linux-hardening - [PATCH] fs: WARN when f_count resurrection is attempted

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Date: Fri,  3 May 2024 13:16:25 -0700
From: Kees Cook <keescook@...omium.org>
To: Christian Brauner <brauner@...nel.org>
Cc: Kees Cook <keescook@...omium.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Jens Axboe <axboe@...nel.dk>,
	Jann Horn <jannh@...gle.com>,
	Jan Kara <jack@...e.cz>,
	linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: [PATCH] fs: WARN when f_count resurrection is attempted

It should never happen that get_file() is called on a file with
f_count equal to zero. If this happens, a use-after-free condition
has happened[1], and we need to attempt a best-effort reporting of
the situation to help find the root cause more easily. Additionally,
this serves as a data corruption indicator that system owners using
warn_limit or panic_on_warn would like to have detected.

Link: https://lore.kernel.org/lkml/7c41cf3c-2a71-4dbb-8f34-0337890906fc@gmail.com/ [1]
Suggested-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Kees Cook <keescook@...omium.org>
---
Cc: Christian Brauner <brauner@...nel.org>
Cc: Alexander Viro <viro@...iv.linux.org.uk>
Cc: Jens Axboe <axboe@...nel.dk>
Cc: Jann Horn <jannh@...gle.com>
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..fa9ea5390f33 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);
+	long prior = atomic_long_fetch_inc_relaxed(&f->f_count);
+	WARN_ONCE(!prior, "struct file::f_count incremented from zero; use-after-free condition present!\n");
 	return f;
 }
 
-- 
2.34.1