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 AB6ADC4345F for ; Thu, 2 May 2024 23:42:19 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 2394E10F297; Thu, 2 May 2024 23:42:15 +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="BNq4ALpK"; 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 22AEF10F233; Thu, 2 May 2024 23:42:12 +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=+jAuGAUMlirWHwpsbSRnjzxGiwK0lb5RSO05/kE9BCw=; b=BNq4ALpKLbJH/KcTZGVVsf3CKJ 6oohbExTsN0l0871Q/7TlyO/laXpsBkWo09az4DGUWLvE3ZnhSutgPPbRrSdmjVD64ociH3mDN1/r gcuF5m3Wd1/GtmS9WdQNT76M7n0pZW4/4mjMhFRhlNHD7QLOPUWnD3I2+RAnpUlOKzb3y7KyOS98x K09oUyMoAR7KSwTEuyYKEXwf8HhZd5wgO8ntlHUFD+3XXf7A8HhJSuBcTLMaabEHDix2ylHnIL6m+ FlrlcoZL1dAfDyMQU82WNe22tv6MCZ5QyTfCry9vkmmNLUa09aNWUKuoG7qNYKGPHCEEaBGvCjE3I mJG5dmsQ==; Received: from viro by zeniv.linux.org.uk with local (Exim 4.96 #2 (Red Hat Linux)) id 1s2g3k-009s3X-2Z; Thu, 02 May 2024 23:41:52 +0000 Date: Fri, 3 May 2024 00:41:52 +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: <20240502234152.GP2118490@ZenIV> References: <20240502222252.work.690-kees@kernel.org> <20240502223341.1835070-5-keescook@chromium.org> <20240502224250.GM2118490@ZenIV> <202405021548.040579B1C@keescook> <20240502231228.GN2118490@ZenIV> <202405021620.C8115568@keescook> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <202405021620.C8115568@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 04:21:13PM -0700, Kees Cook wrote: > On Fri, May 03, 2024 at 12:12:28AM +0100, Al Viro wrote: > > 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. > > But isn't that already handled by __get_file_rcu()? i.e. shouldn't it be > impossible for a simple get_file() to ever see a 0 f_count under normal > conditions? For get_file() it is impossible. The comment about semantics had been about the sane ways to recover if such crap gets detected. __get_file_rcu() is a separate story - consider the comment in there: * atomic_long_inc_not_zero() above provided a full memory * barrier when we acquired a reference. * * This is paired with the write barrier from assigning to the * __rcu protected file pointer so that if that pointer still * matches the current file, we know we have successfully * acquired a reference to the right file. and IIRC, refcount_t is weaker wrt barriers.