Re: [PATCH v4] modules: wait do_free_init correctly

[Luis Chamberlain <mcgrof@kernel.org>]

On Tue, Feb 27, 2024 at 10:35:46AM +0800, Changbin Du wrote:
> The synchronization here is to ensure the ordering of freeing of a module
> init so that it happens before W+X checking. It is worth noting it is not
> that the freeing was not happening, it is just that our sanity checkers
> raced against the permission checkers which assume init memory is already
> gone.
> 
> Commit 1a7b7d922081 ("modules: Use vmalloc special flag") moved
> calling do_free_init() into a global workqueue instead of relying on it
> being called through call_rcu(..., do_free_init), which used to allowed us
> call do_free_init() asynchronously after the end of a subsequent grace
> period. The move to a global workqueue broke the gaurantees for code which
> needed to be sure the do_free_init() would complete with rcu_barrier().
> To fix this callers which used to rely on rcu_barrier() must now instead
> use flush_work(&init_free_wq).
> 
> Without this fix, we still could encounter false positive reports in W+X
> checking since the rcu_barrier() here can not ensure the ordering now.
> 
> Even worse, the rcu_barrier() can introduce significant delay. Eric Chanudet
> reported that the rcu_barrier introduces ~0.1s delay on a PREEMPT_RT kernel.
> 
>   [    0.291444] Freeing unused kernel memory: 5568K
>   [    0.402442] Run /sbin/init as init process
> 
> With this fix, the above delay can be eliminated.
> 
> Fixes: 1a7b7d922081 ("modules: Use vmalloc special flag")
> Signed-off-by: Changbin Du <changbin.du@huawei.com>
> Cc: Xiaoyi Su <suxiaoyi@huawei.com>
> Cc: Eric Chanudet <echanude@redhat.com>
> Cc: Luis Chamberlain <mcgrof@infradead.org>
> Tested-by: Eric Chanudet <echanude@redhat.com>

Acked-by: Luis Chamberlain <mcgrof@kernel.org>

  Luis