Subject: Re: [patch -stable] svcrdma: underflow issue in decode_write_list()

From: walter harms <[email-masked]>
Date: Fri, 12 Jul 2013 10:24:41 +0200

Am 12.07.2013 08:39, schrieb Dan Carpenter:
> My static checker marks everything from ntohl() as untrusted and it
> complains we could have an underflow problem doing:
> 
> 	return (u32 *)&ary->wc_array[nchunks];
> 
> Also on 32 bit systems the upper bound check could overflow.
> 
> Signed-off-by: Dan Carpenter <[email-masked]>
> 
> diff --git a/net/sunrpc/xprtrdma/svc_rdma_marshal.c b/net/sunrpc/xprtrdma/svc_rdma_marshal.c
> index 8d2eddd..65b1462 100644
> --- a/net/sunrpc/xprtrdma/svc_rdma_marshal.c
> +++ b/net/sunrpc/xprtrdma/svc_rdma_marshal.c
> @@ -98,6 +98,7 @@ void svc_rdma_rcl_chunk_counts(struct rpcrdma_read_chunk *ch,
>   */
>  static u32 *decode_write_list(u32 *va, u32 *vaend)
>  {
> +	unsigned long start, end;
>  	int nchunks;
>  
>  	struct rpcrdma_write_array *ary =
> @@ -113,9 +114,12 @@ static u32 *decode_write_list(u32 *va, u32 *vaend)
>  		return NULL;
>  	}
>  	nchunks = ntohl(ary->wc_nchunks);
> -	if (((unsigned long)&ary->wc_array[0] +
> -	     (sizeof(struct rpcrdma_write_chunk) * nchunks)) >
> -	    (unsigned long)vaend) {
> +
> +	start = (unsigned long)&ary->wc_array[0];
> +	end = (unsigned long)vaend;
> +	if (nchunks < 0 ||
> +	    nchunks > (SIZE_MAX - start) / sizeof(struct rpcrdma_write_chunk) ||
> +	    (start + (sizeof(struct rpcrdma_write_chunk) * nchunks)) > end) {
>  		dprintk("svcrdma: ary=%p, wc_nchunks=%d, vaend=%p\n",
>  			ary, nchunks, vaend);

i am struggling to understand what is actually checked here.
Perhaps this improves the readability a bit
 if ( nchunks < 0 ||
       sizeof(struct rpcrdma_write_chunk) * nchunks > (SIZE_MAX - start) ||
       sizeof(struct rpcrdma_write_chunk) * nchunks > (end - start) )

 with that rewrite i would say that (SIZE_MAX - start) is strange.

just my 2 cents,
 wh

>  		return NULL;
> @@ -129,6 +133,7 @@ static u32 *decode_write_list(u32 *va, u32 *vaend)
>  
>  static u32 *decode_reply_array(u32 *va, u32 *vaend)
>  {
> +	unsigned long start, end;
>  	int nchunks;
>  	struct rpcrdma_write_array *ary =
>  		(struct rpcrdma_write_array *)va;
> @@ -143,9 +148,12 @@ static u32 *decode_reply_array(u32 *va, u32 *vaend)
>  		return NULL;
>  	}
>  	nchunks = ntohl(ary->wc_nchunks);
> -	if (((unsigned long)&ary->wc_array[0] +
> -	     (sizeof(struct rpcrdma_write_chunk) * nchunks)) >
> -	    (unsigned long)vaend) {
> +
> +	start = (unsigned long)&ary->wc_array[0];
> +	end = (unsigned long)vaend;
> +	if (nchunks < 0 ||
> +	    nchunks > (SIZE_MAX - start) / sizeof(struct rpcrdma_write_chunk) ||
> +	    (start + (sizeof(struct rpcrdma_write_chunk) * nchunks)) > end) {
>  		dprintk("svcrdma: ary=%p, wc_nchunks=%d, vaend=%p\n",
>  			ary, nchunks, vaend);
>  		return NULL;
> --
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