Re: [PATCH stable-3.2 stable-3.12] net: fix checksum check in skb_copy_and_csum_datagram_iovec()

[Sabrina Dubroca <sd@queasysnail.net>]

Hello Michal,

2015-12-28, 15:01:57 +0100, Michal Kubecek wrote:
> Recent fix "net: add length argument to
> skb_copy_and_csum_datagram_iovec" added to some pre-3.19 stable
> branches, namely
> 
>   stable-3.2.y: commit 127500d724f8
>   stable-3.12.y: commit 3e1ac3aafbd0
> 
> doesn't handle truncated reads correctly. If read length is shorter than
> incoming datagram (but non-zero) and first segment of target iovec is
> sufficient for read length, skb_copy_and_csum_datagram() is used to copy
> checksum the data while copying it. For truncated reads this means only
> the copied part is checksummed (rather than the whole datagram) so that
> the check almost always fails.

I just ran into this issue too, sorry I didn't notice it earlier :(


> Add checksum of the remaining part so that the proper checksum of the
> whole datagram is computed and checked. Special care must be taken if
> the copied length is odd.
> 
> For zero read length, we don't have to copy anything but we still should
> check the checksum so that a peek doesn't return with a datagram which
> is invalid and wouldn't be returned by an actual read.
> 
> Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
> ---
>  net/core/datagram.c | 26 +++++++++++++++++++++-----
>  1 file changed, 21 insertions(+), 5 deletions(-)
> 
> diff --git a/net/core/datagram.c b/net/core/datagram.c
> index f22f120771ef..af4bf368257c 100644
> --- a/net/core/datagram.c
> +++ b/net/core/datagram.c
> @@ -809,13 +809,14 @@ int skb_copy_and_csum_datagram_iovec(struct sk_buff *skb,
>  				     int hlen, struct iovec *iov, int len)
>  {
>  	__wsum csum;
> -	int chunk = skb->len - hlen;
> +	int full_chunk = skb->len - hlen;
> +	int chunk = min_t(int, full_chunk, len);
>  
> -	if (chunk > len)
> -		chunk = len;
> -
> -	if (!chunk)
> +	if (!chunk) {
> +		if (__skb_checksum_complete(skb))
> +			goto csum_error;
>  		return 0;
> +	}
>  
>  	/* Skip filled elements.
>  	 * Pretty silly, look at memcpy_toiovec, though 8)
> @@ -833,6 +834,21 @@ int skb_copy_and_csum_datagram_iovec(struct sk_buff *skb,
>  		if (skb_copy_and_csum_datagram(skb, hlen, iov->iov_base,
>  					       chunk, &csum))
>  			goto fault;
> +		if (full_chunk > chunk) {
> +			if (chunk % 2) {
> +				__be16 odd = 0;
> +
> +				if (skb_copy_bits(skb, hlen + chunk,
> +						  (char *)&odd + 1, 1))
> +					goto fault;
> +				csum = add32_with_carry(odd, csum);
> +				csum = skb_checksum(skb, hlen + chunk + 1,
> +						    full_chunk - chunk - 1,
> +						    csum);
> +			} else
> +				csum = skb_checksum(skb, hlen + chunk,
> +						    full_chunk - chunk, csum);
> +		}
>  		if (csum_fold(csum))
>  			goto csum_error;
>  		if (unlikely(skb->ip_summed == CHECKSUM_COMPLETE))
> -- 
> 2.6.4


This adds quite a bit of complexity.  I am considering a revert of my
buggy patch, and use what Eric Dumazet suggested instead:

https://patchwork.ozlabs.org/patch/543562/

What do you think?
Eric, would you submit your patch formally?


Thanks

-- 
Sabrina