Harden gvfs-helper to validate the packfiles in a multipart prefetch response (#571)

Teach `gvfs-helper` to ignore the optional `.idx` files that may be
included in a `prefetch` response and always use `git index-pack` to
create them from the `.pack` files received in the data stream.

This is a little wasteful in terms of client-side compute and of the
network bandwidth, but allows us to use the full packfile verification
code contained within `git index-pack` to ensure that the received
packfiles are valid.

Author: jeffhostetler

gvfs-helper.c

@@ -105,6 +105,11 @@
 //                       The GVFS Protocol defines this value as a way to
 //                       request cached packfiles NEWER THAN this timestamp.
 //
+//                 --max-retries=<n>     // defaults to "6"
+//
+//                       Number of retries after transient network errors.
+//                       Set to zero to disable such retries.
+//
 //     server
 //
 //            Interactive/sub-process mode.  Listen for a series of commands
@@ -2117,7 +2122,6 @@ static void extract_packfile_from_multipack(
 {
 	struct ph ph;
 	struct tempfile *tempfile_pack = NULL;
-	struct tempfile *tempfile_idx = NULL;
 	int result = -1;
 	int b_no_idx_in_multipack;
 	struct object_id packfile_checksum;
@@ -2151,16 +2155,14 @@ static void extract_packfile_from_multipack(
 	b_no_idx_in_multipack = (ph.idx_len == maximum_unsigned_value_of_type(uint64_t) ||
 				 ph.idx_len == 0);
 
-	if (b_no_idx_in_multipack) {
-		my_create_tempfile(status, 0, "pack", &tempfile_pack, NULL, NULL);
-		if (!tempfile_pack)
-			goto done;
-	} else {
-		/* create a pair of tempfiles with the same basename */
-		my_create_tempfile(status, 0, "pack", &tempfile_pack, "idx", &tempfile_idx);
-		if (!tempfile_pack || !tempfile_idx)
-			goto done;
-	}
+	/*
+	 * We are going to harden `gvfs-helper` here and ignore the .idx file
+	 * if it is provided and always compute it locally so that we get the
+	 * added verification that `git index-pack` provides.
+	 */
+	my_create_tempfile(status, 0, "pack", &tempfile_pack, NULL, NULL);
+	if (!tempfile_pack)
+		goto done;
 
 	/*
 	 * Copy the current packfile from the open stream and capture
@@ -2187,38 +2189,31 @@ static void extract_packfile_from_multipack(
 
 	oid_to_hex_r(hex_checksum, &packfile_checksum);
 
-	if (b_no_idx_in_multipack) {
-		/*
-		 * The server did not send the corresponding .idx, so
-		 * we have to compute it ourselves.
-		 */
-		strbuf_addbuf(&temp_path_idx, &temp_path_pack);
-		strbuf_strip_suffix(&temp_path_idx, ".pack");
-		strbuf_addstr(&temp_path_idx, ".idx");
+	/*
+	 * Always compute the .idx file from the .pack file.
+	 */
+	strbuf_addbuf(&temp_path_idx, &temp_path_pack);
+	strbuf_strip_suffix(&temp_path_idx, ".pack");
+	strbuf_addstr(&temp_path_idx, ".idx");
 
-		my_run_index_pack(params, status,
-				  &temp_path_pack, &temp_path_idx,
-				  NULL);
-		if (status->ec != GH__ERROR_CODE__OK)
-			goto done;
+	my_run_index_pack(params, status,
+			  &temp_path_pack, &temp_path_idx,
+			  NULL);
+	if (status->ec != GH__ERROR_CODE__OK)
+		goto done;
 
-	} else {
+	if (!b_no_idx_in_multipack) {
 		/*
 		 * Server sent the .idx immediately after the .pack in the
-		 * data stream.  I'm tempted to verify it, but that defeats
-		 * the purpose of having it cached...
+		 * data stream.  Skip over it.
 		 */
-		if (my_copy_fd_len(fd_multipack, get_tempfile_fd(tempfile_idx),
-				   ph.idx_len) < 0) {
+		if (lseek(fd_multipack, ph.idx_len, SEEK_CUR) < 0) {
 			strbuf_addf(&status->error_message,
-				    "could not extract index[%d] in multipack",
+				    "could not skip index[%d] in multipack",
 				    k);
 			status->ec = GH__ERROR_CODE__COULD_NOT_INSTALL_PREFETCH;
 			goto done;
 		}
-
-		strbuf_addstr(&temp_path_idx, get_tempfile_path(tempfile_idx));
-		close_tempfile_gently(tempfile_idx);
 	}
 
 	strbuf_addf(&buf_timestamp, "%u", (unsigned int)ph.timestamp);
@@ -2233,7 +2228,6 @@ static void extract_packfile_from_multipack(
 
 done:
 	delete_tempfile(&tempfile_pack);
-	delete_tempfile(&tempfile_idx);
 	strbuf_release(&temp_path_pack);
 	strbuf_release(&temp_path_idx);
 	strbuf_release(&final_path_pack);
@@ -3749,6 +3743,8 @@ static enum gh__error_code do_sub_cmd__prefetch(int argc, const char **argv)
 	static const char *since_str;
 	static struct option prefetch_options[] = {
 		OPT_STRING(0, "since", &since_str, N_("since"), N_("seconds since epoch")),
+		OPT_INTEGER('r', "max-retries", &gh__cmd_opts.max_retries,
+			    N_("retries for transient network errors")),
 		OPT_END(),
 	};
 
@@ -3768,6 +3764,8 @@ static enum gh__error_code do_sub_cmd__prefetch(int argc, const char **argv)
 		if (my_parse_since(since_str, &seconds_since_epoch))
 			die("could not parse 'since' field");
 	}
+	if (gh__cmd_opts.max_retries < 0)
+		gh__cmd_opts.max_retries = 0;
 
 	finish_init(1);
 

t/helper/test-gvfs-protocol.c

@@ -1145,6 +1145,82 @@ static int ct_pack_sort_compare(const void *_a, const void *_b)
 	return (a->ph.timestamp < b->ph.timestamp) ? -1 : (a->ph.timestamp != b->ph.timestamp);
 }
 
+#define MY_MIN(a, b) ((a) < (b) ? (a) : (b))
+
+/*
+ * Like copy.c:copy_fd(), but corrupt part of the trailing SHA (if the
+ * given mayhem key is defined) as we copy it to the destination file.
+ *
+ * We don't know (or care) if the input file is a pack file or idx
+ * file, just that the final bytes are part of a SHA that we can
+ * corrupt.
+ */
+static int copy_fd_with_checksum_mayhem(int ifd, int ofd,
+					const char *mayhem_key,
+					ssize_t nr_wrong_bytes)
+{
+	off_t in_cur, in_len;
+	ssize_t bytes_to_copy;
+	ssize_t bytes_remaining_to_copy;
+	char buffer[8192];
+
+	if (!mayhem_key || !*mayhem_key || !nr_wrong_bytes ||
+	    !string_list_has_string(&mayhem_list, mayhem_key))
+		return copy_fd(ifd, ofd);
+
+	in_cur = lseek(ifd, 0, SEEK_CUR);
+	if (in_cur < 0)
+		return in_cur;
+
+	in_len = lseek(ifd, 0, SEEK_END);
+	if (in_len < 0)
+		return in_len;
+
+	if (lseek(ifd, in_cur, SEEK_SET) < 0)
+		return -1;
+
+	/* Copy the entire file except for the last few bytes. */
+
+	bytes_to_copy = (ssize_t)in_len - nr_wrong_bytes;
+	bytes_remaining_to_copy = bytes_to_copy;
+	while (bytes_remaining_to_copy) {
+		ssize_t to_read = MY_MIN(sizeof(buffer), bytes_remaining_to_copy);
+		ssize_t len = xread(ifd, buffer, to_read);
+
+		if (!len)
+			return -1; /* error on unexpected EOF */
+		if (len < 0)
+			return -1;
+		if (write_in_full(ofd, buffer, len) < 0)
+			return -1;
+
+		bytes_remaining_to_copy -= len;
+	}
+
+	/* Read the trailing bytes so that we can alter them before copying. */
+
+	while (nr_wrong_bytes) {
+		ssize_t to_read = MY_MIN(sizeof(buffer), nr_wrong_bytes);
+		ssize_t len = xread(ifd, buffer, to_read);
+		ssize_t k;
+
+		if (!len)
+			return -1; /* error on unexpected EOF */
+		if (len < 0)
+			return -1;
+
+		for (k = 0; k < len; k++)
+			buffer[k] ^= 0xff;
+
+		if (write_in_full(ofd, buffer, len) < 0)
+			return -1;
+
+		nr_wrong_bytes -= len;
+	}
+
+	return 0;
+}
+
 static enum worker_result send_ct_item(const struct ct_pack_item *item)
 {
 	struct ph ph_le;
@@ -1166,7 +1242,8 @@ static enum worker_result send_ct_item(const struct ct_pack_item *item)
 	trace2_printf("%s: sending prefetch pack '%s'", TR2_CAT, item->path_pack.buf);
 
 	fd_pack = git_open_cloexec(item->path_pack.buf, O_RDONLY);
-	if (fd_pack == -1 || copy_fd(fd_pack, 1)) {
+	if (fd_pack == -1 ||
+	    copy_fd_with_checksum_mayhem(fd_pack, 1, "bad_prefetch_pack_sha", 4)) {
 		logerror("could not send packfile");
 		wr = WR_IO_ERROR;
 		goto done;
@@ -1176,7 +1253,8 @@ static enum worker_result send_ct_item(const struct ct_pack_item *item)
 		trace2_printf("%s: sending prefetch idx '%s'", TR2_CAT, item->path_idx.buf);
 
 		fd_idx = git_open_cloexec(item->path_idx.buf, O_RDONLY);
-		if (fd_idx == -1 || copy_fd(fd_idx, 1)) {
+		if (fd_idx == -1 ||
+		    copy_fd_with_checksum_mayhem(fd_idx, 1, "bad_prefetch_idx_sha", 4)) {
 			logerror("could not send idx");
 			wr = WR_IO_ERROR;
 			goto done;

t/t5799-gvfs-helper.sh

@@ -1299,7 +1299,7 @@ test_expect_success 'duplicate and busy: vfs- packfile' '
 # content matches the requested SHA.
 #
 test_expect_success 'catch corrupted loose object' '
-#	test_when_finished "per_test_cleanup" &&
+	test_when_finished "per_test_cleanup" &&
 	start_gvfs_protocol_server_with_mayhem corrupt_loose &&
 
 	test_must_fail \
@@ -1322,4 +1322,67 @@ test_expect_success 'catch corrupted loose object' '
 	git -C "$REPO_T1" fsck
 '
 
+#################################################################
+# Ensure that we can detect when we receive a corrupted packfile
+# from the server.  This is not concerned with network IO errors,
+# but rather cases when the cache or origin server generates or
+# sends an invalid packfile.
+#
+# For example, if the server throws an exception and writes the
+# stack trace to the socket rather than or in addition to the
+# packfile content.
+#
+# Or for example, if the packfile on the server's disk is corrupt
+# and it sends it correctly, but the original data was already
+# garbage, so the client still has garbage (and retrying won't
+# help).
+#################################################################
+
+# Send corrupt PACK files w/o IDX files (so that `gvfs-helper`
+# must use `index-pack` to create it.  (And as a side-effect,
+# validate the PACK file is not corrupt.)
+test_expect_success 'prefetch corrupt pack without idx' '
+	test_when_finished "per_test_cleanup" &&
+	start_gvfs_protocol_server_with_mayhem \
+		bad_prefetch_pack_sha \
+		no_prefetch_idx &&
+
+	test_must_fail \
+		git -C "$REPO_T1" gvfs-helper \
+			--cache-server=disable \
+			--remote=origin \
+			--no-progress \
+			prefetch \
+			--max-retries=0 \
+			--since="1000000000" \
+			>OUT.output 2>OUT.stderr &&
+
+	stop_gvfs_protocol_server &&
+
+	# Verify corruption detected in pack when building
+	# local idx file for it.
+
+	grep -q "error: .* index-pack failed" <OUT.stderr
+'
+
+# Send corrupt PACK files with IDX files.  Since the cache server
+# sends both, `gvfs-helper` might fail to verify both of them.
+test_expect_success 'prefetch corrupt pack with corrupt idx' '
+	test_when_finished "per_test_cleanup" &&
+	start_gvfs_protocol_server_with_mayhem \
+		bad_prefetch_pack_sha &&
+
+	test_must_fail \
+		git -C "$REPO_T1" gvfs-helper \
+			--cache-server=disable \
+			--remote=origin \
+			--no-progress \
+			prefetch \
+			--max-retries=0 \
+			--since="1000000000" \
+			>OUT.output 2>OUT.stderr &&
+
+	stop_gvfs_protocol_server
+'
+
 test_done