KEYS: restrict /proc/keys by credentials at open time

When checking for permission to view keys whilst reading from
/proc/keys, we should use the credentials with which the /proc/keys file
was opened.  This is because, in a classic type of exploit, it can be
possible to bypass checks for the *current* credentials by passing the
file descriptor to a suid program.

Following commit 34dbbcdb ("Make file credentials available to the
seqfile interfaces") we can finally fix it.  So let's do it.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>

security/keys/proc.c

@@ -187,7 +187,7 @@ static int proc_keys_show(struct seq_file *m, void *v)
 	struct keyring_search_context ctx = {
 		.index_key.type		= key->type,
 		.index_key.description	= key->description,
-		.cred			= current_cred(),
+		.cred			= m->file->f_cred,
 		.match_data.cmp		= lookup_user_key_possessed,
 		.match_data.raw_data	= key,
 		.match_data.lookup_type	= KEYRING_SEARCH_LOOKUP_DIRECT,
@@ -207,11 +207,7 @@ static int proc_keys_show(struct seq_file *m, void *v)
 		}
 	}
 
-	/* check whether the current task is allowed to view the key (assuming
-	 * non-possession)
-	 * - the caller holds a spinlock, and thus the RCU read lock, making our
-	 *   access to __current_cred() safe
-	 */
+	/* check whether the current task is allowed to view the key */
 	rc = key_task_permission(key_ref, ctx.cred, KEY_NEED_VIEW);
 	if (rc < 0)
 		return 0;