From f6bdc7865ef4a7a4393c90a550c728231ebc853e Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Fri, 3 May 2024 13:16:25 -0700 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 Signed-off-by: Kees Cook Link: https://lore.kernel.org/r/20240503201620.work.651-kees@kernel.org Signed-off-by: Christian Brauner --- 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 e5f643e97cfd..5b351c1e6f58 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -1034,7 +1034,8 @@ 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; }