diff options
| -rw-r--r-- | auth-pam.c | 35 |
1 files changed, 31 insertions, 4 deletions
@@ -232,7 +232,6 @@ 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 @@ -795,12 +794,35 @@ sshpam_query(void *ctx, char **name, char **info, 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) { @@ -821,8 +843,11 @@ sshpam_respond(void *ctx, u_int num, char **resp) (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); @@ -1166,6 +1191,7 @@ sshpam_auth_passwd(Authctxt *authctxt, const char *password) { 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 " @@ -1181,7 +1207,7 @@ sshpam_auth_passwd(Authctxt *authctxt, const char *password) */ 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); @@ -1191,6 +1217,7 @@ 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 && authctxt->valid) { debug("PAM: password authentication accepted for %.100s", authctxt->user); |