[PATCH] fs: WARN when f_count resurrection is attempted

[Kees Cook]

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@xxxxxxxxx/ [1]
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
---
 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