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gpg-interface: prefer check_signature() for GPG verification

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This commit refactors the use of verify_signed_buffer() outside of
gpg-interface.c to use check_signature() instead.  It also turns
verify_signed_buffer() into a file-local function since it's now only
invoked internally by check_signature().

There were previously two globally scoped functions used in different
parts of Git to perform GPG signature verification:
verify_signed_buffer() and check_signature().  Now only
check_signature() is used.

The verify_signed_buffer() function doesn't guard against duplicate
signatures as described by Michał Górny [1].  Instead it only ensures a
non-erroneous exit code from GPG and the presence of at least one
GOODSIG status field.  This stands in contrast with check_signature()
that returns an error if more than one signature is encountered.

The lower degree of verification makes the use of verify_signed_buffer()
problematic if callers don't parse and validate the various parts of the
GPG status message themselves.  And processing these messages seems like
a task that should be reserved to gpg-interface.c with the function
check_signature().

Furthermore, the use of verify_signed_buffer() makes it difficult to
introduce new functionality that relies on the content of the GPG status
lines.

Now all operations that does signature verification share a single entry
point to gpg-interface.c.  This makes it easier to propagate changed or
additional functionality in GPG signature verification to all parts of
Git, without having odd edge-cases that don't perform the same degree of
verification.

[1] https://dev.gentoo.org/~mgorny/articles/attack-on-git-signature-verification.html

Signed-off-by: Hans Jerry Illikainen <hji@dyntopia.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Hans Jerry Illikainen
2019-11-27 17:48:21 +00:00
committed by Junio C Hamano
parent 67a6ea6300
commit 72b006f4bf
4 changed files with 72 additions and 75 deletions
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11
builtin/fmt-merge-msg.c
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@ -495,6 +495,7 @@ static void fmt_merge_msg_sigs(struct strbuf *out)
enum object_type type;
unsigned long size, len;
char *buf = read_object_file(oid, &type, &size);
struct signature_check sigc = { 0 };
struct strbuf sig = STRBUF_INIT;
if (!buf || type != OBJ_TAG)
@ -503,10 +504,12 @@ static void fmt_merge_msg_sigs(struct strbuf *out)
if (size == len)
; /* merely annotated */
else if (verify_signed_buffer(buf, len, buf + len, size - len, &sig, NULL)) {
if (!sig.len)
strbuf_addstr(&sig, "gpg verification failed.\n");
}
else if (!check_signature(buf, len, buf + len, size - len,
&sigc)) {
strbuf_addstr(&sig, sigc.gpg_output);
signature_check_clear(&sigc);
} else
strbuf_addstr(&sig, "gpg verification failed.\n");
if (!tag_number++) {
fmt_tag_signature(&tagbuf, &sig, buf, len);