COMPILE
 			/*
-			 * ...and shadow password file entry for this user,
+			 * shadow password file entry for this user,
 			 * if shadowing is enabled
 			 */
-#ifndef HELPER_COMPILE
-			if (geteuid() || SELINUX_ENABLED)
-				return PAM_UNIX_RUN_HELPER;
-#endif
-			*spwdent = pam_modutil_getspnam(pamh, name);
+			*spwdent = getspnam(name);
 			if (*spwdent == NULL || (*spwdent)->sp_pwdp == NULL)
 				return PAM_AUTHINFO_UNAVAIL;
+#else
+			/*
+			 * The helper has to be invoked to deal with
+			 * the shadow password file entry.
+			 */
+			return PAM_UNIX_RUN_HELPER;
+#endif
 		}
 	} else {
 		return PAM_USER_UNKNOWN;


From 8d0c575336ad301cd14e16ad2fdec6fe621764b8 Mon Sep 17 00:00:00 2001
From: Sergei Trofimovich <slyich@gmail.com>
Date: Thu, 28 Mar 2024 21:58:35 +0000
Subject: [PATCH] pam_unix: allow empty passwords with non-empty hashes

Before the change pam_unix has different behaviours for a user with
empty password for these two `/etc/shadow` entries:

    nulloktest:$6$Yy4ty2jJ$bsVQWo8qlXC6UHq1/qTC3UR60ZJKmKApJ3Wj7DreAy8FxlVKtlDnplFQ7jMLVlDqordE7e4t49GvTb.aI59TP0:1::::::
    nulloktest::1::::::

The entry with a hash was rejected and the entry without was accepted.

The rejection happened because 9e74e90147c "pam_unix: avoid determining
if user exists" introduced the following rejection check (slightly
simplified):

        ...
        } else if (p[0] == '\0' && nullok) {
                if (hash[0] != '\0') {
                        retval = PAM_AUTH_ERR;
                }

We should not reject the user with a hash assuming it's non-empty.
The change does that by pushing empty password check into
`verify_pwd_hash()`.

`NixOS` generates such hashed entries for empty passwords as if they
were non-empty using the following perl code:

    sub hashPassword {
        my ($password) = @_;
        my $salt = "";
        my @chars = ('.', '/', 0..9, 'A'..'Z', 'a'..'z');
        $salt .= $chars[rand 64] for (1..8);
        return crypt($password, '$6$' . $salt . '$');
    }

Resolves: https://github.com/linux-pam/linux-pam/issues/758
Fixes: 9e74e90147c "pam_unix: avoid determining if user exists"
Signed-off-by: Sergei Trofimovich <slyich@gmail.com>
---
 modules/pam_unix/passverify.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/modules/pam_unix/passverify.c b/modules/pam_unix/passverify.c
index 30045333..1c83f1aa 100644
--- a/modules/pam_unix/passverify.c
+++ b/modules/pam_unix/passverify.c
@@ -76,9 +76,13 @@ PAMH_ARG_DECL(int verify_pwd_hash,
 
 	strip_hpux_aging(hash);
 	hash_len = strlen(hash);
-	if (!hash_len) {
+
+	if (p && p[0] == '\0' && !nullok) {
+		/* The passed password is empty */
+		retval = PAM_AUTH_ERR;
+	} else if (!hash_len) {
 		/* the stored password is NULL */
-		if (nullok) { /* this means we've succeeded */
+		if (p && p[0] == '\0' && nullok) { /* this means we've succeeded */
 			D(("user has empty password - access granted"));
 			retval = PAM_SUCCESS;
 		} else {
@@ -1109,12 +1113,6 @@ helper_verify_password(const char *name, const char *p, int nullok)
 	if (pwd == NULL || hash == NULL) {
 		helper_log_err(LOG_NOTICE, "check pass; user unknown");
 		retval = PAM_USER_UNKNOWN;
-	} else if (p[0] == '\0' && nullok) {
-		if (hash[0] == '\0') {
-			retval = PAM_SUCCESS;
-		} else {
-			retval = PAM_AUTH_ERR;
-		}
 	} else {
 		retval = verify_pwd_hash(p, hash, nullok);
 	}
-- 
2.47.0