[PATCH v2] credential/libsecret: load secrets explicitly

["Daniel Martí via GitGitGadget" <gitgitgadget@gmail.com>]

From: =?UTF-8?q?Daniel=20Mart=C3=AD?= <mvdan@mvdan.cc>

keyring_get() searches with SECRET_SEARCH_LOAD_SECRETS, then passes
secret_item_get_secret() of the first match unchecked to
secret_value_get_text() and secret_value_unref(). As libsecret
documents, that secret can be NULL: the search does not load secrets
of locked items, such as when SECRET_SEARCH_UNLOCK fails to unlock
them, and it ignores errors from loading secrets. The GNOME keyring
daemon also silently leaves out of its reply any item which is locked
or which was deleted after the search matched it, e.g. by a concurrent
"credential erase" from another git process. We then print

    secret_value_get_text: assertion 'value' failed
    secret_value_unref: assertion 'value != NULL' failed

before git falls back to prompting for the password.

We could keep the flag and load the secret explicitly only when it is
NULL, but SECRET_SEARCH_LOAD_SECRETS is not part of the search call:
libsecret implements it as a separate GetSecrets D-Bus call after
SearchItems. Drop the flag and instead always load the one secret we
use with secret_item_load_secret_sync(), which reports errors. This
takes as many D-Bus calls as before, and leaves a single code path
that runs every time, rather than a fallback that only runs in a rare
race. libsecret's own secret-tool also loads each secret explicitly
after searching.

An inaccessible item now produces a useful error message instead of
the assertion failures, and git still falls back to prompting. The
race needs a concurrent process or a locked keyring to trigger, so
there is no test.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
---
    credential/libsecret: load secrets explicitly

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-2372%2Fmvdan%2Flibsecret-null-secret-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-2372/mvdan/libsecret-null-secret-v2
Pull-Request: https://github.com/git/git/pull/2372

Range-diff vs v1:

 1:  89d3eee91f ! 1:  b9ddf13fa1 credential/libsecret: load secrets explicitly
     @@ Metadata
       ## Commit message ##
          credential/libsecret: load secrets explicitly
      
     -    secret_service_search_sync() can return an item whose secret is not
     -    loaded, despite SECRET_SEARCH_LOAD_SECRETS being set: the search
     -    silently discards secret-loading failures, and the GNOME keyring
     -    daemon silently omits from its GetSecrets reply any item that is
     -    locked or that was deleted after the search matched it, e.g. by a
     -    concurrent "credential erase" from another git process.
     -
     -    secret_item_get_secret() then returns NULL, which we pass unchecked
     -    to secret_value_get_text() and secret_value_unref(), producing
     +    keyring_get() searches with SECRET_SEARCH_LOAD_SECRETS, then passes
     +    secret_item_get_secret() of the first match unchecked to
     +    secret_value_get_text() and secret_value_unref(). As libsecret
     +    documents, that secret can be NULL: the search does not load secrets
     +    of locked items, such as when SECRET_SEARCH_UNLOCK fails to unlock
     +    them, and it ignores errors from loading secrets. The GNOME keyring
     +    daemon also silently leaves out of its reply any item which is locked
     +    or which was deleted after the search matched it, e.g. by a concurrent
     +    "credential erase" from another git process. We then print
      
              secret_value_get_text: assertion 'value' failed
              secret_value_unref: assertion 'value != NULL' failed
      
     -    and losing the password even when the secret is still retrievable.
     +    before git falls back to prompting for the password.
     +
     +    We could keep the flag and load the secret explicitly only when it is
     +    NULL, but SECRET_SEARCH_LOAD_SECRETS is not part of the search call:
     +    libsecret implements it as a separate GetSecrets D-Bus call after
     +    SearchItems. Drop the flag and instead always load the one secret we
     +    use with secret_item_load_secret_sync(), which reports errors. This
     +    takes as many D-Bus calls as before, and leaves a single code path
     +    that runs every time, rather than a fallback that only runs in a rare
     +    race. libsecret's own secret-tool also loads each secret explicitly
     +    after searching.
      
     -    Drop SECRET_SEARCH_LOAD_SECRETS and instead load the secret of the
     -    one item we use with secret_item_load_secret_sync(), which does
     -    report errors. A secret the search would have silently dropped is
     -    now retrieved normally, and a genuinely inaccessible item produces
     -    a useful message instead of assertion spew, with git falling back
     -    to prompting either way. Merely guarding against NULL would avoid
     -    the assertions, but would forfeit a secret that is still available.
     -    The cost is unchanged: the search no longer batch-fetches the
     -    secrets of all matching items, and the explicit load fetches the
     -    one we use.
     +    An inaccessible item now produces a useful error message instead of
     +    the assertion failures, and git still falls back to prompting. The
     +    race needs a concurrent process or a locked keyring to trigger, so
     +    there is no test.
      
          Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
      
     @@ contrib/credential/libsecret/git-credential-libsecret.c: static int keyring_get(
      +
      +		/*
      +		 * Load the secret explicitly rather than via
     -+		 * SECRET_SEARCH_LOAD_SECRETS, which silently discards load
     -+		 * failures and returns items whose secret is NULL.
     ++		 * SECRET_SEARCH_LOAD_SECRETS, which skips locked items and
     ++		 * ignores load failures, leaving the secret NULL.
      +		 */
      +		if (!secret_item_load_secret_sync(item, NULL, &error)) {
      +			g_critical("could not load secret: %s", error->message);


 .../libsecret/git-credential-libsecret.c           | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/contrib/credential/libsecret/git-credential-libsecret.c b/contrib/credential/libsecret/git-credential-libsecret.c
index 941b2afd5e..ad4f60e4d7 100644
--- a/contrib/credential/libsecret/git-credential-libsecret.c
+++ b/contrib/credential/libsecret/git-credential-libsecret.c
@@ -126,7 +126,7 @@ static int keyring_get(struct credential *c)
 	items = secret_service_search_sync(service,
 					   &schema,
 					   attributes,
-					   SECRET_SEARCH_LOAD_SECRETS | SECRET_SEARCH_UNLOCK,
+					   SECRET_SEARCH_UNLOCK,
 					   NULL,
 					   &error);
 	g_hash_table_unref(attributes);
@@ -143,6 +143,18 @@ static int keyring_get(struct credential *c)
 		gchar **parts;
 
 		item = items->data;
+
+		/*
+		 * Load the secret explicitly rather than via
+		 * SECRET_SEARCH_LOAD_SECRETS, which skips locked items and
+		 * ignores load failures, leaving the secret NULL.
+		 */
+		if (!secret_item_load_secret_sync(item, NULL, &error)) {
+			g_critical("could not load secret: %s", error->message);
+			g_error_free(error);
+			g_list_free_full(items, g_object_unref);
+			return EXIT_FAILURE;
+		}
 		secret = secret_item_get_secret(item);
 		attributes = secret_item_get_attributes(item);
 

base-commit: 34f06850c16c7f7ac822b1adc71354f11b0f2ca3
-- 
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