[Libguestfs] [PATCH 04/14] nbd/client: Add safety check on chunk payload length

[Vladimir Sementsov-Ogievskiy]

04.12.2021 02:15, Eric Blake wrote:
> Our existing use of structured replies either reads into a qiov capped
> at 32M (NBD_CMD_READ) or caps allocation to 1000 bytes (see
> NBD_MAX_MALLOC_PAYLOAD in block/nbd.c).  But the existing length
> checks are rather late; if we encounter a buggy (or malicious) server
> that sends a super-large payload length, we should drop the connection
> right then rather than assuming the layer on top will be careful.
> This becomes more important when we permit 64-bit lengths which are
> even more likely to have the potential for attempted denial of service
> abuse.
> 
> Signed-off-by: Eric Blake <eblake at redhat.com>


Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov at virtuozzo.com>

> ---
>   nbd/client.c | 12 ++++++++++++
>   1 file changed, 12 insertions(+)
> 
> diff --git a/nbd/client.c b/nbd/client.c
> index 30d5383cb195..8f137c2320bb 100644
> --- a/nbd/client.c
> +++ b/nbd/client.c
> @@ -1412,6 +1412,18 @@ static int nbd_receive_structured_reply_chunk(QIOChannel *ioc,
>       chunk->handle = be64_to_cpu(chunk->handle);
>       chunk->length = be32_to_cpu(chunk->length);
> 
> +    /*
> +     * Because we use BLOCK_STATUS with REQ_ONE, and cap READ requests
> +     * at 32M, no valid server should send us payload larger than
> +     * this.  Even if we stopped using REQ_ONE, sane servers will cap
> +     * the number of extents they return for block status.
> +     */
> +    if (chunk->length > NBD_MAX_BUFFER_SIZE + sizeof(NBDStructuredReadData)) {
> +        error_setg(errp, "server chunk %" PRIu32 " (%s) payload is too long",
> +                   chunk->type, nbd_rep_lookup(chunk->type));
> +        return -EINVAL;
> +    }
> +
>       return 0;
>   }
> 


-- 
Best regards,
Vladimir