archive: expand only a single %(describe) per archive

Every %(describe) placeholder in $Format:...$ strings in files with the
attribute export-subst is expanded by calling git describe.  This can
potentially result in a lot of such calls per archive.  That's OK for
local repositories under control of the user of git archive, but could
be a problem for hosted repositories.

Expand only a single %(describe) placeholder per archive for now to
avoid denial-of-service attacks.  We can make this limit configurable
later if needed, but let's start out simple.

Reported-by: Jeff King <peff@peff.net>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/pretty.c b/pretty.c
index c612d2ac9b..032e89cd4e 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1247,6 +1247,14 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 		struct child_process cmd = CHILD_PROCESS_INIT;
 		struct strbuf out = STRBUF_INIT;
 		struct strbuf err = STRBUF_INIT;
+		struct pretty_print_describe_status *describe_status;
+
+		describe_status = c->pretty_ctx->describe_status;
+		if (describe_status) {
+			if (!describe_status->max_invocations)
+				return 0;
+			describe_status->max_invocations--;
+		}
 
 		cmd.git_cmd = 1;
 		strvec_push(&cmd.args, "describe");