[PATCH 2/3] cifs.upcall: namespace-aware key authority

spnego key is allocated before switching namespaces (i.e. as root/host
credentials), but instantiation is done only later.

If we have successfully switched user namespaces, key instantiation
returns EPERM because credentials have changed.

Call keyctl_assume_authority() after switching NS/UID/GID so key is
instantiated with correct creds.

Signed-off-by: Enzo Matsumiya <[email protected]>
---
 cifs.upcall.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/cifs.upcall.c b/cifs.upcall.c
index 8a207544cf48..3f56c69cae28 100644
--- a/cifs.upcall.c
+++ b/cifs.upcall.c
@@ -1539,6 +1539,7 @@ int main(const int argc, char *const argv[])
 	krb5_ccache ccache = NULL;
 	unsigned expire_time = DNS_RESOLVER_DEFAULT_TIMEOUT;
 	const char *key_descr = NULL;
+	bool same_ns = true;
 
 	hostbuf[0] = '\0';
 
@@ -1688,6 +1689,7 @@ int main(const int argc, char *const argv[])
 	if (arg->upcall_target == UPTARGET_APP || arg->upcall_target == UPTARGET_UNSPECIFIED) {
 		syslog(LOG_INFO, "upcall_target=app, switching namespaces to application thread");
 		arg->upcall_target = UPTARGET_APP;
+		same_ns = in_same_user_ns(getpid(), arg->pid);
 		rc = switch_to_process_ns(arg->pid);
 		if (rc == -1) {
 			syslog(LOG_ERR, "unable to switch to process namespace: %s", strerror(errno));
@@ -1756,6 +1758,27 @@ int main(const int argc, char *const argv[])
 	if (rc)
 		goto out;
 
+	/*
+	 * Keyring keys' credentials are per user NS.  If we switched user namespaces, we need to
+	 * re-acquire authority (with correct UID/GID) over it so we're able to instantiate it
+	 * later (in setup_key()).
+	 */
+	if (!same_ns) {
+		errno = 0;
+		rc = keyctl_assume_authority(key);
+
+		/*
+		 * EINPROGRESS and ENOENT are non-fatal in most cases.
+		 * keyctl_instantiate() (in setup_key()) will fail if something was really
+		 * unexpected.
+		 */
+		if (rc < 0 && errno != EINPROGRESS && errno != ENOENT) {
+			syslog(LOG_ERR, "keyctl_assume_authority: %s", strerror(errno));
+			rc = 1;
+			goto out;
+		}
+	}
+
 	rc = krb5_init_context(&context);
 	if (rc) {
 		syslog(LOG_ERR, "unable to init krb5 context: %ld", rc);
-- 
2.54.0