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gpg: Make the use of "--verify FILE" for detached sigs harder.

Browse Source 
* g10/openfile.c (open_sigfile): Factor some code out to ...
(get_matching_datafile): new function.
* g10/plaintext.c (hash_datafiles): Do not try to find matching file
in batch mode.
* g10/mainproc.c (check_sig_and_print): Print a warning if a possibly
matching data file is not used by a standard signatures.
--

Allowing to use the abbreviated form for detached signatures is a long
standing bug which has only been noticed by the public with the
release of 2.1.0.  :-(

What we do is to remove the ability to check detached signature in
--batch using the one file abbreviated mode.  This should exhibit
problems in scripts which use this insecure practice.  We also print a
warning if a matching data file exists but was not considered because
the detached signature was actually a standard signature:

  gpgv: Good signature from "Werner Koch (dist sig)"
  gpgv: WARNING: not a detached signature; \
  file 'gnupg-2.1.0.tar.bz2' was NOT verified!

We can only print a warning because it is possible that a standard
signature is indeed to be verified but by coincidence a file with a
matching name is stored alongside the standard signature.

Reported-by: Simon Nicolussi (to gnupg-users on Nov 7)
Signed-off-by: Werner Koch <wk@gnupg.org>

(backported from commit 69384568f66a48eff3968bb1714aa13925580e9f)

Updated doc/gpg.texi.
This commit is contained in:
Werner Koch 2014-11-14 09:36:19 +01:00
parent 42d2474a02
commit fbb50867f8
5 changed files with 129 additions and 47 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

27
doc/gpg.texi
View File

@ -198,16 +198,22 @@ files which don't begin with an encrypted message.
@item --verify
@opindex verify
Assume that the first argument is a signed file or a detached signature
and verify it without generating any output. With no arguments, the
signature packet is read from STDIN. If only a sigfile is given, it may
be a complete signature or a detached signature, in which case the
signed stuff is expected in a file without the ".sig" or ".asc"
extension. With more than 1 argument, the first should be a detached
signature and the remaining files are the signed stuff. To read the
signed stuff from STDIN, use @samp{-} as the second filename. For
security reasons a detached signature cannot read the signed material
from STDIN without denoting it in the above way.
Assume that the first argument is a signed file and verify it without
generating any output. With no arguments, the signature packet is
read from STDIN. If only a one argument is given, it is expected to
be a complete signature.
With more than 1 argument, the first should be a detached signature
and the remaining files ake up the the signed data. To read the signed
data from STDIN, use @samp{-} as the second filename. For security
reasons a detached signature cannot read the signed material from
STDIN without denoting it in the above way.
Note: If the option @option{--batch} is not used, @command{gpg}
may assume that a single argument is a file with a detached signature
and it will try to find a matching data file by stripping certain
suffixes. Using this historical feature to verify a detached
signature is strongly discouraged; always specify the data file too.
Note: When verifying a cleartext signature, @command{gpg} verifies
only what makes up the cleartext signed data and not any extra data
@ -217,6 +223,7 @@ out the actual signed data; but there are other pitfalls with this
format as well. It is suggested to avoid cleartext signatures in
favor of detached signatures.
@item --multifile
@opindex multifile
This modifies certain other commands to accept multiple files for

1
g10/main.h
View File

@ -195,6 +195,7 @@ int overwrite_filep( const char *fname );
char *make_outfile_name( const char *iname );
char *ask_outfile_name( const char *name, size_t namelen );
int open_outfile( const char *iname, int mode, IOBUF *a );
char *get_matching_datafile (const char *sigfilename);
IOBUF open_sigfile( const char *iname, progress_filter_context_t *pfx );
void try_make_homedir( const char *fname );

38
g10/mainproc.c
View File

@ -1940,6 +1940,44 @@ check_sig_and_print( CTX c, KBNODE node )
sig->sig_class==0x01?_("textmode"):_("unknown"),
digest_algo_to_string(sig->digest_algo));
if (!rc && !c->signed_data)
{
/* Signature is basically good but we test whether the
deprecated command
gpg --verify FILE.sig
was used instead of
gpg --verify FILE.sig FILE
to verify a detached signature. If we figure out that a
data file with a matching name exists, we print a warning.
The problem is that the first form would also verify a
standard signature. This behavior could be used to
create a made up .sig file for a tarball by creating a
standard signature from a valid detached signature packet
(for example from a signed git tag). Then replace the
sig file on the FTP server along with a changed tarball.
Using the first form the verify command would correctly
verify the signature but don't even consider the tarball. */
kbnode_t n;
char *dfile;
dfile = get_matching_datafile (c->sigfilename);
if (dfile)
{
for (n = c->list; n; n = n->next)
if (n->pkt->pkttype != PKT_SIGNATURE)
break;
if (n)
{
/* Not only signature packets in the tree thus this
is not a detached signature. */
log_info (_("WARNING: not a detached signature; "
"file '%s' was NOT verified!\n"), dfile);
}
xfree (dfile);
}
}
if( rc )
g10_errors_seen = 1;
if( opt.batch && rc )

61
g10/openfile.c
View File

@ -300,41 +300,70 @@ open_outfile( const char *iname, int mode, IOBUF *a )
}
/* Find a matching data file for the signature file SIGFILENAME and
return it as a malloced string. If no matching data file is found,
return NULL. */
char *
get_matching_datafile (const char *sigfilename)
{
char *fname = NULL;
size_t len;
if (iobuf_is_pipe_filename (sigfilename))
return NULL;
len = strlen (sigfilename);
if (len > 4
&& (!strcmp (sigfilename + len - 4, EXTSEP_S "sig")
|| (len > 5 && !strcmp(sigfilename + len - 5, EXTSEP_S "sign"))
|| !strcmp(sigfilename + len - 4, EXTSEP_S "asc")))
{
fname = xstrdup (sigfilename);
fname[len-(fname[len-1]=='n'?5:4)] = 0 ;
if (access (fname, R_OK ))
{
/* Not found or other error. */
xfree (fname);
fname = NULL;
}
}
return fname;
}
/****************
* Try to open a file without the extension ".sig" or ".asc"
* Return NULL if such a file is not available.
*/
IOBUF
open_sigfile( const char *iname, progress_filter_context_t *pfx )
open_sigfile (const char *sigfilename, progress_filter_context_t *pfx)
{
IOBUF a = NULL;
size_t len;
if( !iobuf_is_pipe_filename (iname) ) {
len = strlen(iname);
if( len > 4 && ( !strcmp(iname + len - 4, EXTSEP_S "sig")
|| ( len > 5 && !strcmp(iname + len - 5, EXTSEP_S "sign") )
|| !strcmp(iname + len - 4, EXTSEP_S "asc")) ) {
iobuf_t a = NULL;
char *buf;
buf = xstrdup(iname);
buf[len-(buf[len-1]=='n'?5:4)] = 0 ;
a = iobuf_open( buf );
buf = get_matching_datafile (sigfilename);
if (buf)
{
a = iobuf_open (buf);
if (a && is_secured_file (iobuf_get_fd (a)))
{
iobuf_close (a);
a = NULL;
errno = EPERM;
}
if( a && opt.verbose )
log_info(_("assuming signed data in `%s'\n"), buf );
if (a)
log_info (_("assuming signed data in `%s'\n"), buf);
if (a && pfx)
handle_progress (pfx, a, buf);
xfree(buf);
}
xfree (buf);
}
return a;
}
/****************
* Copy the option file skeleton to the given directory.
*/

9
g10/plaintext.c
View File

@ -538,13 +538,20 @@ hash_datafiles( MD_HANDLE md, MD_HANDLE md2, STRLIST files,
STRLIST sl;
if( !files ) {
/* check whether we can open the signed material */
/* Check whether we can open the signed material. We avoid
trying to open a file if run in batch mode. This assumed
data file for a sig file feature is just a convenience thing
for the command line and the user needs to read possible
warning messages. */
if (!opt.batch) {
fp = open_sigfile( sigfilename, &pfx );
if( fp ) {
do_hash( md, md2, fp, textmode );
iobuf_close(fp);
return 0;
}
}
log_error (_("no signed data\n"));
return G10ERR_OPEN_FILE;
}