From 283b97ff33ea2c641161950849931bd578de6946 Mon Sep 17 00:00:00 2001 From: Darren Tucker Date: Fri, 15 Jul 2016 13:49:44 +1000 Subject: [PATCH] Mitigate timing of disallowed users PAM logins. When sshd decides to not allow a login (eg PermitRootLogin=no) and it's using PAM, it sends a fake password to PAM so that the timing for the failure is not noticeably different whether or not the password is correct. This behaviour can be detected by sending a very long password string which is slower to hash than the fake password. Mitigate by constructing an invalid password that is the same length as the one from the client and thus takes the same time to hash. Diff from djm@ Upstream-Status: Backport CVE: CVE-2016-6210 patch2 Signed-off-by: Armin Kuster --- auth-pam.c | 35 +++++++++++++++++++++++++++++++---- 1 file changed, 31 insertions(+), 4 deletions(-) Index: openssh-7.1p2/auth-pam.c =================================================================== --- openssh-7.1p2.orig/auth-pam.c +++ openssh-7.1p2/auth-pam.c @@ -231,7 +231,6 @@ static int sshpam_account_status = -1; static char **sshpam_env = NULL; static Authctxt *sshpam_authctxt = NULL; static const char *sshpam_password = NULL; -static char badpw[] = "\b\n\r\177INCORRECT"; /* Some PAM implementations don't implement this */ #ifndef HAVE_PAM_GETENVLIST @@ -809,12 +808,35 @@ sshpam_query(void *ctx, char **name, cha return (-1); } +/* + * Returns a junk password of identical length to that the user supplied. + * Used to mitigate timing attacks against crypt(3)/PAM stacks that + * vary processing time in proportion to password length. + */ +static char * +fake_password(const char *wire_password) +{ + const char junk[] = "\b\n\r\177INCORRECT"; + char *ret = NULL; + size_t i, l = wire_password != NULL ? strlen(wire_password) : 0; + + if (l >= INT_MAX) + fatal("%s: password length too long: %zu", __func__, l); + + ret = malloc(l + 1); + for (i = 0; i < l; i++) + ret[i] = junk[i % (sizeof(junk) - 1)]; + ret[i] = '\0'; + return ret; +} + /* XXX - see also comment in auth-chall.c:verify_response */ static int sshpam_respond(void *ctx, u_int num, char **resp) { Buffer buffer; struct pam_ctxt *ctxt = ctx; + char *fake; debug2("PAM: %s entering, %u responses", __func__, num); switch (ctxt->pam_done) { @@ -835,8 +857,11 @@ sshpam_respond(void *ctx, u_int num, cha (sshpam_authctxt->pw->pw_uid != 0 || options.permit_root_login == PERMIT_YES)) buffer_put_cstring(&buffer, *resp); - else - buffer_put_cstring(&buffer, badpw); + else { + fake = fake_password(*resp); + buffer_put_cstring(&buffer, fake); + free(fake); + } if (ssh_msg_send(ctxt->pam_psock, PAM_AUTHTOK, &buffer) == -1) { buffer_free(&buffer); return (-1); @@ -1180,6 +1205,7 @@ sshpam_auth_passwd(Authctxt *authctxt, c { int flags = (options.permit_empty_passwd == 0 ? PAM_DISALLOW_NULL_AUTHTOK : 0); + char *fake = NULL; if (!options.use_pam || sshpam_handle == NULL) fatal("PAM: %s called when PAM disabled or failed to " @@ -1195,7 +1221,7 @@ sshpam_auth_passwd(Authctxt *authctxt, c */ if (!authctxt->valid || (authctxt->pw->pw_uid == 0 && options.permit_root_login != PERMIT_YES)) - sshpam_password = badpw; + sshpam_password = fake = fake_password(password); sshpam_err = pam_set_item(sshpam_handle, PAM_CONV, (const void *)&passwd_conv); @@ -1205,6 +1231,7 @@ sshpam_auth_passwd(Authctxt *authctxt, c sshpam_err = pam_authenticate(sshpam_handle, flags); sshpam_password = NULL; + free(fake); if (sshpam_err == PAM_SUCCESS && authctxt->valid) { debug("PAM: password authentication accepted for %.100s", authctxt->user);