From 9b3c1867c8fb53c73cdad85dc73578261df481af Mon Sep 17 00:00:00 2001
From: Simon McVittie <smcv@debian.org>
Date: Mon, 26 Dec 2016 18:45:02 +0000
Subject: [PATCH] Try revert operations (on a branch) before approving them

Otherwise, we have a time-of-check/time-of-use vulnerability:
rcs_preprevert previously looked at what changed in the commit we are
reverting, not at what would result from reverting it now. In
particular, if some files were renamed since the commit we are
reverting, a revert of changes that were within the designated
subdirectory and allowed by check_canchange() might now affect
files that are outside the designated subdirectory or disallowed
by check_canchange().

OVE-20161226-0002
---
 IkiWiki/Plugin/git.pm | 85 ++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 80 insertions(+), 5 deletions(-)

diff --git a/IkiWiki/Plugin/git.pm b/IkiWiki/Plugin/git.pm
index 5d7415ffa..021ee726c 100644
--- a/IkiWiki/Plugin/git.pm
+++ b/IkiWiki/Plugin/git.pm
@@ -425,6 +425,16 @@ sub parse_diff_tree ($) {
 	}
 	shift @{ $dt_ref } if $dt_ref->[0] =~ /^$/;
 
+	$ci{details} = [parse_changed_files($dt_ref)];
+
+	return \%ci;
+}
+
+sub parse_changed_files {
+	my $dt_ref = shift;
+
+	my @files;
+
 	# Modified files.
 	while (my $line = shift @{ $dt_ref }) {
 		if ($line =~ m{^
@@ -442,7 +452,7 @@ sub parse_diff_tree ($) {
 			my $status = shift(@tmp);
 
 			if (length $file) {
-				push @{ $ci{'details'} }, {
+				push @files, {
 					'file'      => decode_git_file($file),
 					'sha1_from' => $sha1_from[0],
 					'sha1_to'   => $sha1_to,
@@ -456,7 +466,7 @@ sub parse_diff_tree ($) {
 		last;
 	}
 
-	return \%ci;
+	return @files;
 }
 
 sub git_commit_info ($;$) {
@@ -955,10 +965,14 @@ sub rcs_preprevert ($) {
 	my $rev=shift;
 	my ($sha1) = $rev =~ /^($sha1_pattern)$/; # untaint
 
+	my @undo;      # undo stack for cleanup in case of an error
+
+	ensure_committer();
+
 	# Examine changes from root of git repo, not from any subdir,
 	# in order to see all changes.
 	my ($subdir, $rootdir) = git_find_root();
-	in_git_dir($rootdir, sub {
+	return in_git_dir($rootdir, sub {
 		my @commits=git_commit_info($sha1, 1);
 	
 		if (! @commits) {
@@ -971,7 +985,68 @@ sub rcs_preprevert ($) {
 			error gettext("you are not allowed to revert a merge");
 		}
 
+		# Due to the presence of rename-detection, we cannot actually
+		# see what will happen in a revert without trying it.
+		# But we can guess, which is enough to rule out most changes
+		# that we won't allow reverting.
 		git_parse_changes(1, @commits);
+
+		my $failure;
+		my @ret;
+		# If it looks OK, do it for real, on a branch.
+		eval {
+			IkiWiki::disable_commit_hook();
+			push @undo, sub {
+				IkiWiki::enable_commit_hook();
+			};
+			my $branch = "ikiwiki_revert_${sha1}"; # supposed to be unique
+
+			push @undo, sub {
+				run_or_cry('git', 'branch', '-D', $branch) if $failure;
+			};
+			if (run_or_non('git', 'rev-parse', '--quiet', '--verify', $branch)) {
+				run_or_non('git', 'branch', '-D', $branch);
+			}
+			run_or_die('git', 'branch', $branch, $config{gitmaster_branch});
+
+			push @undo, sub {
+				if (!run_or_cry('git', 'checkout', '--quiet', $config{gitmaster_branch})) {
+					run_or_cry('git', 'checkout','-f', '--quiet', $config{gitmaster_branch});
+				}
+			};
+			run_or_die('git', 'checkout', '--quiet', $branch);
+
+			run_or_die('git', 'revert', '--no-commit', $sha1);
+			run_or_non('git', 'commit', '-m', "revert $sha1", '-a');
+
+			# Re-switch to master.
+			run_or_die('git', 'checkout', '--quiet', $config{gitmaster_branch});
+
+			my @raw_lines;
+			@raw_lines = run_or_die('git', 'diff', '--pretty=raw',
+				'--raw', '--abbrev=40', '--always', '--no-renames',
+				"ikiwiki_revert_${sha1}..");
+
+			my $ci = {
+				details => [parse_changed_files(\@raw_lines)],
+			};
+
+			@ret = git_parse_changes(0, $ci);
+		};
+		$failure = $@;
+
+		# Process undo stack (in reverse order).  By policy cleanup
+		# actions should normally print a warning on failure.
+		while (my $handle = pop @undo) {
+			$handle->();
+		}
+
+		if ($failure) {
+			my $message = sprintf(gettext("Failed to revert commit %s"), $sha1);
+			error("$message\n$failure\n");
+		}
+
+		return @ret;
 	});
 }
 
@@ -982,11 +1057,11 @@ sub rcs_revert ($) {
 
 	ensure_committer();
 
-	if (run_or_non('git', 'revert', '--no-commit', $sha1)) {
+	if (run_or_non('git', 'merge', '--ff-only', "ikiwiki_revert_$sha1")) {
 		return undef;
 	}
 	else {
-		run_or_die('git', 'reset', '--hard');
+		run_or_non('git', 'branch', '-D', "ikiwiki_revert_$sha1");
 		return sprintf(gettext("Failed to revert commit %s"), $sha1);
 	}
 }
-- 
2.39.5