packed-backend: check whether the refname contains NUL characters

"packed-backend.c::next_record" will use "check_refname_format" to check
the consistency of the refname. If it is not OK, the program will die.
However, it is reported in [1], we cannot catch some corruption. But we
already have the code path and we must miss out something.

We use the following code to get the refname:

    strbuf_add(&iter->refname_buf, p, eol - p);
    iter->base.refname = iter->refname_buf.buf

In the above code, `p` is the start pointer of the refname and `eol` is
the next newline pointer. We calculate the length of the refname by
subtracting the two pointers. Then we add the memory range between `p`
and `eol` to get the refname.

However, if there are some NUL characters in the memory range between `p`
and `eol`, we will see the refname as a valid ref name as long as the
memory range between `p` and first occurred NUL character is valid.

In order to catch above corruption, create a new function
"refname_contains_nul" by searching the first NUL character. If it is
not at the end of the string, there must be some NUL characters in the
refname.

Use this function in "next_record" function to die the program if
"refname_contains_nul" returns true.

[1] https://lore.kernel.org/git/[email protected]/

Reported-by: R. Diez <[email protected]>
Mentored-by: Patrick Steinhardt <[email protected]>
Mentored-by: Karthik Nayak <[email protected]>
Signed-off-by: shejialuo <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: shejialuo

refs/packed-backend.c

@@ -494,6 +494,21 @@ static void verify_buffer_safe(struct snapshot *snapshot)
 				 last_line, eof - last_line);
 }
 
+/*
+ * When parsing the "packed-refs" file, we will parse it line by line.
+ * Because we know the start pointer of the refname and the next
+ * newline pointer, we could calculate the length of the refname by
+ * subtracting the two pointers. However, there is a corner case where
+ * the refname contains corrupted embedded NUL characters. And
+ * `check_refname_format()` will not catch this when the truncated
+ * refname is still a valid refname. To prevent this, we need to check
+ * whether the refname contains the NUL characters.
+ */
+static int refname_contains_nul(struct strbuf *refname)
+{
+	return !!memchr(refname->buf, '\0', refname->len);
+}
+
 #define SMALL_FILE_SIZE (32*1024)
 
 /*
@@ -895,6 +910,9 @@ static int next_record(struct packed_ref_iterator *iter)
 	strbuf_add(&iter->refname_buf, p, eol - p);
 	iter->base.refname = iter->refname_buf.buf;
 
+	if (refname_contains_nul(&iter->refname_buf))
+		die("packed refname contains embedded NULL: %s", iter->base.refname);
+
 	if (check_refname_format(iter->base.refname, REFNAME_ALLOW_ONELEVEL)) {
 		if (!refname_is_safe(iter->base.refname))
 			die("packed refname is dangerous: %s",