Re: [PATCH v1] test_sysctl: unregister tables before freeing bitmap

[Joel Granados]

On Sun, Sep 20, 2026 at 02:51:10AM +0800, Yibo Tan wrote:
> The table registered for bitmap_0001 retains a pointer to
> test_data.bitmap_0001. unregister_sysctl_table() prevents new handlers
> from starting and waits for active handlers to finish.
> 
> test_sysctl_exit() currently frees the bitmap before unregistering the
> table. A concurrent read during module removal can therefore enter
> proc_do_large_bitmap() while the bitmap is freed. KASAN reports a
> slab-use-after-free in _find_next_bit().
> 
> Unregister all tables before freeing the bitmap so the unregister rundown
> provides the required lifetime boundary.
> 
> The unmodified module reproduced the use-after-free in three KASAN runs.
> With this change, three runs each completed 100 load, read and unload race
> rounds without a kernel diagnostic.
> 
> Fixes: 2ea622b887e7 ("tools/testing/selftests/sysctl/sysctl.sh: add proc_do_large_bitmap() test case")
> Assisted-by: Codex:GPT-5

I'll change this to Assisted-by: LLM like it says in Documentation/process/coding-assistants.rst

> Signed-off-by: Yibo Tan <lhfff@xxxxxxxxxx>
> ---
>  lib/test_sysctl.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/test_sysctl.c b/lib/test_sysctl.c
> index 909cfcf76dbfc..dd795979558a2 100644
> --- a/lib/test_sysctl.c
> +++ b/lib/test_sysctl.c
> @@ -321,11 +321,11 @@ module_init(test_sysctl_init);
>  
>  static void __exit test_sysctl_exit(void)
>  {
> -	kfree(test_data.bitmap_0001);
>  	for (int i = 0; i < TEST_H_SIZE; i++) {
>  		if (ctl_headers[i])
>  			unregister_sysctl_table(ctl_headers[i]);
>  	}
> +	kfree(test_data.bitmap_0001);
>  }
>  
>  module_exit(test_sysctl_exit);
> 
> base-commit: 40288c9206c17eb66a603262e06a58d300d0f279

I'll leave the Fixes tag but will add this to sysctl-next as opposed to
pushing it through as a fix for 7.3. Lets avoid making noise just for
the testing module.

Thx for the fix

Best

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