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

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Date: Fri, 3 May 2024 11:37:25 +0200
From: Christian Brauner <brauner@...nel.org>
To: Kees Cook <keescook@...omium.org>
Cc: 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>, 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 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.