Re: [PATCH] fs/aio: Check IOCB_AIO_RW before the struct aio_kiocb conversion

[Christian Brauner <brauner@kernel.org>]

On Mon, Mar 04, 2024 at 03:57:15PM -0800, Bart Van Assche wrote:
> The first kiocb_set_cancel_fn() argument may point at a struct kiocb
> that is not embedded inside struct aio_kiocb. With the current code,
> depending on the compiler, the req->ki_ctx read happens either before
> the IOCB_AIO_RW test or after that test. Move the req->ki_ctx read such
> that it is guaranteed that the IOCB_AIO_RW test happens first.
> 
> Reported-by: Eric Biggers <ebiggers@kernel.org>
> Cc: Benjamin LaHaise <ben@communityfibre.ca>
> Cc: Eric Biggers <ebiggers@google.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Avi Kivity <avi@scylladb.com>
> Cc: Sandeep Dhavale <dhavale@google.com>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Kent Overstreet <kent.overstreet@linux.dev>
> Cc: stable@vger.kernel.org
> Fixes: b820de741ae4 ("fs/aio: Restrict kiocb_set_cancel_fn() to I/O submitted via libaio")
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> ---

Can I please get a review from Eric and/or Benjamin, please?

>  fs/aio.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/aio.c b/fs/aio.c
> index da18dbcfcb22..9cdaa2faa536 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -589,8 +589,8 @@ static int aio_setup_ring(struct kioctx *ctx, unsigned int nr_events)
>  
>  void kiocb_set_cancel_fn(struct kiocb *iocb, kiocb_cancel_fn *cancel)
>  {
> -	struct aio_kiocb *req = container_of(iocb, struct aio_kiocb, rw);
> -	struct kioctx *ctx = req->ki_ctx;
> +	struct aio_kiocb *req;
> +	struct kioctx *ctx;
>  	unsigned long flags;
>  
>  	/*
> @@ -600,9 +600,13 @@ void kiocb_set_cancel_fn(struct kiocb *iocb, kiocb_cancel_fn *cancel)
>  	if (!(iocb->ki_flags & IOCB_AIO_RW))
>  		return;
>  
> +	req = container_of(iocb, struct aio_kiocb, rw);
> +
>  	if (WARN_ON_ONCE(!list_empty(&req->ki_list)))
>  		return;
>  
> +	ctx = req->ki_ctx;
> +
>  	spin_lock_irqsave(&ctx->ctx_lock, flags);
>  	list_add_tail(&req->ki_list, &ctx->active_reqs);
>  	req->ki_cancel = cancel;