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

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Date: Fri, 3 May 2024 12:36:14 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Christian Brauner <brauner@...nel.org>
Cc: Kees Cook <keescook@...omium.org>, Al Viro <viro@...iv.linux.org.uk>,
	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>, 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 Fri, May 03, 2024 at 11:37:25AM +0200, Christian Brauner wrote:
> On Thu, May 02, 2024 at 05:41:23PM -0700, Kees Cook wrote:
> > On Fri, May 03, 2024 at 01:14:45AM +0100, Al Viro wrote:
> > > On Thu, May 02, 2024 at 05:10:18PM -0700, Kees Cook wrote:
> > > 
> > > > 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...
> > > 
> > > ... which brings back the question of what's the sane damage mitigation
> > > for that.  Adding arseloads of never-exercised failure exits is generally
> > > a bad idea - it's asking for bitrot and making the thing harder to review
> > > in future.
> > 
> > Linus seems to prefer best-effort error recovery to sprinkling BUG()s
> > around.  But if that's really the solution, then how about get_file()
> > switching to to use inc_not_zero and BUG on 0?
> 
> Making get_file() return an error is not an option. For all current
> callers that's pointless churn for a condition that's not supposed to
> happen at all.
> 
> Additionally, iirc *_inc_not_zero() variants are implemented with
> try_cmpxchg() which scales poorly under contention for a condition
> that's not supposed to happen.

	unsigned long old = atomic_long_fetch_inc_relaxed(&f->f_count);
	WARN_ON(!old);

Or somesuch might be an option?