Re: [PATCH] io_uring: fix provide_buffers sign extension

[Pavel Begunkov <asml.silence@gmail.com>]

On 19/03/2021 10:21, Pavel Begunkov wrote:
> io_provide_buffers_prep()'s "p->len * p->nbufs" to sign extension
> problems. Not a huge problem as it's only used for access_ok() and
> increases the checked length, but better to keep typing right.

Forgot to mention, I suggest it for 5.12, simple enough and
may be a problem for userspace in some rare cases.


> Reported-by: Colin Ian King <colin.king@canonical.com>
> Fixes: efe68c1ca8f49 ("io_uring: validate the full range of provided buffers for access")
> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> ---
>  fs/io_uring.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index c2489b463eb9..4f1c98502a09 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -3978,6 +3978,7 @@ static int io_remove_buffers(struct io_kiocb *req, unsigned int issue_flags)
>  static int io_provide_buffers_prep(struct io_kiocb *req,
>  				   const struct io_uring_sqe *sqe)
>  {
> +	unsigned long size;
>  	struct io_provide_buf *p = &req->pbuf;
>  	u64 tmp;
>  
> @@ -3991,7 +3992,8 @@ static int io_provide_buffers_prep(struct io_kiocb *req,
>  	p->addr = READ_ONCE(sqe->addr);
>  	p->len = READ_ONCE(sqe->len);
>  
> -	if (!access_ok(u64_to_user_ptr(p->addr), (p->len * p->nbufs)))
> +	size = (unsigned long)p->len * p->nbufs;
> +	if (!access_ok(u64_to_user_ptr(p->addr), size))
>  		return -EFAULT;
>  
>  	p->bgid = READ_ONCE(sqe->buf_group);
> 

-- 
Pavel Begunkov