From patchwork Thu Apr 16 16:49:06 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Stefan Hajnoczi X-Patchwork-Id: 11493511 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 1F20515AB for ; Thu, 16 Apr 2020 16:50:34 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id E9A822076D for ; Thu, 16 Apr 2020 16:50:33 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="ERNQKaO8" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E9A822076D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+patchwork-qemu-devel=patchwork.kernel.org@nongnu.org Received: from localhost ([::1]:37122 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jP7ij-0008CH-1A for patchwork-qemu-devel@patchwork.kernel.org; Thu, 16 Apr 2020 12:50:33 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:32934) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jP7hg-0006g1-PI for qemu-devel@nongnu.org; Thu, 16 Apr 2020 12:49:29 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1jP7hf-0000IN-MW for qemu-devel@nongnu.org; Thu, 16 Apr 2020 12:49:28 -0400 Received: from us-smtp-2.mimecast.com ([207.211.31.81]:55620 helo=us-smtp-delivery-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1jP7hf-0000Ee-J5 for qemu-devel@nongnu.org; Thu, 16 Apr 2020 12:49:27 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1587055765; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=QZbd1wkLddIQWvgwrrzUlnRVaKwv8Al0ckS8kzN+P3s=; b=ERNQKaO8GdCybNTIzLiBWBniegHwRJcXBIprTI0Eu7OOdGMlhd9HUDSphr1YX+95INZ8m0 V+MSuYjJjLKglHPpgNvrGJX2ExmN+MjyJQKG/irzNa0gokxWmbeJLIF2iDq14msooVLS7U SpCzHqEuS5PzkXg1Jpl3ewVOKA4XIZ8= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-155-QD5d7MkdNyeKHbZO96_yzA-1; Thu, 16 Apr 2020 12:49:23 -0400 X-MC-Unique: QD5d7MkdNyeKHbZO96_yzA-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 8D38B108839B for ; Thu, 16 Apr 2020 16:49:22 +0000 (UTC) Received: from localhost (ovpn-114-223.ams2.redhat.com [10.36.114.223]) by smtp.corp.redhat.com (Postfix) with ESMTP id 930CC5D9E2; Thu, 16 Apr 2020 16:49:16 +0000 (UTC) From: Stefan Hajnoczi To: qemu-devel@nongnu.org Subject: [PATCH 1/2] virtiofsd: only retain file system capabilities Date: Thu, 16 Apr 2020 17:49:06 +0100 Message-Id: <20200416164907.244868-2-stefanha@redhat.com> In-Reply-To: <20200416164907.244868-1-stefanha@redhat.com> References: <20200416164907.244868-1-stefanha@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.81 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: virtio-fs@redhat.com, Stefan Hajnoczi , "Dr. David Alan Gilbert" , Vivek Goyal Errors-To: qemu-devel-bounces+patchwork-qemu-devel=patchwork.kernel.org@nongnu.org Sender: "Qemu-devel" virtiofsd runs as root but only needs a subset of root's Linux capabilities(7). As a file server its purpose is to create and access files on behalf of a client. It needs to be able to access files with arbitrary uid/gid owners. It also needs to be create device nodes. Introduce a Linux capabilities(7) whitelist and drop all capabilities that we don't need, making the virtiofsd process less powerful than a regular uid root process. # cat /proc/PID/status ... Before After CapInh: 0000000000000000 0000000000000000 CapPrm: 0000003fffffffff 00000000880000df CapEff: 0000003fffffffff 00000000880000df CapBnd: 0000003fffffffff 0000000000000000 CapAmb: 0000000000000000 0000000000000000 Note that file capabilities cannot be used to achieve the same effect on the virtiofsd executable because mount is used during sandbox setup. Therefore we drop capabilities programmatically at the right point during startup. This patch only affects the sandboxed child process. The parent process that sits in waitpid(2) still has full root capabilities and will be addressed in the next patch. Signed-off-by: Stefan Hajnoczi Reviewed-by: Dr. David Alan Gilbert --- tools/virtiofsd/passthrough_ll.c | 38 ++++++++++++++++++++++++++++++++ 1 file changed, 38 insertions(+) diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c index 4c35c95b25..af97ba1c41 100644 --- a/tools/virtiofsd/passthrough_ll.c +++ b/tools/virtiofsd/passthrough_ll.c @@ -2695,6 +2695,43 @@ static void setup_mounts(const char *source) close(oldroot); } +/* + * Only keep whitelisted capabilities that are needed for file system operation + */ +static void setup_capabilities(void) +{ + pthread_mutex_lock(&cap.mutex); + capng_restore_state(&cap.saved); + + /* + * Whitelist file system-related capabilities that are needed for a file + * server to act like root. Drop everything else like networking and + * sysadmin capabilities. + * + * Exclusions: + * 1. CAP_LINUX_IMMUTABLE is not included because it's only used via ioctl + * and we don't support that. + * 2. CAP_MAC_OVERRIDE is not included because it only seems to be + * used by the Smack LSM. Omit it until there is demand for it. + */ + capng_setpid(syscall(SYS_gettid)); + capng_clear(CAPNG_SELECT_BOTH); + capng_updatev(CAPNG_ADD, CAPNG_PERMITTED | CAPNG_EFFECTIVE, + CAP_CHOWN, + CAP_DAC_OVERRIDE, + CAP_DAC_READ_SEARCH, + CAP_FOWNER, + CAP_FSETID, + CAP_SETGID, + CAP_SETUID, + CAP_MKNOD, + CAP_SETFCAP); + capng_apply(CAPNG_SELECT_BOTH); + + cap.saved = capng_save_state(); + pthread_mutex_unlock(&cap.mutex); +} + /* * Lock down this process to prevent access to other processes or files outside * source directory. This reduces the impact of arbitrary code execution bugs. @@ -2705,6 +2742,7 @@ static void setup_sandbox(struct lo_data *lo, struct fuse_session *se, setup_namespaces(lo, se); setup_mounts(lo->source); setup_seccomp(enable_syslog); + setup_capabilities(); } /* Raise the maximum number of open file descriptors */