Re: [PATCH] aio: fix the "too late munmap()" race

[Jeff Moyer]

Al Viro <viro@xxxxxxxxxxxxxxxxxx> writes:

> Current code has put_ioctx() called asynchronously from aio_fput_routine();
> that's done *after* we have killed the request that used to pin ioctx,
> so there's nothing to stop io_destroy() waiting in wait_for_all_aios()
> from progressing.  As the result, we can end up with async call of
> put_ioctx() being the last one and possibly happening during exit_mmap()
> or elf_core_dump(), neither of which expects stray munmap() being done
> to them...
>
> We do need to prevent _freeing_ ioctx until aio_fput_routine() is done
> with that, but that's all we care about - neither io_destroy() nor
> exit_aio() will progress past wait_for_all_aios() until aio_fput_routine()
> does really_put_req(), so the ioctx teardown won't be done until then
> and we don't care about the contents of ioctx past that point.
>
> Since actual freeing of these suckers is RCU-delayed, we don't need to
> bump ioctx refcount when request goes into list for async removal.
> All we need is rcu_read_lock held just over the ->ctx_lock-protected
> area in aio_fput_routine().

Looks good to me.

Reviewed-by: Jeff Moyer <jmoyer@xxxxxxxxxx>

>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> ---
> [on top of the previous patch, applies to anything past 2008 when freeing
> via call_rcu() went in; earlier versions just need kmem_cache_free() done
> via call_rcu() as commit abf137dd7712132ee56d5b3143c2ff61a72a5faa does,
> except that we don't really need to delay playing with aio_nr - mainline
> also will be better off by not bothering with that]
> diff --git a/fs/aio.c b/fs/aio.c
> index f6578cb..8cf3796 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -228,12 +228,6 @@ static void __put_ioctx(struct kioctx *ctx)
>  	call_rcu(&ctx->rcu_head, ctx_rcu_free);
>  }
>  
> -static inline void get_ioctx(struct kioctx *kioctx)
> -{
> -	BUG_ON(atomic_read(&kioctx->users) <= 0);
> -	atomic_inc(&kioctx->users);
> -}
> -
>  static inline int try_get_ioctx(struct kioctx *kioctx)
>  {
>  	return atomic_inc_not_zero(&kioctx->users);
> @@ -609,11 +603,16 @@ static void aio_fput_routine(struct work_struct *data)
>  			fput(req->ki_filp);
>  
>  		/* Link the iocb into the context's free list */
> +		rcu_read_lock();
>  		spin_lock_irq(&ctx->ctx_lock);
>  		really_put_req(ctx, req);
> +		/*
> +		 * at that point ctx might've been killed, but actual
> +		 * freeing is RCU'd
> +		 */
>  		spin_unlock_irq(&ctx->ctx_lock);
> +		rcu_read_unlock();
>  
> -		put_ioctx(ctx);
>  		spin_lock_irq(&fput_lock);
>  	}
>  	spin_unlock_irq(&fput_lock);
> @@ -644,7 +643,6 @@ static int __aio_put_req(struct kioctx *ctx, struct kiocb *req)
>  	 * this function will be executed w/out any aio kthread wakeup.
>  	 */
>  	if (unlikely(!fput_atomic(req->ki_filp))) {
> -		get_ioctx(ctx);
>  		spin_lock(&fput_lock);
>  		list_add(&req->ki_list, &fput_head);
>  		spin_unlock(&fput_lock);
> --
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