blame: harden ignore-revs parser and tag peeling

Currently, blame.ignoreRevsFile and --ignore-revs-file only read paths
explicitly configured by the user, which cannot point to an
attacker-controlled file under Git's threat model. An upcoming commit
will teach git-blame(1) and git-annotate(1) to automatically read the
HEAD:.git-blame-ignore-revs blob by default if it exists, exposing the
ignore-revs parser (oidset_parse_file_carefully() in oidset.c) and its
tag-peeling callback (peel_to_commit_oid() in builtin/blame.c) to
upstream-controlled content at a well-known path.

Harden both code paths before enabling the default blob:

- In oidset_parse_file_carefully(), strbuf_getline() reads up to the
  next newline and records the full line length in sb.len, including
  any embedded NUL bytes. However, strchr(sb.buf, '#') and
  parse_oid_hex_algop(sb.buf, &oid, &p, algop) treat sb.buf as a
  NUL-terminated string. If a line contains an embedded NUL byte after
  a valid object name (such as "<oid>\0garbage" or "<oid>\0# comment"),
  *p is '\0' and trailing bytes on the line are silently ignored.
  Reject any line containing an embedded NUL byte via memchr() before
  stripping comments and whitespace.
- In peel_to_commit_oid(), odb_read_object_info() is called without
  OBJECT_INFO_SKIP_FETCH_OBJECT or OBJECT_INFO_QUICK, and deref_tag()
  calls parse_object() on tag targets without checking whether the
  target object exists locally first. In a partial clone, any missing
  commit OID or tag target listed in the ignore-revs file would trigger
  lazy promisor fetches and pack directory rescans during git-blame(1).
  Use odb_read_object_info_extended() with OBJECT_INFO_LOOKUP_REPLACE |
  OBJECT_INFO_SKIP_FETCH_OBJECT | OBJECT_INFO_QUICK and peel OBJ_TAG
  objects one layer per iteration, verifying that each target object
  exists locally and matches the tag's declared type before parsing it.

Signed-off-by: Ravi Mistry <rmistry@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/blame.c

@@ -911,21 +911,37 @@ static int is_a_rev(const char *name)
 static int peel_to_commit_oid(struct object_id *oid_ret, void *cbdata)
 {
 	struct repository *r = ((struct blame_scoreboard *)cbdata)->repo;
+	enum object_type expected_type = OBJ_ANY;
 	struct object_id oid;
 
 	oidcpy(&oid, oid_ret);
 	while (1) {
+		unsigned flags = OBJECT_INFO_LOOKUP_REPLACE |
+				 OBJECT_INFO_SKIP_FETCH_OBJECT |
+				 OBJECT_INFO_QUICK;
+		struct object_info oi = OBJECT_INFO_INIT;
+		enum object_type kind;
 		struct object *obj;
-		int kind = odb_read_object_info(r->objects, &oid, NULL);
+
+		oi.typep = &kind;
+		if (odb_read_object_info_extended(r->objects, &oid, &oi,
+						  flags) < 0)
+			return -1;
+		if (expected_type != OBJ_ANY && kind != expected_type)
+			return -1;
 		if (kind == OBJ_COMMIT) {
 			oidcpy(oid_ret, &oid);
 			return 0;
 		}
 		if (kind != OBJ_TAG)
 			return -1;
-		obj = deref_tag(r, parse_object(r, &oid), NULL, 0);
+		obj = parse_object(r, &oid);
+		if (!obj || obj->type != OBJ_TAG)
+			return -1;
+		obj = ((struct tag *)obj)->tagged;
 		if (!obj)
 			return -1;
+		expected_type = obj->type;
 		oidcpy(&oid, &obj->oid);
 	}
 }

oidset.c

@@ -85,6 +85,9 @@ void oidset_parse_file_carefully(struct oidset *set, const char *path,
 		const char *p;
 		const char *name;
 
+		if (memchr(sb.buf, '\0', sb.len))
+			die("invalid object name: %s", sb.buf);
+
 		/*
 		 * Allow trailing comments, leading whitespace
 		 * (including before commits), and empty or whitespace

t/t8013-blame-ignore-revs.sh

@@ -327,4 +327,42 @@ test_expect_success ignore_merge '
 	test_cmp expect actual
 '
 
+test_expect_success 'ignore-revs-file rejects lines with embedded NUL bytes' '
+	rev_b=$(git rev-parse B) &&
+	printf "%sQgarbage\n" "$rev_b" | q_to_nul >ignore_nul &&
+	test_must_fail git blame file --ignore-revs-file ignore_nul 2>err &&
+	test_grep "invalid object name:" err &&
+
+	printf "%sQ# comment\n" "$rev_b" | q_to_nul >ignore_nul_comment &&
+	test_must_fail git blame file --ignore-revs-file ignore_nul_comment 2>err &&
+	test_grep "invalid object name:" err
+'
+
+test_expect_success 'ignore-revs-file peels chained tags and skips missing tag targets' '
+	test_write_lines BB L2-modified L3 L4 L5 L6 L7 L8 CC >file &&
+	git add file &&
+	test_tick &&
+	git commit -m D &&
+	git tag -a -m "tag 1" D_TAG1 HEAD &&
+	git tag -a -m "tag 2" D_TAG2 D_TAG1 &&
+	git rev-parse D_TAG2 >ignore_tag_chain &&
+	git blame --line-porcelain file --ignore-revs-file ignore_tag_chain >blame_raw &&
+	sed -ne "/^[0-9a-f][0-9a-f]* [0-9][0-9]* 2/s/ .*//p" blame_raw >actual &&
+	git rev-parse A >expect &&
+	test_cmp expect actual &&
+
+	test_config extensions.partialClone origin &&
+	test_config remote.origin.promisor true &&
+	test_config remote.origin.url /nonexistent &&
+	missing_oid=$(test_oid deadbeef) &&
+	bad_tag=$(printf "object %s\ntype commit\ntag bad-tag\ntagger T <t@example.com> 0 +0000\n\nmsg\n" "$missing_oid" |
+		git hash-object -t tag -w --stdin) &&
+	test_write_lines "$missing_oid" "$bad_tag" >ignore_bad_tag &&
+	git blame --line-porcelain file --ignore-revs-file ignore_bad_tag >blame_raw 2>err &&
+	test_must_be_empty err &&
+	sed -ne "/^[0-9a-f][0-9a-f]* [0-9][0-9]* 2/s/ .*//p" blame_raw >actual &&
+	git rev-parse HEAD >expect &&
+	test_cmp expect actual
+'
+
 test_done