linux-kernel - [PATCH 1/5] fs: Do not allow get_file() to resurrect 0 f

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Message-Id: <20240502223341.1835070-1-keescook@chromium.org>
Date: Thu,  2 May 2024 15:33:36 -0700
From: Kees Cook <keescook@...omium.org>
To: Christian Brauner <brauner@...nel.org>
Cc: Kees Cook <keescook@...omium.org>,
	Alexander 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: [PATCH 1/5] fs: Do not allow get_file() to resurrect 0 f_count

If f_count reaches 0, calling get_file() should be a failure. Adjust to
use atomic_long_inc_not_zero() and return NULL on failure. In the future
get_file() can be annotated with __must_check, though that is not
currently possible.

Signed-off-by: Kees Cook <keescook@...omium.org>
---
Cc: Christian Brauner <brauner@...nel.org>
Cc: Alexander Viro <viro@...iv.linux.org.uk>
Cc: Jan Kara <jack@...e.cz>
Cc: linux-fsdevel@...r.kernel.org
---
 include/linux/fs.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 00fc429b0af0..210bbbfe9b83 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1038,7 +1038,8 @@ struct file_handle {
 
 static inline struct file *get_file(struct file *f)
 {
-	atomic_long_inc(&f->f_count);
+	if (unlikely(!atomic_long_inc_not_zero(&f->f_count)))
+		return NULL;
 	return f;
 }
 
-- 
2.34.1