AOH :: HP Unsorted G :: C07-1193.HTM

GnuPG: remotely controllable function pointer



GnuPG: remotely controllable function pointer
GnuPG: remotely controllable function pointer



--=Croatian-csim-Bosnia-global-wire-transfer-LABLINK-AGT.-AMME-class=st
Content-Type: multipart/mixed; boundary="=InfoSec-AMW-military-Rule-Psix-BROMURE-Consul-morse-Firewalls-CID=Cr"

--=InfoSec-AMW-military-Rule-Psix-BROMURE-Consul-morse-Firewalls-CID=Cr
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable

     GnuPG: remotely controllable function pointer [CVE-2006-6235]
    ==============================================================                              2006-12-04

Summary
======
Tavis Ormandy of the Gentoo security team identified a severe and
exploitable bug in the processing of encrypted packets in GnuPG.

[ Please do not send private mail in response to this message.  The
  mailing list gnupg-devel is the best place to discuss this problem
  (please subscribe first so you don't need moderator approval [1]). ]


Impact
=====
Using malformed OpenPGP packets an attacker is able to modify and
dereference a function pointer in GnuPG.  This is a remotely
exploitable bug and affects any use of GnuPG where an attacker can
control the data processed by GnuPG.  It is not necessary limited to
encrypted data, also signed data may be affected.

Affected versions: All versions of GnuPG   < 1.4.6 
                   All versions of GnuPG-2 < 2.0.2
                   All beta versions of GnuPG-2 (1.9.0 .. 1.9.95)
Affected tools: gpg, gpgv, gpg2 and gpgv2.
Affected platforms: All.

gpg-agent, gpgsm as well as other tools are not affected.

A workaround is not known. 


Solution
=======
If you are using a vendor supplied version of GnuPG:

 * Wait for an update from your vendor.  Vendors have been informed on
   Saturday December 2, less than a day after this bug has been reported.

If you are using GnuPG 1.4: 

 * Update as soon as possible to GnuPG 1.4.6. It has been uploaded to
the usual location: ftp://ftp.gnupg.org/gcrypt/gnupg/. This version 
   was due to be released anyway this week.  See
http://www.gnupg.org/download/ for details. 

 * Or: As another and less intrusive option, apply the attached patch
   to GnuPG 1.4.5.  This is the smallest possible fix.

If you are using GnuPG 2.0:

 * Apply the attached patch against GnuPG 2.0.1.

 * Or: Stop using gpg2 and gpgv2, install GnuPG 1.4.6 and use gpg and gpgv
   instead.

If you are using a binary Windows version of GnuPG:

 * A binary version of GnuPG 1.4.6 for Windows is available as usual.

 * Gpg4win 1.0.8, including GnuPG 1.4.6, is available.  Please go to
http://www.gpg4win.org . 




Background
=========
GnuPG uses data structures called filters to process OpenPGP messages.
These filters ware used in a similar way as a pipelines in the shell.
For communication between these filters context structures are used.
These are usually allocated on the stack and passed to the filter
functions.  At most places the OpenPGP data stream fed into these
filters is closed before the context structure gets deallocated.
While decrypting encrypted packets, this may not happen in all cases
and the filter may use a void contest structure filled with garbage.
An attacker may control this garbage.  The filter context includes
another context used by the low-level decryption to access the
decryption algorithm.  This is done using a function pointer.  By
carefully crafting an OpenPGP message, an attacker may control this
function pointer and call an arbitrary function of the process.
Obviously an exploit needs to prepared for a specific version,
compiler, libc, etc to be successful - but it is definitely doable.

Fixing this is obvious: We need to allocate the context on the heap
and use a reference count to keep it valid as long as either the
controlling code or the filter code needs it.

We have checked all other usages of such a stack based filter contexts
but fortunately found no other vulnerable places.  This allows to
release a relatively small patch.  However, for reasons of code
cleanness and easier audits we will soon start to change all these
stack based filter contexts to heap based ones.


Support 
======
g10 Code GmbH, a Duesseldorf based company owned and headed by GnuPG's
principal author, is currently funding GnuPG development.  As evident
by the two vulnerabilities found within a week, a review of the entire
code base should be undertaken as soon as possible.  As maintainers we
try to do our best and are working slowly through the code.  The long
standing plan is to scrutinize the 2.0 code base, write more test
cases and to backport new fixes and cleanups to 1.4.  However, as a
small company our resources are limited and we need to prioritize
other projects which get us actual revenues.  Support contracts or
other financial backing would greatly help us to improve the quality
of GnuPG.


Thanks
=====
Tavis Ormandy found this vulnerability.




[1] See http://lists.gnupg.org/mailman/listinfo/gnupg-devel . 

=2D- 
g10 Code GmbH http://g10code.com AmtsGer. Wuppertal HRB 14459 
H=C3=BCttenstr. 61                               Gesch=C3=A4ftsf=C3=BChrung Werner Koch
D-40699 Erkrath  -=- The GnuPG Experts -=-  USt-Id DE215605608

--=InfoSec-AMW-military-Rule-Psix-BROMURE-Consul-morse-Firewalls-CID=Cr
Content-Disposition: attachment; filename=filter-context-14-small.diff
Content-Transfer-Encoding: quoted-printable
Content-Description: Patch against 1.4.5

This is a patch against GnuPG 1.4.5.  Change the directory to g10/ and
apply this patch.

2006-12-02 Werner Koch  

	* encr-data.c: Allocate DFX context on the heap and not on the
	stack.  Changes at several places.  Fixes CVE-2006-6235.
=09


=2D-- encr-data.c.orig	2006-05-16 14:34:26.000000000 +0200
+++ encr-data.c	2006-12-04 11:58:53.000000000 +0100
@@ -44,7 +44,27 @@ typedef struct {
     char defer[20];
     int  defer_filled;
     int  eof_seen;
=2D} decode_filter_ctx_t;
+    int  refcount;
+} *decode_filter_ctx_t;
+
+
+/* Helper to release the decode context.  */
+static void
+release_dfx_context (decode_filter_ctx_t dfx)
+{
+  if (!dfx)
+    return;
+
+  assert (dfx->refcount);
+  if ( !--dfx->refcount )
+    {
+      cipher_close (dfx->cipher_hd);
+      dfx->cipher_hd = NULL;
+      md_close (dfx->mdc_hash);
+      dfx->mdc_hash = NULL;
+      xfree (dfx);
+    }
+}
 
 
 /****************
@@ -60,7 +80,10 @@ decrypt_data( void *procctx, PKT_encrypt
     unsigned blocksize;
     unsigned nprefix;
 
=2D    memset( &dfx, 0, sizeof dfx );
+
+    dfx = xcalloc (1, sizeof *dfx);
+    dfx->refcount = 1;
+
     if( opt.verbose && !dek->algo_info_printed ) {
 	const char *s = cipher_algo_to_string( dek->algo );
 	if( s )
@@ -79,15 +102,15 @@ decrypt_data( void *procctx, PKT_encrypt
 	BUG();
 
     if( ed->mdc_method ) {
=2D	dfx.mdc_hash = md_open( ed->mdc_method, 0 );
+	dfx->mdc_hash = md_open ( ed->mdc_method, 0 );
 	if ( DBG_HASHING )
=2D	    md_start_debug(dfx.mdc_hash, "checkmdc");
+	    md_start_debug (dfx->mdc_hash, "checkmdc");
     }
=2D    dfx.cipher_hd = cipher_open( dek->algo,
=2D				 ed->mdc_method? CIPHER_MODE_CFB
=2D					       : CIPHER_MODE_AUTO_CFB, 1 );
+    dfx->cipher_hd = cipher_open ( dek->algo,
+                                   ed->mdc_method? CIPHER_MODE_CFB
+                                                 : CIPHER_MODE_AUTO_CFB, 1 );
     /* log_hexdump( "thekey", dek->key, dek->keylen );*/
=2D    rc = cipher_setkey( dfx.cipher_hd, dek->key, dek->keylen );
+    rc = cipher_setkey ( dfx->cipher_hd, dek->key, dek->keylen );
     if( rc == G10ERR_WEAK_KEY )
       {
 	log_info(_("WARNING: message was encrypted with"
@@ -105,7 +128,7 @@ decrypt_data( void *procctx, PKT_encrypt
         goto leave;
     }
 
=2D    cipher_setiv( dfx.cipher_hd, NULL, 0 );
+    cipher_setiv ( dfx->cipher_hd, NULL, 0 );
 
     if( ed->len ) {
 	for(i=0; i < (nprefix+2) && ed->len; i++, ed->len-- ) {
@@ -122,8 +145,8 @@ decrypt_data( void *procctx, PKT_encrypt
 	    else
 		temp[i] = c;
     }
=2D    cipher_decrypt( dfx.cipher_hd, temp, temp, nprefix+2);
=2D    cipher_sync( dfx.cipher_hd );
+    cipher_decrypt ( dfx->cipher_hd, temp, temp, nprefix+2);
+    cipher_sync ( dfx->cipher_hd );
     p = temp;
 /* log_hexdump( "prefix", temp, nprefix+2 ); */
     if(dek->symmetric
@@ -133,34 +156,34 @@ decrypt_data( void *procctx, PKT_encrypt
 	goto leave;
       }
 
=2D    if( dfx.mdc_hash )
=2D	md_write( dfx.mdc_hash, temp, nprefix+2 );
+    if ( dfx->mdc_hash )
+	md_write ( dfx->mdc_hash, temp, nprefix+2 );
 
=2D    if( ed->mdc_method )
=2D	iobuf_push_filter( ed->buf, mdc_decode_filter, &dfx );
+    dfx->refcount++;
+    if ( ed->mdc_method )
+	iobuf_push_filter( ed->buf, mdc_decode_filter, dfx );
     else
=2D	iobuf_push_filter( ed->buf, decode_filter, &dfx );
+	iobuf_push_filter( ed->buf, decode_filter, dfx );
 
     proc_packets( procctx, ed->buf );
     ed->buf = NULL;
=2D    if( ed->mdc_method && dfx.eof_seen == 2 )
+    if( ed->mdc_method && dfx->eof_seen == 2 )
 	rc = G10ERR_INVALID_PACKET;
     else if( ed->mdc_method ) { /* check the mdc */
 	int datalen = md_digest_length( ed->mdc_method );
 
=2D	cipher_decrypt( dfx.cipher_hd, dfx.defer, dfx.defer, 20);
=2D	md_final( dfx.mdc_hash );
+	cipher_decrypt ( dfx->cipher_hd, dfx->defer, dfx->defer, 20);
+	md_final ( dfx->mdc_hash );
 	if( datalen != 20
=2D	    || memcmp(md_read( dfx.mdc_hash, 0 ), dfx.defer, datalen) )
+	    || memcmp(md_read( dfx->mdc_hash, 0 ), dfx->defer, datalen) )
 	    rc = G10ERR_BAD_SIGN;
=2D	/*log_hexdump("MDC calculated:", md_read( dfx.mdc_hash, 0), datalen);*/
=2D	/*log_hexdump("MDC message   :", dfx.defer, 20);*/
+	/*log_hexdump("MDC calculated:",md_read( dfx->mdc_hash, 0), datalen);*/
+	/*log_hexdump("MDC message   :", dfx->defer, 20);*/
     }
     
 
   leave:
=2D    cipher_close(dfx.cipher_hd);
=2D    md_close( dfx.mdc_hash );
+    release_dfx_context (dfx);
     return rc;
 }
 
@@ -171,7 +194,7 @@ static int
 mdc_decode_filter( void *opaque, int control, IOBUF a,
 					      byte *buf, size_t *ret_len)
 {
=2D    decode_filter_ctx_t *dfx = opaque;
+    decode_filter_ctx_t dfx = opaque;
     size_t n, size = *ret_len;
     int rc = 0;
     int c;
@@ -226,8 +249,10 @@ mdc_decode_filter( void *opaque, int con
 	}
 
 	if( n ) {
=2D	    cipher_decrypt( dfx->cipher_hd, buf, buf, n);
=2D	    md_write( dfx->mdc_hash, buf, n );
+            if (dfx->cipher_hd)
+                cipher_decrypt( dfx->cipher_hd, buf, buf, n);
+            if (dfx->mdc_hash)
+                md_write( dfx->mdc_hash, buf, n );
 	}
 	else {
 	    assert( dfx->eof_seen );
@@ -235,6 +260,9 @@ mdc_decode_filter( void *opaque, int con
 	}
 	*ret_len = n;
     }
+    else if ( control == IOBUFCTRL_FREE ) {
+        release_dfx_context (dfx);
+    }
     else if( control == IOBUFCTRL_DESC ) {
 	*(char**)buf = "mdc_decode_filter";
     }
@@ -244,7 +272,7 @@ mdc_decode_filter( void *opaque, int con
 static int
 decode_filter( void *opaque, int control, IOBUF a, byte *buf, size_t *ret_len)
 {
=2D    decode_filter_ctx_t *fc = opaque;
+    decode_filter_ctx_t fc = opaque;
     size_t n, size = *ret_len;
     int rc = 0;
 
@@ -252,12 +280,17 @@ decode_filter( void *opaque, int control
 	assert(a);
 	n = iobuf_read( a, buf, size );
 	if( n == -1 ) n = 0;
=2D	if( n )
=2D	    cipher_decrypt( fc->cipher_hd, buf, buf, n);
+	if( n ) {
+            if (fc->cipher_hd)
+                cipher_decrypt( fc->cipher_hd, buf, buf, n);
+        }
 	else
 	    rc = -1; /* eof */
 	*ret_len = n;
     }
+    else if ( control == IOBUFCTRL_FREE ) {
+        release_dfx_context (fc);
+    }
     else if( control == IOBUFCTRL_DESC ) {
 	*(char**)buf = "decode_filter";
     }

--=InfoSec-AMW-military-Rule-Psix-BROMURE-Consul-morse-Firewalls-CID=Cr
Content-Disposition: attachment; filename=filter-context-20-small.diff
Content-Transfer-Encoding: quoted-printable
Content-Description: Patch against 2.0.1

This is a patch against GnuPG 2.0.1. Change the directory to g10/ and
apply this patch.

2006-12-02 Werner Koch  

	* encr-data.c: Allocate DFX context on the heap and not on the
	stack.  Changes at several places.  Fixes CVE-2006-6235.
=09

Index: encr-data.c
===================================================================2D-- encr-data.c	(revision 4352)
+++ encr-data.c	(working copy)
@@ -39,16 +39,37 @@
 static int decode_filter ( void *opaque, int control, IOBUF a,
 					byte *buf, size_t *ret_len);
 
=2Dtypedef struct 
+typedef struct decode_filter_context_s
 {
   gcry_cipher_hd_t cipher_hd;
   gcry_md_hd_t mdc_hash;
   char defer[22];
   int  defer_filled;
   int  eof_seen;
=2D} decode_filter_ctx_t;
+  int  refcount;
+} *decode_filter_ctx_t;
 
 
+/* Helper to release the decode context.  */
+static void
+release_dfx_context (decode_filter_ctx_t dfx)
+{
+  if (!dfx)
+    return;
+
+  assert (dfx->refcount);
+  if ( !--dfx->refcount )
+    {
+      gcry_cipher_close (dfx->cipher_hd);
+      dfx->cipher_hd = NULL;
+      gcry_md_close (dfx->mdc_hash);
+      dfx->mdc_hash = NULL;
+      xfree (dfx);
+    }
+}
+
+
+
 /****************
  * Decrypt the data, specified by ED with the key DEK.
  */
@@ -62,7 +83,11 @@
   unsigned blocksize;
   unsigned nprefix;
   
=2D  memset( &dfx, 0, sizeof dfx );
+  dfx = xtrycalloc (1, sizeof *dfx);
+  if (!dfx)
+    return gpg_error_from_syserror ();
+  dfx->refcount = 1;
+
   if ( opt.verbose && !dek->algo_info_printed )
     {
       const char *s = gcry_cipher_algo_name (dek->algo);
@@ -77,20 +102,20 @@
     goto leave;
   blocksize = gcry_cipher_get_algo_blklen (dek->algo);
   if ( !blocksize || blocksize > 16 )
=2D    log_fatal("unsupported blocksize %u\n", blocksize );
+    log_fatal ("unsupported blocksize %u\n", blocksize );
   nprefix = blocksize;
   if ( ed->len && ed->len < (nprefix+2) )
     BUG();
 
   if ( ed->mdc_method ) 
     {
=2D      if (gcry_md_open (&dfx.mdc_hash, ed->mdc_method, 0 ))
+      if (gcry_md_open (&dfx->mdc_hash, ed->mdc_method, 0 ))
         BUG ();
       if ( DBG_HASHING )
=2D        gcry_md_start_debug (dfx.mdc_hash, "checkmdc");
+        gcry_md_start_debug (dfx->mdc_hash, "checkmdc");
     }
 
=2D  rc = gcry_cipher_open (&dfx.cipher_hd, dek->algo,
+  rc = gcry_cipher_open (&dfx->cipher_hd, dek->algo,
                          GCRY_CIPHER_MODE_CFB,
                          (GCRY_CIPHER_SECURE
                           | ((ed->mdc_method || dek->algo >= 100)?
@@ -104,7 +129,7 @@
 
 
   /* log_hexdump( "thekey", dek->key, dek->keylen );*/
=2D  rc = gcry_cipher_setkey (dfx.cipher_hd, dek->key, dek->keylen);
+  rc = gcry_cipher_setkey (dfx->cipher_hd, dek->key, dek->keylen);
   if ( gpg_err_code (rc) == GPG_ERR_WEAK_KEY )
     {
       log_info(_("WARNING: message was encrypted with"
@@ -123,7 +148,7 @@
       goto leave;
     }
 
=2D  gcry_cipher_setiv (dfx.cipher_hd, NULL, 0);
+  gcry_cipher_setiv (dfx->cipher_hd, NULL, 0);
 
   if ( ed->len )
     {
@@ -144,8 +169,8 @@
           temp[i] = c;
     }
   
=2D  gcry_cipher_decrypt (dfx.cipher_hd, temp, nprefix+2, NULL, 0);
=2D  gcry_cipher_sync (dfx.cipher_hd);
+  gcry_cipher_decrypt (dfx->cipher_hd, temp, nprefix+2, NULL, 0);
+  gcry_cipher_sync (dfx->cipher_hd);
   p = temp;
   /* log_hexdump( "prefix", temp, nprefix+2 ); */
   if (dek->symmetric
@@ -155,17 +180,18 @@
       goto leave;
     }
   
=2D  if ( dfx.mdc_hash )
=2D    gcry_md_write (dfx.mdc_hash, temp, nprefix+2);
=2D  
+  if ( dfx->mdc_hash )
+    gcry_md_write (dfx->mdc_hash, temp, nprefix+2);
+
+  dfx->refcount++;
   if ( ed->mdc_method )
=2D    iobuf_push_filter( ed->buf, mdc_decode_filter, &dfx );
+    iobuf_push_filter ( ed->buf, mdc_decode_filter, dfx );
   else
=2D    iobuf_push_filter( ed->buf, decode_filter, &dfx );
+    iobuf_push_filter ( ed->buf, decode_filter, dfx );
 
   proc_packets ( procctx, ed->buf );
   ed->buf = NULL;
=2D  if ( ed->mdc_method && dfx.eof_seen == 2 )
+  if ( ed->mdc_method && dfx->eof_seen == 2 )
     rc = gpg_error (GPG_ERR_INV_PACKET);
   else if ( ed->mdc_method )
     { 
@@ -184,26 +210,28 @@
          bytes are appended.  */
       int datalen = gcry_md_get_algo_dlen (ed->mdc_method);
 
=2D      gcry_cipher_decrypt (dfx.cipher_hd, dfx.defer, 22, NULL, 0);
=2D      gcry_md_write (dfx.mdc_hash, dfx.defer, 2);
=2D      gcry_md_final (dfx.mdc_hash);
+      assert (dfx->cipher_hd);
+      assert (dfx->mdc_hash);
+      gcry_cipher_decrypt (dfx->cipher_hd, dfx->defer, 22, NULL, 0);
+      gcry_md_write (dfx->mdc_hash, dfx->defer, 2);
+      gcry_md_final (dfx->mdc_hash);
 
=2D      if (dfx.defer[0] != '\xd3' || dfx.defer[1] != '\x14' )
+      if (dfx->defer[0] != '\xd3' || dfx->defer[1] != '\x14' )
         {
           log_error("mdc_packet with invalid encoding\n");
           rc = gpg_error (GPG_ERR_INV_PACKET);
         }
       else if (datalen != 20
=2D               || memcmp (gcry_md_read (dfx.mdc_hash, 0),dfx.defer+2,datalen))
+               || memcmp (gcry_md_read (dfx->mdc_hash, 0),
+                          dfx->defer+2,datalen ))
         rc = gpg_error (GPG_ERR_BAD_SIGNATURE);
=2D      /* log_printhex("MDC message:", dfx.defer, 22); */
=2D      /* log_printhex("MDC calc:", gcry_md_read (dfx.mdc_hash,0), datalen); */
+      /* log_printhex("MDC message:", dfx->defer, 22); */
+      /* log_printhex("MDC calc:", gcry_md_read (dfx->mdc_hash,0), datalen); */
     }
   
   
  leave:
=2D  gcry_cipher_close (dfx.cipher_hd);
=2D  gcry_md_close (dfx.mdc_hash);
+  release_dfx_context (dfx);
   return rc;
 }
 
@@ -214,7 +242,7 @@
 mdc_decode_filter (void *opaque, int control, IOBUF a,
                    byte *buf, size_t *ret_len)
 {
=2D  decode_filter_ctx_t *dfx = opaque;
+  decode_filter_ctx_t dfx = opaque;
   size_t n, size = *ret_len;
   int rc = 0;
   int c;
@@ -226,11 +254,11 @@
     }
   else if( control == IOBUFCTRL_UNDERFLOW )
     {
=2D      assert(a);
=2D      assert( size > 44 );
+      assert (a);
+      assert ( size > 44 );
       
       /* Get at least 22 bytes and put it somewhere ahead in the buffer. */
=2D      for(n=22; n < 44 ; n++ )
+      for (n=22; n < 44 ; n++ )
         {
           if( (c = iobuf_get(a)) == -1 )
             break;
@@ -279,8 +307,10 @@
 
       if ( n )
         {
=2D          gcry_cipher_decrypt (dfx->cipher_hd, buf, n, NULL, 0);
=2D          gcry_md_write (dfx->mdc_hash, buf, n);
+          if ( dfx->cipher_hd )
+            gcry_cipher_decrypt (dfx->cipher_hd, buf, n, NULL, 0);
+          if ( dfx->mdc_hash )
+            gcry_md_write (dfx->mdc_hash, buf, n);
 	}
       else
         {
@@ -289,6 +319,10 @@
 	}
       *ret_len = n;
     }
+  else if ( control == IOBUFCTRL_FREE ) 
+    {
+      release_dfx_context (dfx);
+    }
   else if ( control == IOBUFCTRL_DESC ) 
     {
       *(char**)buf = "mdc_decode_filter";
@@ -300,7 +334,7 @@
 static int
 decode_filter( void *opaque, int control, IOBUF a, byte *buf, size_t *ret_len)
 {
=2D  decode_filter_ctx_t *fc = opaque;
+  decode_filter_ctx_t fc = opaque;
   size_t n, size = *ret_len;
   int rc = 0;
   
@@ -311,11 +345,18 @@
       if ( n == -1 )
         n = 0;
       if ( n )
=2D        gcry_cipher_decrypt (fc->cipher_hd, buf, n, NULL, 0);
+        {
+          if (fc->cipher_hd)
+            gcry_cipher_decrypt (fc->cipher_hd, buf, n, NULL, 0);
+        }
       else
         rc = -1; /* EOF */
       *ret_len = n;
     }
+  else if ( control == IOBUFCTRL_FREE ) 
+    {
+      release_dfx_context (fc);
+    }
   else if ( control == IOBUFCTRL_DESC )
     {
       *(char**)buf = "decode_filter";

--=InfoSec-AMW-military-Rule-Psix-BROMURE-Consul-morse-Firewalls-CID=Cr--

--=Croatian-csim-Bosnia-global-wire-transfer-LABLINK-AGT.-AMME-class=st
Content-Type: application/pgp-signature

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Version: GnuPG v2.0.1 (GNU/Linux)

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=/s5x
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--=Croatian-csim-Bosnia-global-wire-transfer-LABLINK-AGT.-AMME-class=st--


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