Received-SPF: pass (google.com: domain of linux-kernel+bounces-167088-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
Date: Thu, 2 May 2024 17:10:18 -0700
From: Kees Cook <keescook@chromium.org>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>, Jan Kara <jack@suse.cz>,
	linux-fsdevel@vger.kernel.org, Zack Rusin <zack.rusin@broadcom.com>,
	Broadcom internal kernel review list <bcm-kernel-feedback-list@broadcom.com>,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Maxime Ripard <mripard@kernel.org>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	David Airlie <airlied@gmail.com>, Daniel Vetter <daniel@ffwll.ch>,
	Jani Nikula <jani.nikula@linux.intel.com>,
	Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
	Rodrigo Vivi <rodrigo.vivi@intel.com>,
	Tvrtko Ursulin <tursulin@ursulin.net>,
	Andi Shyti <andi.shyti@linux.intel.com>,
	Lucas De Marchi <lucas.demarchi@intel.com>,
	Matt Atwood <matthew.s.atwood@intel.com>,
	Matthew Auld <matthew.auld@intel.com>,
	Nirmoy Das <nirmoy.das@intel.com>,
	Jonathan Cavitt <jonathan.cavitt@intel.com>,
	Will Deacon <will@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Boqun Feng <boqun.feng@gmail.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Kent Overstreet <kent.overstreet@linux.dev>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Nicolas Schier <nicolas@fjasle.eu>,
	Andrew Morton <akpm@linux-foundation.org>,
	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: <202405021708.267B02842@keescook>
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>
 <20240502234152.GP2118490@ZenIV>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20240502234152.GP2118490@ZenIV>

On Fri, May 03, 2024 at 12:41:52AM +0100, Al Viro wrote:
> 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.

I think that was also fixed for refcount_t. I'll need to go dig out the
commit...

But anyway, there needs to be a general "oops I hit 0"-aware form of
get_file(), and it seems like it should just be get_file() itself...

-- 
Kees Cook