Description of problem:
This issue has been found during analysing of bug #845283.
Let's have NIS client and server configured. glibc's functions like yp_match then send UDP packets to NIS server to ask for network information like login names, passwords etc. Requests are byte strings containing e.g. "...<domainname length><domainname string>...".
When we change length field of the xdr request, while the rest of the string is unchanged and pass such malformed request to the ypserv daemon for example, glibc routine allocates huge memory and then tries to read huge amount of such unallocated memory.
ypserv has a bug #845283, that causes not freeing memory when request is not parsed properly, which results in huge memory leaks and daemon failure. While freeing memory after failed parsing has to be fixed in ypserv, glibc should prevent such behaviour and not allocate more memory than necessary to avoid reading unallocated memory.
There are already limits for NIS fields like domainname, mapname, etc., but they're not used in the parsing functions. Valgrind prints then the following error:
==16566== Conditional jump or move depends on uninitialised value(s)
==16566== at 0x4A077F9: __GI_strlen (mc_replace_strmem.c:284)
==16566== by 0x51587F6: xdr_string (xdr.c:680)
==16566== by 0x4E2D5AD: xdr_ypreq_key (yp_xdr.c:50)
==16566== by 0x10B092: ypprog_2 (ypserv.c:222)
==16566== by 0x5156398: svc_getreq_common (svc.c:485)
==16566== by 0x5155ED0: svc_getreq_poll (svc.c:415)
==16566== by 0x5156A02: svc_run (svc_run.c:94)
==16566== by 0x10B84C: main (ypserv.c:726)
Using the following sane limits to NIS fields should prevent such behaviour:
#define YPMAXRECORD 1024
#define YPMAXDOMAIN 64
#define YPMAXMAP 64
#define YPMAXPEER 64
Version-Release number of selected component (if applicable):
Steps to Reproduce:
1. start ypserv daemon
2. configure ypbind to use that ypserv daemon
3. create a malformed request string (example of such request will be attached as ypserv-packet-bin)
4. cat ypserv-packet-bin |nc -u 192.168.122.70 811 (192.168.122.70:811 is where ypserv listens for UDP requests)
huge amount of memory is allocated and then uninitialized memory is read during parsing request
amount of memory allocated during request parsing is limited to a sane amount
Created attachment 604885 [details]
malformed request that can cause huge memory allocations
Use this like this (having ypserv listening for UDP on 192.168.122.70:788:
cat ypserv-packet-bin |nc -u 192.168.122.70 788
Created attachment 604886 [details]
proposed patch that uses sane limits while parsing xdr request
Resetting flags for 6.5 consideration.
This approach causes problems with the slapi-nis plugin in IPA.
We'll be looking at this issue as part of the review process for the next release. We'll keep the issue updated.
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.