From owner-bugtraq@SECURITYFOCUS.COM Tue May 2 17:37:00 2000 Return-Path: Received: from lists.securityfocus.com (lists.securityfocus.com [207.126.127.68]) by smoke.dope.org (8.9.3/8.9.3_smoke_dope!) with ESMTP id RAA21967 for ; Tue, 2 May 2000 17:37:00 -0500 Received: from lists.securityfocus.com (lists.securityfocus.com [207.126.127.68]) by lists.securityfocus.com (Postfix) with ESMTP id 2715F1F306; Tue, 2 May 2000 12:25:55 -0700 (PDT) Received: from LISTS.SECURITYFOCUS.COM by LISTS.SECURITYFOCUS.COM (LISTSERV-TCP/IP release 1.8d) with spool id 8534163 for BUGTRAQ@LISTS.SECURITYFOCUS.COM; Tue, 2 May 2000 12:24:31 -0700 Approved-By: aleph1@SECURITYFOCUS.COM Delivered-To: bugtraq@lists.securityfocus.com Received: from securityfocus.com (mail.securityfocus.com [207.126.127.78]) by lists.securityfocus.com (Postfix) with SMTP id 407231F393 for ; Mon, 1 May 2000 13:27:21 -0700 (PDT) Received: (qmail 6619 invoked by alias); 1 May 2000 20:27:05 -0000 Delivered-To: bugtraq@securityfocus.com Received: (qmail 6613 invoked from network); 1 May 2000 20:27:05 -0000 Received: from ferret.lmh.ox.ac.uk (163.1.138.204) by mail.securityfocus.com with SMTP; 1 May 2000 20:27:05 -0000 Received: (qmail 14530 invoked by uid 501); 1 May 2000 20:27:18 -0000 Received: from localhost (sendmail-bs@127.0.0.1) by localhost with SMTP; 1 May 2000 20:27:18 -0000 MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Message-ID: Date: Mon, 1 May 2000 21:27:18 +0100 Reply-To: Chris Evans Sender: Bugtraq List From: Chris Evans Subject: Linux knfsd DoS issue X-To: bugtraq@securityfocus.com To: BUGTRAQ@SECURITYFOCUS.COM Status: RO Content-Length: 3951 Lines: 144 Hi, I was going to wait until a 2.2 kernel incorporating the fix was released, but that's taking a while, and details have already leaked by several routes. ISSUE ===== A DoS condition exists in the Linux kernel knfsd server. Remote, unauthenticated users (i.e. those with neither a directory mounted nor permission to mount one) can OOPS the host kernel. The OOPS does not bring down the target host, but it is possible to render the NFS service inoperable until a reboot. FIX === The fix is present in the following kernels - 2.3.99pre7-pre1 - 2.2.15pre20 [1] - The kernel update recently release by RedHat (AFAIK) Fix is also included at the bottom of this post. [1] Which, AFAIK, is only available as a patch on top of 2.2.15pre19. Please see ftp://ftp.kernel.org/pub/linux/kernel/people/alan/pre-patch-2.2.15-19to20.bz2 DISCUSSION OF ISSUE =================== This issue is caused by signed/unsigned issues. A size check is performed on an "int" (i.e. signed) variable. This size check does not check for negative values. As a consequence it is possible to point an internal buffer pointer _way_ before the beginning of the buffer. The kernel will try to read a memory address in limbo, and OOPS. The faulty code is in kernel/net/sunrpc/svcauth.c, svcauth_unix() int slen; ... slen = ntohl(*bufp++); /* gids length */ if (slen > 16 || (len -= slen + 2) < 0) goto badcred; ... bufp += (slen - i); As a general note, signed/unsigned errors like the above are _almost_ as prevalent as buffer overflows. Once you start looking for them. Re-auditing previously audited software with signed/unsigned issues in mind often yields new problems. Note that the userland sunrpc implementation suffered from a very similar flaw a while back. No, it certainly wasn't a copy/paste error. Some signed/unsigned errors can lead to exploitable conditions rather than just DoS. The following code fragment should illustrate this: func() { char name[NAMELEN]; int len = get_from_network(); if (len > NAMELEN) goto i_dont_think_so; memcpy(name, get_from_network(), len); /* -1 implicitly converted to 4Gb */ } An interesting variant on this (that HAS been observed) is func() { char* bufp; char* bufmax = bufp + 100; /* Or whatever */ unsigned int len = get_from_network(); if (bufp + len > bufmax) goto i_dont_think_so; ... bufp += len; } On machines with a small 4Gb address space, e.g. Intel x86, a suitable large value of len will wrap bufp around the address space, leading to a position outside of the buffer boundary, but satisfying the "bufp + len <= bufmax" constraint. Programs which have in their history been found to include signed/unsigned issues include - knfsd - sunrpc part of glibc - xfs - libORBIT - issues at syscall API of various kernels - issues in networking stacks PATCH TO FIX ============ --- net/sunrpc/svcauth.c.old Tue Apr 18 05:13:47 2000 +++ net/sunrpc/svcauth.c Tue Apr 18 06:36:20 2000 @@ -4,6 +4,9 @@ * The generic interface for RPC authentication on the server side. * * Copyright (C) 1995, 1996 Olaf Kirch + * + * CHANGES + * 19-Apr-2000 Chris Evans - Security fix */ #include @@ -117,7 +120,8 @@ struct svc_buf *resp = &rqstp->rq_resbuf; struct svc_cred *cred = &rqstp->rq_cred; u32 *bufp = argp->buf; - int len = argp->len, slen, i; + int len = argp->len; + u32 slen, i; if ((len -= 3) < 0) { *statp = rpc_garbage_args; @@ -127,7 +131,7 @@ bufp++; /* length */ bufp++; /* time stamp */ slen = (ntohl(*bufp++) + 3) >> 2; /* machname length */ - if (slen > 64 || (len -= slen) < 0) + if (slen > 64 || (len -= slen + 3) < 0) goto badcred; bufp += slen; /* skip machname */ EXPLOIT ======= Not today! In fact, don't bother. DoS is very passe... very 90's :-) Cheers Chris