Re: [PATCH 1/1] io_uring: prevent reg-wait speculations

[Jens Axboe <axboe@kernel.dk>]

On 11/18/24 6:29 PM, Pavel Begunkov wrote:
> With *ENTER_EXT_ARG_REG instead of passing a user pointer with arguments
> for the waiting loop the user can specify an offset into a pre-mapped
> region of memory, in which case the
> [offset, offset + sizeof(io_uring_reg_wait)) will be intepreted as the
> argument.
> 
> As we address a kernel array using a user given index, it'd be a subject
> to speculation type of exploits.
> 
> Fixes: d617b3147d54c ("io_uring: restore back registered wait arguments")
> Fixes: aa00f67adc2c0 ("io_uring: add support for fixed wait regions")
> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> ---
>  io_uring/io_uring.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
> index da8fd460977b..3a3e4fca1545 100644
> --- a/io_uring/io_uring.c
> +++ b/io_uring/io_uring.c
> @@ -3207,6 +3207,7 @@ static struct io_uring_reg_wait *io_get_ext_arg_reg(struct io_ring_ctx *ctx,
>  		     end > ctx->cq_wait_size))
>  		return ERR_PTR(-EFAULT);
>  
> +	barrier_nospec();
>  	return ctx->cq_wait_arg + offset;

We need something better than that, barrier_nospec() is a big slow
hammer...

-- 
Jens Axboe