SECURITY UPDATE: Fix out of bounds read in .ar and .tar implementation (

CVE-2020-3810)

When normalizing ar member names by removing trailing whitespace
and slashes, an out-out-bound read can be caused if the ar member
name consists only of such characters, because the code did not
stop at 0, but would wrap around and continue reading from the
stack, without any limit.

Add a check to abort if we reached the first character in the
name, effectively rejecting the use of names consisting just
of slashes and spaces.

Furthermore, certain error cases in arfile.cc and extracttar.cc have
included member names in the output that were not checked at all and
might hence not be nul terminated, leading to further out of bound reads.

Fixes Debian/apt#111
LP: #1878177

apt-pkg/contrib/arfile.cc

@@ -92,7 +92,7 @@ bool ARArchive::LoadHeaders()
 	  StrToNum(Head.Size,Memb->Size,sizeof(Head.Size)) == false)
       {
 	 delete Memb;
-	 return _error->Error(_("Invalid archive member header %s"), Head.Name);
+	 return _error->Error(_("Invalid archive member header"));
       }
 
       // Check for an extra long name string
@@ -119,7 +119,14 @@ bool ARArchive::LoadHeaders()
       else
       {
 	 unsigned int I = sizeof(Head.Name) - 1;
-	 for (; Head.Name[I] == ' ' || Head.Name[I] == '/'; I--);
+	 for (; Head.Name[I] == ' ' || Head.Name[I] == '/'; I--)
+	 {
+	    if (I == 0)
+	    {
+	       delete Memb;
+	       return _error->Error(_("Invalid archive member header"));
+	    }
+	 }
 	 Memb->Name = std::string(Head.Name,I+1);
       }
 

apt-pkg/contrib/extracttar.cc

@@ -254,7 +254,7 @@ bool ExtractTar::Go(pkgDirStream &Stream)
 
 	 default:
 	 BadRecord = true;
-	 _error->Warning(_("Unknown TAR header type %u, member %s"),(unsigned)Tar->LinkFlag,Tar->Name);
+	 _error->Warning(_("Unknown TAR header type %u"), (unsigned)Tar->LinkFlag);
 	 break;
       }
 

test/integration/test-github-111-invalid-armember

@@ -0,0 +1,88 @@
+#!/bin/sh
+set -e
+
+TESTDIR="$(readlink -f "$(dirname "$0")")"
+. "$TESTDIR/framework"
+setupenvironment
+configarchitecture "amd64"
+setupaptarchive
+
+# this used to crash, but it should treat it as an invalid member header
+touch ' '
+ar -q test.deb ' '
+testsuccessequal "E: Invalid archive member header" ${BUILDDIRECTORY}/../test/interactive-helper/testdeb test.deb
+
+
+rm test.deb
+touch 'x'
+ar -q test.deb 'x'
+testsuccessequal "E: This is not a valid DEB archive, missing 'debian-binary' member" ${BUILDDIRECTORY}/../test/interactive-helper/testdeb test.deb
+
+
+# <name><size> [ other fields] - name is not nul terminated here, it ends in .
+msgmsg "Unterminated ar member name"
+printf '!<arch>\0120123456789ABCDE.A123456789A.01234.01234.0123456.012345678.0.' > test.deb
+testsuccessequal "E: Invalid archive member header" ${BUILDDIRECTORY}/../test/interactive-helper/testdeb test.deb
+
+
+# unused source code for generating $tar below
+maketar() {
+	cat > maketar.c << EOF
+	#include <stdio.h>
+	#include <string.h>
+	struct tar {
+	   char Name[100];
+	   char Mode[8];
+	   char UserID[8];
+	   char GroupID[8];
+	   char Size[12];
+	   char MTime[12];
+	   char Checksum[8];
+	   char LinkFlag;
+	   char LinkName[100];
+	   char MagicNumber[8];
+	   char UserName[32];
+	   char GroupName[32];
+	   char Major[8];
+	   char Minor[8];
+	};
+
+	int main(void)
+	{
+		union {
+			struct tar t;
+			char buf[512];
+		} t;
+		for (int i = 0; i < sizeof(t.buf); i++)
+			t.buf[i] = '7';
+		memcpy(t.t.Name, "unterminatedName", 16);
+		memcpy(t.t.UserName, "userName", 8);
+		memcpy(t.t.GroupName, "thisIsAGroupNamethisIsAGroupName", 32);
+		t.t.LinkFlag = 'X'; // I AM BROKEN
+		memcpy(t.t.Size, "000000000000", sizeof(t.t.Size));
+		memset(t.t.Checksum,' ',sizeof(t.t.Checksum));
+
+		unsigned long sum = 0;
+		for (int i = 0; i < sizeof(t.buf); i++)
+			sum += t.buf[i];
+
+		int written = sprintf(t.t.Checksum, "%lo", sum);
+		for (int i = written; i < sizeof(t.t.Checksum); i++)
+			t.t.Checksum[i] = ' ';
+		fwrite(t.buf, sizeof(t.buf), 1, stdout);
+	}
+EOF
+
+	gcc maketar.c -o maketar -Wall
+	./maketar
+}
+
+
+#
+tar="unterminatedName77777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777700000000000077777777777773544   X777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777userName777777777777777777777777thisIsAGroupNamethisIsAGroupName777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777"
+printf '%s' "$tar" | gzip > control.tar.gz
+cp control.tar.gz data.tar.gz
+touch debian-binary
+rm test.deb
+ar -q test.deb debian-binary control.tar.gz data.tar.gz
+testsuccessequal "W: Unknown TAR header type 88" ${BUILDDIRECTORY}/../test/interactive-helper/testdeb test.deb