Dpkg::Source::Patch: add more sanity checks on patches

patch will happily accept filenames like "../../../../stuff" and modify
files outside of the expected destination directory. To avoid problems
we error out when we detect a filename that contains "/../". Any leading
"../" is not a problem since patches are applied with -p1 and it's
stripped.

We also verify that the file to be modified is not accessed through a
symlink as a compromised source package could also provide a symlink in
the orig.tar.gz that points outside of the destination directory.

debian/changelog

@@ -67,6 +67,12 @@ dpkg (1.15.6) UNRELEASED; urgency=low
     spaces. Closes: #572030
   * dpkg-source does no longer fallback to other source formats if the
     requested one is not usable. Closes: #557459
+  * Modify dpkg-source to error out when it would apply patches containing
+    insecure paths (with "/../") and also error out when it would apply a
+    patch through a symlink. Those checks are required as patch will happily
+    modify files outside of the target directory and unpacking a source package
+    should not be able to have any side-effect outside of the target
+    directory. Fixes CVE-2010-0396.
 
   [ Guillem Jover ]
   * Handle argument parsing in dpkg-checkbuilddeps and dpkg-scanpackages

scripts/Dpkg/Source/Patch.pm

@@ -322,8 +322,9 @@ sub analyze {
 	    error(_g("expected ^--- in line %d of diff `%s'"), $., $diff);
 	}
         $_ = strip_ts($_);
-        if ($_ eq '/dev/null' or s{^(\./)?[^/]+/}{$destdir/}) {
+        if ($_ eq '/dev/null' or s{^[^/]+/}{$destdir/}) {
             $fn = $_;
+	    error(_g("%s contains an insecure path: %s"), $diff, $_) if m{/\.\./};
         }
 	if (/\.dpkg-orig$/) {
 	    error(_g("diff `%s' patches file with name ending .dpkg-orig"), $diff);
@@ -336,8 +337,9 @@ sub analyze {
 	    error(_g("line after --- isn't as expected in diff `%s' (line %d)"), $diff, $.);
 	}
         $_ = strip_ts($_);
-        if ($_ eq '/dev/null' or s{^(\./)?[^/]+/}{$destdir/}) {
+        if ($_ eq '/dev/null' or s{^[^/]+/}{$destdir/}) {
             $fn2 = $_;
+	    error(_g("%s contains an insecure path: %s"), $diff, $_) if m{/\.\./};
         } else {
             unless (defined $fn) {
                 error(_g("none of the filenames in ---/+++ are relative in diff `%s' (line %d)"),
@@ -363,6 +365,17 @@ sub analyze {
 	if ($dirname =~ s{/[^/]+$}{} && not -d $dirname) {
 	    $dirtocreate{$dirname} = 1;
 	}
+
+	# Sanity check, refuse to patch through a symlink
+	$dirname = $fn;
+	while (1) {
+	    if (-l $dirname) {
+		error(_g("diff %s modifies file %s through a symlink: %s"),
+		      $diff, $fn, $dirname);
+	    }
+	    last unless $dirname =~ s{/[^/]+$}{};
+	}
+
 	if (-e $fn and not -f _) {
 	    error(_g("diff `%s' patches something which is not a plain file"), $diff);
 	}