KEYS: always initialize keyring_index_key::desc_len

syzbot hit the 'BUG_ON(index_key->desc_len == 0);' in __key_link_begin()
called from construct_alloc_key() during sys_request_key(), because the
length of the key description was never calculated.

The problem is that we rely on ->desc_len being initialized by
search_process_keyrings(), specifically by search_nested_keyrings().
But, if the process isn't subscribed to any keyrings that never happens.

Fix it by always initializing keyring_index_key::desc_len as soon as the
description is set, like we already do in some places.

The following program reproduces the BUG_ON() when it's run as root and
no session keyring has been installed.  If it doesn't work, try removing
pam_keyinit.so from /etc/pam.d/login and rebooting.

    #include <stdlib.h>
    #include <unistd.h>
    #include <keyutils.h>

    int main(void)
    {
            int id = add_key("keyring", "syz", NULL, 0, KEY_SPEC_USER_KEYRING);

            keyctl_setperm(id, KEY_OTH_WRITE);
            setreuid(5000, 5000);
            request_key("user", "desc", "", id);
    }

Reported-by: syzbot+ec24e95ea483de0a24da@syzkaller.appspotmail.com
Fixes: b2a4df20 ("KEYS: Expand the capacity of a keyring")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: James Morris <james.morris@microsoft.com>

security/keys/keyring.c

@@ -661,9 +661,6 @@ static bool search_nested_keyrings(struct key *keyring,
 	BUG_ON((ctx->flags & STATE_CHECKS) == 0 ||
 	       (ctx->flags & STATE_CHECKS) == STATE_CHECKS);
 
-	if (ctx->index_key.description)
-		ctx->index_key.desc_len = strlen(ctx->index_key.description);
-
 	/* Check to see if this top-level keyring is what we are looking for
 	 * and whether it is valid or not.
 	 */
@@ -914,6 +911,7 @@ key_ref_t keyring_search(key_ref_t keyring,
 	struct keyring_search_context ctx = {
 		.index_key.type		= type,
 		.index_key.description	= description,
+		.index_key.desc_len	= strlen(description),
 		.cred			= current_cred(),
 		.match_data.cmp		= key_default_cmp,
 		.match_data.raw_data	= description,

security/keys/proc.c

@@ -165,8 +165,7 @@ static int proc_keys_show(struct seq_file *m, void *v)
 	int rc;
 
 	struct keyring_search_context ctx = {
-		.index_key.type		= key->type,
-		.index_key.description	= key->description,
+		.index_key		= key->index_key,
 		.cred			= m->file->f_cred,
 		.match_data.cmp		= lookup_user_key_possessed,
 		.match_data.raw_data	= key,

security/keys/request_key.c

@@ -531,6 +531,7 @@ struct key *request_key_and_link(struct key_type *type,
 	struct keyring_search_context ctx = {
 		.index_key.type		= type,
 		.index_key.description	= description,
+		.index_key.desc_len	= strlen(description),
 		.cred			= current_cred(),
 		.match_data.cmp		= key_default_cmp,
 		.match_data.raw_data	= description,

security/keys/request_key_auth.c

@@ -247,7 +247,7 @@ struct key *key_get_instantiation_authkey(key_serial_t target_id)
 	struct key *authkey;
 	key_ref_t authkey_ref;
 
-	sprintf(description, "%x", target_id);
+	ctx.index_key.desc_len = sprintf(description, "%x", target_id);
 
 	authkey_ref = search_process_keyrings(&ctx);