ipc: don't audit capability check in ipc_permissions()

The IPC sysctls implement the ctl_table_root::permissions hook and
they override the file access mode based on the CAP_CHECKPOINT_RESTORE
capability, which is being checked regardless of whether any access is
actually denied or not, so if an LSM denies the capability, an audit
record may be logged even when access is in fact granted.

It wouldn't be viable to restructure the sysctl permission logic to only
check the capability when the access would be actually denied if it's
not granted. Thus, do the same as in net_ctl_permissions()
(net/sysctl_net.c) - switch from ns_capable() to ns_capable_noaudit(),
so that the check never emits an audit record.

Fixes: 0889f44 ("ipc: Check permissions for checkpoint_restart sysctls at open time")
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Acked-by: Alexey Gladkov <legion@kernel.org>
Acked-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Serge Hallyn <sergeh@kernel.org>

Author: WOnder93

include/linux/capability.h

@@ -203,6 +203,12 @@ static inline bool checkpoint_restore_ns_capable(struct user_namespace *ns)
 		ns_capable(ns, CAP_SYS_ADMIN);
 }
 
+static inline bool checkpoint_restore_ns_capable_noaudit(struct user_namespace *ns)
+{
+	return ns_capable_noaudit(ns, CAP_CHECKPOINT_RESTORE) ||
+		ns_capable_noaudit(ns, CAP_SYS_ADMIN);
+}
+
 /* audit system wants to get cap info from files as well */
 int get_vfs_caps_from_disk(struct mnt_idmap *idmap,
 			   const struct dentry *dentry,

ipc/ipc_sysctl.c

@@ -214,7 +214,7 @@ static int ipc_permissions(struct ctl_table_header *head, const struct ctl_table
 	if (((table->data == &ns->ids[IPC_SEM_IDS].next_id) ||
 	     (table->data == &ns->ids[IPC_MSG_IDS].next_id) ||
 	     (table->data == &ns->ids[IPC_SHM_IDS].next_id)) &&
-	    checkpoint_restore_ns_capable(ns->user_ns))
+	    checkpoint_restore_ns_capable_noaudit(ns->user_ns))
 		mode = 0666;
 	else
 #endif