Re: [PATCH v1 3/6] virtio 9p: Fix an overflow

[Michael S. Tsirkin]

On Thu, Jan 19, 2023 at 03:57:18PM +0200, Alexander Shishkin wrote:
> From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> 
> tag_len is read as a u16 from the untrusted host. It could overflow
> in the memory allocation, which would lead to a too small buffer.
> 
> Some later loops use it when extended to 32bit, so they could overflow
> the too small buffer.
> 
> Make sure to do the arithmetic for the buffer size in 32bit to avoid
> wrapping.
> 
> Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Signed-off-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> Reviewed-by: Christian Schoenebeck <linux_oss@xxxxxxxxxxxxx>
> Cc: Eric Van Hensbergen <ericvh@xxxxxxxxx>
> Cc: Latchesar Ionkov <lucho@xxxxxxxxxx>
> Cc: Dominique Martinet <asmadeus@xxxxxxxxxxxxx>
> Cc: v9fs-developer@xxxxxxxxxxxxxxxxxxxxx
> ---
>  net/9p/trans_virtio.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
> index 3c27ffb781e3..a78e4d80e5ba 100644
> --- a/net/9p/trans_virtio.c
> +++ b/net/9p/trans_virtio.c
> @@ -629,7 +629,7 @@ static int p9_virtio_probe(struct virtio_device *vdev)
>  		err = -EINVAL;
>  		goto out_free_vq;
>  	}
> -	tag = kzalloc(tag_len + 1, GFP_KERNEL);
> +	tag = kzalloc((u32)tag_len + 1, GFP_KERNEL);
>  	if (!tag) {
>  		err = -ENOMEM;
>  		goto out_free_vq;

Hmm are you sure there's a difference in behaviour? I thought C will just
extend the integer to int.

> -- 
> 2.39.0