[PATCH] fs: WARN when f_count resurrection is attempted

[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@gmail.com/ [1]
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
Cc: Christian Brauner <brauner@kernel.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jann Horn <jannh@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: linux-fsdevel@vger.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

Re: [PATCH] fs: WARN when f_count resurrection is attempted

[Christian Brauner]

On Fri, 03 May 2024 13:16:25 -0700, Kees Cook wrote:
> 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.
> 
> [...]

Applied to the vfs.misc branch of the vfs/vfs.git tree.
Patches in the vfs.misc branch should appear in linux-next soon.

Please report any outstanding bugs that were missed during review in a
new review to the original patch series allowing us to drop it.

It's encouraged to provide Acked-bys and Reviewed-bys even though the
patch has now been applied. If possible patch trailers will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. If in doubt, please check the listed branch.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
branch: vfs.misc

[1/1] fs: WARN when f_count resurrection is attempted
      https://git.kernel.org/vfs/vfs/c/f6bdc7865ef4

Re: [PATCH] fs: WARN when f_count resurrection is attempted

[Peter Zijlstra]

On Fri, May 03, 2024 at 01:16:25PM -0700, Kees Cook wrote:
> 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>
> ---
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: Jann Horn <jannh@google.com>
> Cc: Jan Kara <jack@suse.cz>
> Cc: linux-fsdevel@vger.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");

This reminds me, I should some day try and fix the horrible code-gen for
WARN() :/ WARN_ON_*() and friends turn into a single trap instruction,
but the WARN() and friends thing turns into a horrible piece of crap for
the printk().