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fs: WARN when f_count resurrection is attempted

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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@infradead.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20240503201620.work.651-kees@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Kees Cook 2024-05-03 13:16:25 -07:00 committed by Christian Brauner
parent e035af9f6e
commit f6bdc7865e
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1 changed files with 2 additions and 1 deletions
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include/linux/fs.h
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@ -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;
}