From 140bae1df2b7246bb43439d039bf994159973585 Mon Sep 17 00:00:00 2001
From: =?utf8?q?Marco=20Trevisan=20=28Trevi=C3=B1o=29?= <mail@3v1n0.net>
Date: Mon, 30 Sep 2024 13:14:11 +0200
Subject: [PATCH] auth-pam: Check the user didn't change during PAM transaction

PAM modules can change the user during their execution, in such case ssh
would still use the user that has been provided giving potentially
access to another user with the credentials of another one.

So prevent this to happen, by ensuring that the final PAM user is
matching the one that initiated the transaction.
---
 auth-pam.c | 41 +++++++++++++++++++++++++++++++++--------
 1 file changed, 33 insertions(+), 8 deletions(-)

diff --git a/auth-pam.c b/auth-pam.c
index 13c0a792e..2481db45f 100644
--- a/auth-pam.c
+++ b/auth-pam.c
@@ -467,6 +467,32 @@ sshpam_thread_conv(int n, sshpam_const struct pam_message **msg,
 	return (PAM_CONV_ERR);
 }
 
+static int
+check_pam_user(Authctxt *authctxt)
+{
+	const char *pam_user;
+
+	if (authctxt == NULL || authctxt->pw == NULL ||
+	    authctxt->pw->pw_name == NULL)
+		fatal("%s: PAM authctxt user not initialized", __func__);
+
+	if ((sshpam_err = pam_get_item(sshpam_handle, PAM_USER,
+	    (sshpam_const void **) &pam_user)) != PAM_SUCCESS)
+		return sshpam_err;
+
+	if (pam_user == NULL) {
+		debug("PAM error: PAM_USER is NULL");
+		return PAM_USER_UNKNOWN;
+	}
+
+	if (strcmp(authctxt->pw->pw_name, pam_user) != 0) {
+		debug("PAM user \"%s\" does not match expected \"%s\"",
+		      pam_user, authctxt->pw->pw_name);
+		return PAM_USER_UNKNOWN;
+	}
+	return PAM_SUCCESS;
+}
+
 /*
  * Authentication thread.
  */
@@ -521,6 +547,8 @@ sshpam_thread(void *ctxtp)
 		sshpam_set_maxtries_reached(1);
 	if (sshpam_err != PAM_SUCCESS)
 		goto auth_fail;
+	if ((sshpam_err = check_pam_user(sshpam_authctxt)) != PAM_SUCCESS)
+		goto auth_fail;
 
 	if (!do_pam_account()) {
 		sshpam_err = PAM_ACCT_EXPIRED;
@@ -686,8 +714,7 @@ sshpam_cleanup(void)
 static int
 sshpam_init(struct ssh *ssh, Authctxt *authctxt)
 {
-	const char *pam_user, *user = authctxt->user;
-	const char **ptr_pam_user = &pam_user;
+	const char *user = authctxt->user;
 	int r;
 
 	if (options.pam_service_name == NULL)
@@ -706,12 +733,8 @@ sshpam_init(struct ssh *ssh, Authctxt *authctxt)
 	}
 	if (sshpam_handle != NULL) {
 		/* We already have a PAM context; check if the user matches */
-		sshpam_err = pam_get_item(sshpam_handle,
-		    PAM_USER, (sshpam_const void **)ptr_pam_user);
-		if (sshpam_err == PAM_SUCCESS && strcmp(user, pam_user) == 0)
-			return (0);
-		pam_end(sshpam_handle, sshpam_err);
-		sshpam_handle = NULL;
+		if ((sshpam_err = check_pam_user(authctxt)) != PAM_SUCCESS)
+			fatal("PAM user mismatch");
 	}
 	debug("PAM: initializing for \"%s\" with service \"%s\"", user,
 	    options.pam_service_name);
@@ -1378,6 +1401,8 @@ sshpam_auth_passwd(Authctxt *authctxt, const char *password)
 	sshpam_err = pam_authenticate(sshpam_handle, flags);
 	sshpam_password = NULL;
 	free(fake);
+	if (sshpam_err == PAM_SUCCESS)
+		sshpam_err = check_pam_user(authctxt);
 	if (sshpam_err == PAM_MAXTRIES)
 		sshpam_set_maxtries_reached(1);
 	if (sshpam_err == PAM_SUCCESS && authctxt->valid) {
-- 
2.47.2