linux-hardening - Re: [PATCH 5/5] fs: Convert struct file::f_count to refcount_long

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Date: Thu, 2 May 2024 15:52:21 -0700
From: Kees Cook <keescook@...omium.org>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	linux-fsdevel@...r.kernel.org, Zack Rusin <zack.rusin@...adcom.com>,
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
	Jani Nikula <jani.nikula@...ux.intel.com>,
	Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
	Rodrigo Vivi <rodrigo.vivi@...el.com>,
	Tvrtko Ursulin <tursulin@...ulin.net>,
	Andi Shyti <andi.shyti@...ux.intel.com>,
	Lucas De Marchi <lucas.demarchi@...el.com>,
	Matt Atwood <matthew.s.atwood@...el.com>,
	Matthew Auld <matthew.auld@...el.com>,
	Nirmoy Das <nirmoy.das@...el.com>,
	Jonathan Cavitt <jonathan.cavitt@...el.com>,
	Will Deacon <will@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Mark Rutland <mark.rutland@....com>,
	Kent Overstreet <kent.overstreet@...ux.dev>,
	Masahiro Yamada <masahiroy@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nicolas Schier <nicolas@...sle.eu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	intel-gfx@...ts.freedesktop.org, linux-kbuild@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH 5/5] fs: Convert struct file::f_count to refcount_long_t

On Thu, May 02, 2024 at 11:42:50PM +0100, Al Viro wrote:
> On Thu, May 02, 2024 at 03:33:40PM -0700, Kees Cook wrote:
> > Underflow of f_count needs to be more carefully detected than it
> > currently is. The results of get_file() should be checked, but the
> > first step is detection. Redefine f_count from atomic_long_t to
> > refcount_long_t.
> 
> 	It is used on fairly hot paths.  What's more, it's not
> at all obvious what the hell would right semantics be.

I think we've put performance concerns between refcount_t and atomic_t
to rest long ago. If there is a real workload where it's a problem,
let's find it! :)

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.

-- 
Kees Cook