From d3b8c0723d0d691585474b0e14982f62b115a672 Mon Sep 17 00:00:00 2001
From: Tobias Stoeckmann <tobias@stoeckmann.org>
Date: Mon, 8 Jan 2024 21:59:23 +0100
Subject: [PATCH] pam_unix: do not truncate user names

This could allow users with very long names to impersonate a user
with a 255 characters long name.

The check if the argument argv[1] actually matches the user name
implies that "user" can unconditionally be set to argv[1]: If they are
equal, the strings are obviously equal. If they are not or if null is
returned by getuidname, "user" is set to argv[1] anyway.

This way, the static buffer can be safely removed because the result
of getpwuid() is not stored, which means that subsequent calls to
such functions can safely overwrite their internal buffers.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>
---
 modules/pam_unix/passverify.c | 6 +-----
 modules/pam_unix/unix_chkpwd.c | 2 +-
 2 files changed, 2 insertions(+), 6 deletions(-)

diff --git a/modules/pam_unix/passverify.c b/modules/pam_unix/passverify.c
index c48e3c5a79..c6515a65c0 100644
--- a/modules/pam_unix/passverify.c
+++ b/modules/pam_unix/passverify.c
@@ -1190,16 +1190,12 @@ char *
 getuidname(uid_t uid)
 {
 struct passwd *pw;
- static char username[256];
 
 pw = getpwuid(uid);
 if (pw == NULL)
 return NULL;
 
- strncpy(username, pw->pw_name, sizeof(username));
- username[sizeof(username) - 1] = '0円';
-
- return username;
+ return pw->pw_name;
 }
 
 #endif
diff --git a/modules/pam_unix/unix_chkpwd.c b/modules/pam_unix/unix_chkpwd.c
index 556a2e2cc5..50570dbc84 100644
--- a/modules/pam_unix/unix_chkpwd.c
+++ b/modules/pam_unix/unix_chkpwd.c
@@ -138,11 +138,11 @@ int main(int argc, char *argv[])
 	 /* if the caller specifies the username, verify that user
 	 matches it */
 	 if (user == NULL || strcmp(user, argv[1])) {
-	 user = argv[1];
 	 /* no match -> permanently change to the real user and proceed */
 	 if (setuid(getuid()) != 0)
 		return PAM_AUTH_ERR;
 	 }
+	 user = argv[1];
 	}
 
 	option=argv[2];