From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 4C06BC04FFE for ; Thu, 2 May 2024 23:12:53 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 6F47A10E906; Thu, 2 May 2024 23:12:50 +0000 (UTC) Authentication-Results: gabe.freedesktop.org; dkim=fail reason="signature verification failed" (2048-bit key; unprotected) header.d=linux.org.uk header.i=@linux.org.uk header.b="nLUBYPMO"; dkim-atps=neutral Received: from zeniv.linux.org.uk (zeniv.linux.org.uk [62.89.141.173]) by gabe.freedesktop.org (Postfix) with ESMTPS id 2240B10E1A2; Thu, 2 May 2024 23:12:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=linux.org.uk; s=zeniv-20220401; h=Sender:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=O+ovtRI+qjFoRKgW6PbWKtfCDqZHsyNloCpeAE5jAfE=; b=nLUBYPMOxAkFlz7J3+shxAx8uh 9sx8gN2/C/59+EGGOXikIkpjCG85w8YVO5UrIBPF6YZxW18luIXpfopq1JRvQO9oRKMhCPdFbUqCy AG2C+Q/fTic5lf6IymQNB/RBRnQWBiiEbFiUv+p8WPpggILEUV4isEa76TxgHiClmGqQqy+vUV+98 GY2WS1jkLAYEjOUUH1CuxEjTwvyVy6AYdg5Ad/AYDlpxql0gyNAyDNfPyKQDxYI7xb49Y+7QRiubk GJzJusN31jWemm9+PTjKpw09m+qztnOzgZXzOsxUo7cj6ndTvtPQWu/RCxI5dzesVbQ4Z/azyCNWR OtxWt1fg==; Received: from viro by zeniv.linux.org.uk with local (Exim 4.96 #2 (Red Hat Linux)) id 1s2fbI-009qeI-1T; Thu, 02 May 2024 23:12:28 +0000 Date: Fri, 3 May 2024 00:12:28 +0100 From: Al Viro To: Kees Cook Cc: Christian Brauner , Jan Kara , linux-fsdevel@vger.kernel.org, Zack Rusin , Broadcom internal kernel review list , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Daniel Vetter , Jani Nikula , Joonas Lahtinen , Rodrigo Vivi , Tvrtko Ursulin , Andi Shyti , Lucas De Marchi , Matt Atwood , Matthew Auld , Nirmoy Das , Jonathan Cavitt , Will Deacon , Peter Zijlstra , Boqun Feng , Mark Rutland , Kent Overstreet , Masahiro Yamada , Nathan Chancellor , Nicolas Schier , Andrew Morton , linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org, linux-kbuild@vger.kernel.org, linux-hardening@vger.kernel.org Subject: Re: [PATCH 5/5] fs: Convert struct file::f_count to refcount_long_t Message-ID: <20240502231228.GN2118490@ZenIV> References: <20240502222252.work.690-kees@kernel.org> <20240502223341.1835070-5-keescook@chromium.org> <20240502224250.GM2118490@ZenIV> <202405021548.040579B1C@keescook> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <202405021548.040579B1C@keescook> X-BeenThere: intel-gfx@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Intel graphics driver community testing & development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: intel-gfx-bounces@lists.freedesktop.org Sender: "Intel-gfx" On Thu, May 02, 2024 at 03:52:21PM -0700, Kees Cook wrote: > As for semantics, what do you mean? Detecting dec-below-zero means we > catch underflow, and detected inc-from-zero means we catch resurrection > attempts. In both cases we avoid double-free, but we have already lost > to a potential dangling reference to a freed struct file. But just > letting f_count go bad seems dangerous. Detected inc-from-zero can also mean an RCU lookup detecting a descriptor in the middle of getting closed. And it's more subtle than that, actually, thanks to SLAB_TYPESAFE_BY_RCU for struct file.