Re: [PATCH 1/2] 9p/xen: check logical size for buffer size

[Christian Schoenebeck <linux_oss@crudebyte.com>]

On Friday, November 18, 2022 2:55:41 PM CET Dominique Martinet wrote:
> trans_xen did not check the data fits into the buffer before copying
> from the xen ring, but we probably should.
> Add a check that just skips the request and return an error to
> userspace if it did not fit
> 
> Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
> ---
> 
> This comes more or less as a follow up of a fix for trans_fd:
> https://lkml.kernel.org/r/20221117091159.31533-1-guozihua@huawei.com
> Where msize should be replaced by capacity check, except trans_xen
> did not actually use to check the size fits at all.
> 
> While we normally trust the hypervisor (they can probably do whatever
> they want with our memory), a bug in the 9p server is always possible so
> sanity checks never hurt, especially now buffers got drastically smaller
> with a recent patch.
> 
> My setup for xen is unfortunately long dead so I cannot test this:
> Stefano, you've tested v9fs xen patches in the past, would you mind
> verifying this works as well?
> 
>  net/9p/trans_xen.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/net/9p/trans_xen.c b/net/9p/trans_xen.c
> index b15c64128c3e..66ceb3b3ae30 100644
> --- a/net/9p/trans_xen.c
> +++ b/net/9p/trans_xen.c
> @@ -208,6 +208,14 @@ static void p9_xen_response(struct work_struct *work)
>  			continue;
>  		}
>  
> +		if (h.size > req->rc.capacity) {
> +			dev_warn(&priv->dev->dev,
> +				 "requested packet size too big: %d for tag %d with capacity %zd\n",
> +		                 h.size, h.tag, rreq->rc.capacity);
> +			req->status = REQ_STATUS_ERROR;
> +			goto recv_error;
> +		}
> +

Looks good (except of s/rreq/req/ mentioned by Stefano already).

>  		memcpy(&req->rc, &h, sizeof(h));

Is that really OK?

1. `h` is of type xen_9pfs_header and declared as packed, whereas `rc` is of 
   type p9_fcall not declared as packed.

2. Probably a bit dangerous to assume the layout of xen_9pfs_header being in
   sync with the starting layout of p9_fcall without any compile-time 
   assertion?

>  		req->rc.offset = 0;
>  
> @@ -217,6 +225,7 @@ static void p9_xen_response(struct work_struct *work)
>  				     masked_prod, &masked_cons,
>  				     XEN_9PFS_RING_SIZE(ring));
>  
> +recv_error:
>  		virt_mb();
>  		cons += h.size;
>  		ring->intf->in_cons = cons;
>