forked from rpms/openssh
CVE-2016-6210 is fixed upstream
This commit is contained in:
Jakub Jelen
parent
13a7aaf5e3
commit
70c2ac20bd
@ -1,215 +0,0 @@
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From 9286875a73b2de7736b5e50692739d314cd8d9dc Mon Sep 17 00:00:00 2001
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From: Darren Tucker <dtucker@zip.com.au>
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Date: Fri, 15 Jul 2016 13:32:45 +1000
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Subject: [PATCH] Determine appropriate salt for invalid users.
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When sshd is processing a non-PAM login for a non-existent user it uses
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the string from the fakepw structure as the salt for crypt(3)ing the
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password supplied by the client. That string has a Blowfish prefix, so on
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systems that don't understand that crypt will fail fast due to an invalid
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salt, and even on those that do it may have significantly different timing
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from the hash methods used for real accounts (eg sha512). This allows
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user enumeration by, eg, sending large password strings. This was noted
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by EddieEzra.Harari at verint.com (CVE-2016-6210).
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To mitigate, use the same hash algorithm that root uses for hashing
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passwords for users that do not exist on the system. ok djm@
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---
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auth-passwd.c | 12 ++++++++----
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openbsd-compat/xcrypt.c | 34 ++++++++++++++++++++++++++++++++++
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2 files changed, 42 insertions(+), 4 deletions(-)
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diff --git a/auth-passwd.c b/auth-passwd.c
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index 63ccf3c..530b5d4 100644
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--- a/auth-passwd.c
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+++ b/auth-passwd.c
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@@ -193,7 +193,7 @@ int
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sys_auth_passwd(Authctxt *authctxt, const char *password)
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{
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struct passwd *pw = authctxt->pw;
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- char *encrypted_password;
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+ char *encrypted_password, *salt = NULL;
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/* Just use the supplied fake password if authctxt is invalid */
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char *pw_password = authctxt->valid ? shadow_pw(pw) : pw->pw_passwd;
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@@ -202,9 +202,13 @@ sys_auth_passwd(Authctxt *authctxt, const char *password)
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if (strcmp(pw_password, "") == 0 && strcmp(password, "") == 0)
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return (1);
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- /* Encrypt the candidate password using the proper salt. */
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- encrypted_password = xcrypt(password,
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- (pw_password[0] && pw_password[1]) ? pw_password : "xx");
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+ /*
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+ * Encrypt the candidate password using the proper salt, or pass a
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+ * NULL and let xcrypt pick one.
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+ */
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+ if (authctxt->valid && pw_password[0] && pw_password[1])
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+ salt = pw_password;
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+ encrypted_password = xcrypt(password, salt);
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/*
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* Authentication is accepted if the encrypted passwords
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diff --git a/openbsd-compat/xcrypt.c b/openbsd-compat/xcrypt.c
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index 8577cbd..8913bb8 100644
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--- a/openbsd-compat/xcrypt.c
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+++ b/openbsd-compat/xcrypt.c
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@@ -25,6 +25,7 @@
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#include "includes.h"
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#include <sys/types.h>
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+#include <string.h>
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#include <unistd.h>
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#include <pwd.h>
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@@ -62,11 +63,44 @@
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# define crypt DES_crypt
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# endif
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+/*
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+ * Pick an appropriate password encryption type and salt for the running
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+ * system.
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+ */
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+static const char *
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+pick_salt(void)
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+{
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+ struct passwd *pw;
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+ char *passwd, *p;
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+ size_t typelen;
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+ static char salt[32];
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+
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+ if (salt[0] != '\0')
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+ return salt;
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+ strlcpy(salt, "xx", sizeof(salt));
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+ if ((pw = getpwuid(0)) == NULL)
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+ return salt;
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+ passwd = shadow_pw(pw);
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+ if (passwd[0] != '$' || (p = strrchr(passwd + 1, '$')) == NULL)
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+ return salt; /* no $, DES */
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+ typelen = p - passwd + 1;
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+ strlcpy(salt, passwd, MIN(typelen, sizeof(salt)));
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+ explicit_bzero(passwd, strlen(passwd));
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+ return salt;
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+}
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+
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char *
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xcrypt(const char *password, const char *salt)
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{
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char *crypted;
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+ /*
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+ * If we don't have a salt we are encrypting a fake password for
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+ * for timing purposes. Pick an appropriate salt.
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+ */
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+ if (salt == NULL)
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+ salt = pick_salt();
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+
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# ifdef HAVE_MD5_PASSWORDS
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if (is_md5_salt(salt))
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crypted = md5_crypt(password, salt);
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From 283b97ff33ea2c641161950849931bd578de6946 Mon Sep 17 00:00:00 2001
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From: Darren Tucker <dtucker@zip.com.au>
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Date: Fri, 15 Jul 2016 13:49:44 +1000
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Subject: [PATCH] Mitigate timing of disallowed users PAM logins.
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When sshd decides to not allow a login (eg PermitRootLogin=no) and
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it's using PAM, it sends a fake password to PAM so that the timing for
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the failure is not noticeably different whether or not the password
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is correct. This behaviour can be detected by sending a very long
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password string which is slower to hash than the fake password.
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Mitigate by constructing an invalid password that is the same length
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as the one from the client and thus takes the same time to hash.
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Diff from djm@
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---
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auth-pam.c | 35 +++++++++++++++++++++++++++++++----
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1 file changed, 31 insertions(+), 4 deletions(-)
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diff --git a/auth-pam.c b/auth-pam.c
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index 451de78..465b5a7 100644
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--- a/auth-pam.c
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+++ b/auth-pam.c
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@@ -232,7 +232,6 @@ static int sshpam_account_status = -1;
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static char **sshpam_env = NULL;
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static Authctxt *sshpam_authctxt = NULL;
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static const char *sshpam_password = NULL;
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-static char badpw[] = "\b\n\r\177INCORRECT";
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/* Some PAM implementations don't implement this */
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#ifndef HAVE_PAM_GETENVLIST
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@@ -795,12 +794,35 @@ sshpam_query(void *ctx, char **name, char **info,
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return (-1);
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}
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+/*
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+ * Returns a junk password of identical length to that the user supplied.
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+ * Used to mitigate timing attacks against crypt(3)/PAM stacks that
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+ * vary processing time in proportion to password length.
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+ */
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+static char *
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+fake_password(const char *wire_password)
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+{
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+ const char junk[] = "\b\n\r\177INCORRECT";
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+ char *ret = NULL;
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+ size_t i, l = wire_password != NULL ? strlen(wire_password) : 0;
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+
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+ if (l >= INT_MAX)
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+ fatal("%s: password length too long: %zu", __func__, l);
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+
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+ ret = malloc(l + 1);
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+ for (i = 0; i < l; i++)
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+ ret[i] = junk[i % (sizeof(junk) - 1)];
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+ ret[i] = '\0';
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+ return ret;
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+}
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+
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/* XXX - see also comment in auth-chall.c:verify_response */
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static int
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sshpam_respond(void *ctx, u_int num, char **resp)
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{
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Buffer buffer;
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struct pam_ctxt *ctxt = ctx;
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+ char *fake;
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debug2("PAM: %s entering, %u responses", __func__, num);
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switch (ctxt->pam_done) {
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@@ -821,8 +843,11 @@ sshpam_respond(void *ctx, u_int num, char **resp)
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(sshpam_authctxt->pw->pw_uid != 0 ||
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options.permit_root_login == PERMIT_YES))
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buffer_put_cstring(&buffer, *resp);
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- else
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- buffer_put_cstring(&buffer, badpw);
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+ else {
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+ fake = fake_password(*resp);
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+ buffer_put_cstring(&buffer, fake);
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+ free(fake);
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+ }
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if (ssh_msg_send(ctxt->pam_psock, PAM_AUTHTOK, &buffer) == -1) {
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buffer_free(&buffer);
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return (-1);
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@@ -1166,6 +1191,7 @@ sshpam_auth_passwd(Authctxt *authctxt, const char *password)
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{
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int flags = (options.permit_empty_passwd == 0 ?
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PAM_DISALLOW_NULL_AUTHTOK : 0);
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+ char *fake = NULL;
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if (!options.use_pam || sshpam_handle == NULL)
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fatal("PAM: %s called when PAM disabled or failed to "
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@@ -1181,7 +1207,7 @@ sshpam_auth_passwd(Authctxt *authctxt, const char *password)
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*/
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if (!authctxt->valid || (authctxt->pw->pw_uid == 0 &&
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options.permit_root_login != PERMIT_YES))
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- sshpam_password = badpw;
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+ sshpam_password = fake = fake_password(password);
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sshpam_err = pam_set_item(sshpam_handle, PAM_CONV,
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(const void *)&passwd_conv);
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@@ -1191,6 +1217,7 @@ sshpam_auth_passwd(Authctxt *authctxt, const char *password)
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sshpam_err = pam_authenticate(sshpam_handle, flags);
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sshpam_password = NULL;
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+ free(fake);
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if (sshpam_err == PAM_SUCCESS && authctxt->valid) {
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debug("PAM: password authentication accepted for %.100s",
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authctxt->user);
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@ -221,10 +221,6 @@ Patch939: openssh-7.2p2-s390-closefrom.patch
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# expose more information to PAM
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# https://github.com/openssh/openssh-portable/pull/47
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Patch940: openssh-7.2p2-expose-pam.patch
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# Prevent user enumeration via covert timing channel (#1357443)
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# https://github.com/openssh/openssh-portable/commit/9286875a
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# https://github.com/openssh/openssh-portable/commit/283b97ff
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Patch941: openssh-7.2p2-user-enumeration.patch
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# Rework SELinux context handling with chroot (#1357860)
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Patch942: openssh-7.2p2-chroot-capabilities.patch
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@ -460,7 +456,6 @@ popd
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%patch933 -p1 -b .fingerprint
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%patch939 -p1 -b .s390-dev
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%patch940 -p1 -b .expose-pam
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%patch941 -p1 -b .user-enumeration
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%patch942 -p1 -b .chroot-cap
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%patch200 -p1 -b .audit
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