Add explicit message for unsupported binary signature

Verifying the content of Release.gpg made us fail on binary signatures
which were never officially supported (apt-secure manpage only documents
only the generation of ASCII armored), but silently accepted by gpgv as
we passed it on unchecked before.

The binary format is complex and is itself split into old and new
formats so adding support for this would not only add lots of code but
also a good opportunity for bugs and dubious benefit.

Reporting this issue explicitly should help repository creators figure
out the problem faster than the default NODATA message hinting at
captive portals.

Given that the binary format has no file magic or any other clear and
simple indication that this is a detached signature we guess based on
the first two bits only – and by that only supporting the "old" binary
format which seems to be the only one generated by gnupg in this case.

References: e2965b0b6bdd68ffcad0e06d11755412a7e16e50
Closes: #921685

2 files changed, 38 insertions, 2 deletions

diff --git a/apt-pkg/contrib/gpgv.cc b/apt-pkg/contrib/gpgv.cc
index 35d859849..d956eaf00 100644
--- a/apt-pkg/contrib/gpgv.cc
+++ b/apt-pkg/contrib/gpgv.cc
@@ -297,10 +297,24 @@ void ExecGPGV(std::string const &File, std::string const &FileGPG,
       }
       if (found_signatures == 0 && statusfd != -1)
       {
-	 // This is not an attack attempt but a file even gpgv would complain about
-	 // likely the result of a paywall which is covered by the gpgv method
 	 auto const errtag = "[GNUPG:] NODATA\n";
 	 FileFd::Write(fd[1], errtag, strlen(errtag));
+	 // guess if this is a binary signature, we never officially supported them,
+	 // but silently accepted them via passing them unchecked to gpgv
+	 if (found_badcontent)
+	 {
+	    rewind(detached.get());
+	    auto ptag = fgetc(detached.get());
+	    // §4.2 says that the first bit is always set and gpg seems to generate
+	    // only old format which is indicated by the second bit not set
+	    if (ptag != EOF && (ptag & 0x80) != 0 && (ptag & 0x40) == 0)
+	    {
+	       apt_error(std::cerr, statusfd, fd, "Detached signature file '%s' is in unsupported binary format", FileGPG.c_str());
+	       local_exit(112);
+	    }
+	 }
+	 // This is not an attack attempt but a file even gpgv would complain about
+	 // likely the result of a paywall which is covered by the gpgv method
 	 local_exit(113);
       }
       else if (found_badcontent)
diff --git a/test/integration/test-bug-921685-binary-detached-signature b/test/integration/test-bug-921685-binary-detached-signature
new file mode 100755
index 000000000..df863197a
--- /dev/null
+++ b/test/integration/test-bug-921685-binary-detached-signature
@@ -0,0 +1,22 @@
+#!/bin/sh
+set -e
+
+TESTDIR="$(readlink -f "$(dirname "$0")")"
+. "$TESTDIR/framework"
+setupenvironment
+configarchitecture 'amd64'
+
+insertpackage 'unstable' 'foo' 'all' '1'
+
+buildaptarchive
+setupdistsaptarchive
+
+for RELEASE in $(find aptarchive -name 'Release'); do
+	# note the missing --armor
+	dosigning "keys/joesixpack" --detach-sign --sign --output "${RELEASE}.gpg" "${RELEASE}"
+done
+
+testfailure apt show foo
+testfailure aptget update
+testsuccess grep 'W: .* Detached signature file .* is in unsupported binary format' rootdir/tmp/testfailure.output
+testfailure apt show foo