From patchwork Mon Aug 2 13:44:12 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Greg Kroah-Hartman X-Patchwork-Id: 490827 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-19.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER, INCLUDES_PATCH, MAILING_LIST_MULTI, SPF_HELO_NONE, SPF_PASS, URIBL_BLOCKED, USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0B5E4C4338F for ; Mon, 2 Aug 2021 13:46:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id ECA8E60FF2 for ; Mon, 2 Aug 2021 13:46:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234165AbhHBNq3 (ORCPT ); Mon, 2 Aug 2021 09:46:29 -0400 Received: from mail.kernel.org ([198.145.29.99]:55918 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233981AbhHBNqW (ORCPT ); Mon, 2 Aug 2021 09:46:22 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 9EBC160555; Mon, 2 Aug 2021 13:46:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1627911972; bh=onN+AG9ioMbM04VJN/xrnWqij2LxuOIGiSndtJGtRuo=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=mXqrwsWs2ukx0+zfBYx3tm/qTLK5SCL26U93gu1kQvCIZmBpYeUKajHCJIdkDz6X2 RwiXRDvlFoqc33X9tOfL22BZMxL4YrohM6PU7VN0Oh1CR0gl6NrLVs/sKa+ZqQMpeL bu6eSEF8m44PN/z4LEXzaosPT3J4GumvrCbk2zOk= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Miklos Szeredi , Linus Torvalds Subject: [PATCH 4.4 02/26] af_unix: fix garbage collect vs MSG_PEEK Date: Mon, 2 Aug 2021 15:44:12 +0200 Message-Id: <20210802134332.110702961@linuxfoundation.org> X-Mailer: git-send-email 2.32.0 In-Reply-To: <20210802134332.033552261@linuxfoundation.org> References: <20210802134332.033552261@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: Miklos Szeredi commit cbcf01128d0a92e131bd09f1688fe032480b65ca upstream. unix_gc() assumes that candidate sockets can never gain an external reference (i.e. be installed into an fd) while the unix_gc_lock is held. Except for MSG_PEEK this is guaranteed by modifying inflight count under the unix_gc_lock. MSG_PEEK does not touch any variable protected by unix_gc_lock (file count is not), yet it needs to be serialized with garbage collection. Do this by locking/unlocking unix_gc_lock: 1) increment file count 2) lock/unlock barrier to make sure incremented file count is visible to garbage collection 3) install file into fd This is a lock barrier (unlike smp_mb()) that ensures that garbage collection is run completely before or completely after the barrier. Cc: Signed-off-by: Miklos Szeredi Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- net/unix/af_unix.c | 51 +++++++++++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 49 insertions(+), 2 deletions(-) --- a/net/unix/af_unix.c +++ b/net/unix/af_unix.c @@ -1506,6 +1506,53 @@ out: return err; } +static void unix_peek_fds(struct scm_cookie *scm, struct sk_buff *skb) +{ + scm->fp = scm_fp_dup(UNIXCB(skb).fp); + + /* + * Garbage collection of unix sockets starts by selecting a set of + * candidate sockets which have reference only from being in flight + * (total_refs == inflight_refs). This condition is checked once during + * the candidate collection phase, and candidates are marked as such, so + * that non-candidates can later be ignored. While inflight_refs is + * protected by unix_gc_lock, total_refs (file count) is not, hence this + * is an instantaneous decision. + * + * Once a candidate, however, the socket must not be reinstalled into a + * file descriptor while the garbage collection is in progress. + * + * If the above conditions are met, then the directed graph of + * candidates (*) does not change while unix_gc_lock is held. + * + * Any operations that changes the file count through file descriptors + * (dup, close, sendmsg) does not change the graph since candidates are + * not installed in fds. + * + * Dequeing a candidate via recvmsg would install it into an fd, but + * that takes unix_gc_lock to decrement the inflight count, so it's + * serialized with garbage collection. + * + * MSG_PEEK is special in that it does not change the inflight count, + * yet does install the socket into an fd. The following lock/unlock + * pair is to ensure serialization with garbage collection. It must be + * done between incrementing the file count and installing the file into + * an fd. + * + * If garbage collection starts after the barrier provided by the + * lock/unlock, then it will see the elevated refcount and not mark this + * as a candidate. If a garbage collection is already in progress + * before the file count was incremented, then the lock/unlock pair will + * ensure that garbage collection is finished before progressing to + * installing the fd. + * + * (*) A -> B where B is on the queue of A or B is on the queue of C + * which is on the queue of listening socket A. + */ + spin_lock(&unix_gc_lock); + spin_unlock(&unix_gc_lock); +} + static int unix_scm_to_skb(struct scm_cookie *scm, struct sk_buff *skb, bool send_fds) { int err = 0; @@ -2131,7 +2178,7 @@ static int unix_dgram_recvmsg(struct soc sk_peek_offset_fwd(sk, size); if (UNIXCB(skb).fp) - scm.fp = scm_fp_dup(UNIXCB(skb).fp); + unix_peek_fds(&scm, skb); } err = (flags & MSG_TRUNC) ? skb->len - skip : size; @@ -2376,7 +2423,7 @@ unlock: /* It is questionable, see note in unix_dgram_recvmsg. */ if (UNIXCB(skb).fp) - scm.fp = scm_fp_dup(UNIXCB(skb).fp); + unix_peek_fds(&scm, skb); sk_peek_offset_fwd(sk, chunk);