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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 2020-03-04 11:48:04 +00:00 committed by Junio C Hamano
parent f1e3df3169
commit 6794898198
4 changed files with 73 additions and 78 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

11
builtin/fmt-merge-msg.c
View File

@ -494,6 +494,7 @@ 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)
@ -502,10 +503,12 @@ 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) &&
!sigc.gpg_output)
strbuf_addstr(&sig, "gpg verification failed.\n");
else
strbuf_addstr(&sig, sigc.gpg_output);
signature_check_clear(&sigc);
if (!tag_number++) {
fmt_tag_signature(&tagbuf, &sig, buf, len);

97
gpg-interface.c
View File

@ -256,6 +256,55 @@ error:
FREE_AND_NULL(sigc->key);
}
static int verify_signed_buffer(const char *payload, size_t payload_size,
const char *signature, size_t signature_size,
struct strbuf *gpg_output,
struct strbuf *gpg_status)
{
struct child_process gpg = CHILD_PROCESS_INIT;
struct gpg_format *fmt;
struct tempfile *temp;
int ret;
struct strbuf buf = STRBUF_INIT;
temp = mks_tempfile_t(".git_vtag_tmpXXXXXX");
if (!temp)
return error_errno(_("could not create temporary file"));
if (write_in_full(temp->fd, signature, signature_size) < 0 ||
close_tempfile_gently(temp) < 0) {
error_errno(_("failed writing detached signature to '%s'"),
temp->filename.buf);
delete_tempfile(&temp);
return -1;
}
fmt = get_format_by_sig(signature);
if (!fmt)
BUG("bad signature '%s'", signature);
argv_array_push(&gpg.args, fmt->program);
argv_array_pushv(&gpg.args, fmt->verify_args);
argv_array_pushl(&gpg.args,
"--status-fd=1",
"--verify", temp->filename.buf, "-",
NULL);
if (!gpg_status)
gpg_status = &buf;
sigchain_push(SIGPIPE, SIG_IGN);
ret = pipe_command(&gpg, payload, payload_size,
gpg_status, 0, gpg_output, 0);
sigchain_pop(SIGPIPE);
delete_tempfile(&temp);
ret |= !strstr(gpg_status->buf, "\n[GNUPG:] GOODSIG ");
strbuf_release(&buf); /* no matter it was used or not */
return ret;
}
int check_signature(const char *payload, size_t plen, const char *signature,
size_t slen, struct signature_check *sigc)
{
@ -418,51 +467,3 @@ int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *sig
return 0;
}
int verify_signed_buffer(const char *payload, size_t payload_size,
const char *signature, size_t signature_size,
struct strbuf *gpg_output, struct strbuf *gpg_status)
{
struct child_process gpg = CHILD_PROCESS_INIT;
struct gpg_format *fmt;
struct tempfile *temp;
int ret;
struct strbuf buf = STRBUF_INIT;
temp = mks_tempfile_t(".git_vtag_tmpXXXXXX");
if (!temp)
return error_errno(_("could not create temporary file"));
if (write_in_full(temp->fd, signature, signature_size) < 0 ||
close_tempfile_gently(temp) < 0) {
error_errno(_("failed writing detached signature to '%s'"),
temp->filename.buf);
delete_tempfile(&temp);
return -1;
}
fmt = get_format_by_sig(signature);
if (!fmt)
BUG("bad signature '%s'", signature);
argv_array_push(&gpg.args, fmt->program);
argv_array_pushv(&gpg.args, fmt->verify_args);
argv_array_pushl(&gpg.args,
"--status-fd=1",
"--verify", temp->filename.buf, "-",
NULL);
if (!gpg_status)
gpg_status = &buf;
sigchain_push(SIGPIPE, SIG_IGN);
ret = pipe_command(&gpg, payload, payload_size,
gpg_status, 0, gpg_output, 0);
sigchain_pop(SIGPIPE);
delete_tempfile(&temp);
ret |= !strstr(gpg_status->buf, "\n[GNUPG:] GOODSIG ");
strbuf_release(&buf); /* no matter it was used or not */
return ret;
}

9
gpg-interface.h
View File

@ -54,15 +54,6 @@ size_t parse_signature(const char *buf, size_t size);
int sign_buffer(struct strbuf *buffer, struct strbuf *signature,
const char *signing_key);
/*
* Run "gpg" to see if the payload matches the detached signature.
* gpg_output, when set, receives the diagnostic output from GPG.
* gpg_status, when set, receives the status output from GPG.
*/
int verify_signed_buffer(const char *payload, size_t payload_size,
const char *signature, size_t signature_size,
struct strbuf *gpg_output, struct strbuf *gpg_status);
int git_gpg_config(const char *, const char *, void *);
void set_signing_key(const char *);
const char *get_signing_key(void);

34
log-tree.c
View File

@ -449,22 +449,21 @@ static void show_signature(struct rev_info *opt, struct commit *commit)
{
struct strbuf payload = STRBUF_INIT;
struct strbuf signature = STRBUF_INIT;
struct strbuf gpg_output = STRBUF_INIT;
struct signature_check sigc = { 0 };
int status;
if (parse_signed_commit(commit, &payload, &signature) <= 0)
goto out;
status = verify_signed_buffer(payload.buf, payload.len,
signature.buf, signature.len,
&gpg_output, NULL);
if (status && !gpg_output.len)
strbuf_addstr(&gpg_output, "No signature\n");
show_sig_lines(opt, status, gpg_output.buf);
status = check_signature(payload.buf, payload.len, signature.buf,
signature.len, &sigc);
if (status && !sigc.gpg_output)
show_sig_lines(opt, status, "No signature\n");
else
show_sig_lines(opt, status, sigc.gpg_output);
signature_check_clear(&sigc);
out:
strbuf_release(&gpg_output);
strbuf_release(&payload);
strbuf_release(&signature);
}
@ -497,8 +496,9 @@ static int show_one_mergetag(struct commit *commit,
struct object_id oid;
struct tag *tag;
struct strbuf verify_message;
struct signature_check sigc = { 0 };
int status, nth;
size_t payload_size, gpg_message_offset;
size_t payload_size;
hash_object_file(the_hash_algo, extra->value, extra->len,
type_name(OBJ_TAG), &oid);
@ -520,19 +520,19 @@ static int show_one_mergetag(struct commit *commit,
else
strbuf_addf(&verify_message,
"parent #%d, tagged '%s'\n", nth + 1, tag->tag);
gpg_message_offset = verify_message.len;
payload_size = parse_signature(extra->value, extra->len);
status = -1;
if (extra->len > payload_size) {
/* could have a good signature */
if (!verify_signed_buffer(extra->value, payload_size,
extra->value + payload_size,
extra->len - payload_size,
&verify_message, NULL))
status = 0; /* good */
else if (verify_message.len <= gpg_message_offset)
status = check_signature(extra->value, payload_size,
extra->value + payload_size,
extra->len - payload_size, &sigc);
if (sigc.gpg_output)
strbuf_addstr(&verify_message, sigc.gpg_output);
else
strbuf_addstr(&verify_message, "No signature\n");
signature_check_clear(&sigc);
/* otherwise we couldn't verify, which is shown as bad */
}