cifs: Create dedicated keyring for spnego operations

The session key is the default keyring set for request_key operations.
This session key is revoked when the user owning the session logs out.
Any long running daemon processes started by this session ends up with
revoked session keyring which prevents these processes from using the
request_key mechanism from obtaining the krb5 keys.

The problem has been reported by a large number of autofs users. The
problem is also seen with multiuser mounts where the share may be used
by processes run by a user who has since logged out. A reproducer using
automount is available on the Red Hat bz.

The patch creates a new keyring which is used to cache cifs spnego
upcalls.

Red Hat bz: 1267754

Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>
Reported-by: Scott Mayhew <smayhew@redhat.com>
Reviewed-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
CC: Stable <stable@vger.kernel.org>
Signed-off-by: Steve French <smfrench@gmail.com>

3 files changed, 71 insertions, 2 deletions

diff --git a/fs/cifs/cifs_spnego.c b/fs/cifs/cifs_spnego.c
index 6908080e9b6d..b611fc2e8984 100644
--- a/fs/cifs/cifs_spnego.c
+++ b/fs/cifs/cifs_spnego.c
@@ -24,10 +24,13 @@
 #include <linux/string.h>
 #include <keys/user-type.h>
 #include <linux/key-type.h>
+#include <linux/keyctl.h>
 #include <linux/inet.h>
 #include "cifsglob.h"
 #include "cifs_spnego.h"
 #include "cifs_debug.h"
+#include "cifsproto.h"
+static const struct cred *spnego_cred;
 
 /* create a new cifs key */
 static int
@@ -102,6 +105,7 @@ cifs_get_spnego_key(struct cifs_ses *sesInfo)
 	size_t desc_len;
 	struct key *spnego_key;
 	const char *hostname = server->hostname;
+	const struct cred *saved_cred;
 
 	/* length of fields (with semicolons): ver=0xyz ip4=ipaddress
 	   host=hostname sec=mechanism uid=0xFF user=username */
@@ -163,7 +167,9 @@ cifs_get_spnego_key(struct cifs_ses *sesInfo)
 	sprintf(dp, ";pid=0x%x", current->pid);
 
 	cifs_dbg(FYI, "key description = %s\n", description);
+	saved_cred = override_creds(spnego_cred);
 	spnego_key = request_key(&cifs_spnego_key_type, description, "");
+	revert_creds(saved_cred);
 
 #ifdef CONFIG_CIFS_DEBUG2
 	if (cifsFYI && !IS_ERR(spnego_key)) {
@@ -177,3 +183,64 @@ out:
 	kfree(description);
 	return spnego_key;
 }
+
+int
+init_cifs_spnego(void)
+{
+	struct cred *cred;
+	struct key *keyring;
+	int ret;
+
+	cifs_dbg(FYI, "Registering the %s key type\n",
+		 cifs_spnego_key_type.name);
+
+	/*
+	 * Create an override credential set with special thread keyring for
+	 * spnego upcalls.
+	 */
+
+	cred = prepare_kernel_cred(NULL);
+	if (!cred)
+		return -ENOMEM;
+
+	keyring = keyring_alloc(".cifs_spnego",
+				GLOBAL_ROOT_UID, GLOBAL_ROOT_GID, cred,
+				(KEY_POS_ALL & ~KEY_POS_SETATTR) |
+				KEY_USR_VIEW | KEY_USR_READ,
+				KEY_ALLOC_NOT_IN_QUOTA, NULL, NULL);
+	if (IS_ERR(keyring)) {
+		ret = PTR_ERR(keyring);
+		goto failed_put_cred;
+	}
+
+	ret = register_key_type(&cifs_spnego_key_type);
+	if (ret < 0)
+		goto failed_put_key;
+
+	/*
+	 * instruct request_key() to use this special keyring as a cache for
+	 * the results it looks up
+	 */
+	set_bit(KEY_FLAG_ROOT_CAN_CLEAR, &keyring->flags);
+	cred->thread_keyring = keyring;
+	cred->jit_keyring = KEY_REQKEY_DEFL_THREAD_KEYRING;
+	spnego_cred = cred;
+
+	cifs_dbg(FYI, "cifs spnego keyring: %d\n", key_serial(keyring));
+	return 0;
+
+failed_put_key:
+	key_put(keyring);
+failed_put_cred:
+	put_cred(cred);
+	return ret;
+}
+
+void
+exit_cifs_spnego(void)
+{
+	key_revoke(spnego_cred->thread_keyring);
+	unregister_key_type(&cifs_spnego_key_type);
+	put_cred(spnego_cred);
+	cifs_dbg(FYI, "Unregistered %s key type\n", cifs_spnego_key_type.name);
+}
diff --git a/fs/cifs/cifsfs.c b/fs/cifs/cifsfs.c
index 08fa36e5b2bc..67f622df0858 100644
--- a/fs/cifs/cifsfs.c
+++ b/fs/cifs/cifsfs.c
@@ -1303,7 +1303,7 @@ init_cifs(void)
 		goto out_destroy_mids;
 
 #ifdef CONFIG_CIFS_UPCALL
-	rc = register_key_type(&cifs_spnego_key_type);
+	rc = init_cifs_spnego();
 	if (rc)
 		goto out_destroy_request_bufs;
 #endif /* CONFIG_CIFS_UPCALL */
@@ -1326,7 +1326,7 @@ out_init_cifs_idmap:
 out_register_key_type:
 #endif
 #ifdef CONFIG_CIFS_UPCALL
-	unregister_key_type(&cifs_spnego_key_type);
+	exit_cifs_spnego();
 out_destroy_request_bufs:
 #endif
 	cifs_destroy_request_bufs();
diff --git a/fs/cifs/cifsproto.h b/fs/cifs/cifsproto.h
index 0f9a6bc4ba43..1243bd326591 100644
--- a/fs/cifs/cifsproto.h
+++ b/fs/cifs/cifsproto.h
@@ -58,6 +58,8 @@ do {								\
 } while (0)
 extern int init_cifs_idmap(void);
 extern void exit_cifs_idmap(void);
+extern int init_cifs_spnego(void);
+extern void exit_cifs_spnego(void);
 extern char *build_path_from_dentry(struct dentry *);
 extern char *cifs_build_path_to_root(struct smb_vol *vol,
 				     struct cifs_sb_info *cifs_sb,