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Re: easy DoS in most RPC appsBill Paul (wpaulCTR.COLUMBIA.EDU)
Sun, 17 May 1998 15:48:55 -0400
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Of all the gin joints in all the towns in all the world, Scott Stone had to walk into mine and say: > On Sun, 17 May 1998, David LeBlanc wrote: > > > At 02:35 AM 5/15/98 +0200, Peter van Dijk wrote: > > >Finally, I'm quite sure of this: the bug is in Sun's RPC code. > > >Investigations show Linux, FreeBSD, SunOS, System V and NeXTstep machines > > >are affected, which means we've got a _big_ problem here. > > > > If that's the case, then any ports of these utilities running on Windows NT > > would also exhibit the same problem - we're all running off of pretty much > > the same Sun ONC RPC code. > > > > The FreeBSD people have already made a patch for this, check their home > site. I'm going to attempt to port the patch to Linux, as the base code > should be about the same.. the fix is to a couple of rpc-related files in > the C libraries. (I thought I sent a message about this to the list on Friday but it seems not to have made it. Either I sent it off into /dev/null without realizing it or gremlins ate it. In either case, I'll try again.) The modifications I made to the FreeBSD RPC library prevent an attacker from completely wedging a stream based RPC service for an indefinite period, however there really ought to be more done to avoid the problem completely. The real problem is in the XDR record marking code which is used for the TCP transport. (In RPC 4.0, TCP is the only transport affected. In TI-RPC, any 'virtual circuit' transport including but not limited to TCP is affected.) The set_input_fragment() routine in src/lib/libc/xdr/xdr_rec.c attempts to read a record header which is supposed to specify the size of the record that follows. Unfortunately, this routine performs no sanity checking: if you telnet to a TCP service and send a few carriage returns, set_input_fragment() misinterprets them as a ridiculously large record size. This in turn causes the fill_input_buffer() routine to try reading a ridiculously large amount of data from the network. This is why the service stays wedged until you disconnect. The patch I made to fix this is as follows: *** xdr_rec.c.orig Fri May 15 17:43:57 1998 --- xdr_rec.c Fri May 15 17:47:58 1998 *************** *** 550,555 **** --- 550,561 ---- return (FALSE); header = (long)ntohl(header); rstrm->last_frag = ((header & LAST_FRAG) == 0) ? FALSE : TRUE; + /* + * Sanity check. Try not to accept wildly incorrect + * record sizes. + */ + if ((header & (~LAST_FRAG)) > rstrm->recvsize) + return(FALSE); rstrm->fbtbc = header & (~LAST_FRAG); return (TRUE); } The next change relates to the svc_tcp.c module directly. The svctcp_recv() routine calls xdr_callmsg() to attempt to decode the RPC message header that should accompany every RPC request. With the UDP transport, a datagram that doesn't contain a valid header is dropped on the floor. With TCP, the connection is left open to attempt to receive another request that may be pending. In my view, if no valid message header is found where there should have been one, the connection should be dropped. The following patch to src/lib/libc/rpc/svc_tcp.c does this: *** svc_tcp.c.orig Fri May 15 17:11:21 1998 --- svc_tcp.c Fri May 15 17:09:02 1998 *************** *** 404,409 **** --- 404,410 ---- cd->x_id = msg->rm_xid; return (TRUE); } + cd->strm_stat = XPRT_DIED; /* XXXX */ return (FALSE); } This marks the transport handle as dead if xdr_callmsg() fails, which in turn will cause the dispatcher to drop the connection. With these patches, you have 35 seconds to supply a valid record containing an RPC message header and request, otherwise the session is disconnected. If you enter garbage data, the connection is dropped immediately. As far as I know, this bug is likely present in all Sun-derived ONC RPC implementations, including TI-RPC from ONC+, which is what you'll find in Solaris 2.x and I think AIX 4.2 and up. TI-RPC uses the same XDR record marking code, although it has an svc_vc.c module to handle virtual circuit transports as opposed to a transport-specific svc_tcp.c module. Mind you, this observation is based on the TI-RPC 2.3 source, which is quite old. I do not consider the bug completely fixed though. These patches only work around the immediate problem. A proper fix would allow the service to continue to handle new requests even while waiting for the 35 second timeout to expire, and would apply a more intelligent sanity check in set_input_fragment(). I think one solution would be to modify readtcp() so that it monitors the other transport handles in addition to the current socket that it's reading from, but I still have to do some tests to see if this idea is really practical. -Bill -- ============================================================================= -Bill Paul (212) 854-6020 | System Manager, Master of Unix-Fu Work: wpaulctr.columbia.edu | Center for Telecommunications Research Home: wpaulskynet.ctr.columbia.edu | Columbia University, New York City ============================================================================= "Now, that's "Open" as used in the sentence "Open your wallet", right?" =============================================================================