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Date:   Tue, 24 Jul 2018 05:13:26 +0000
From:   Vakul Garg <vakul.garg@....com>
To:     Doron Roberts-Kedes <doronrk@...com>,
        "David S . Miller" <davem@...emloft.net>
CC:     Dave Watson <davejwatson@...com>, Matt Mullins <mmullins@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH net-next] tls: Fix improper revert in zerocopy_from_iter



> -----Original Message-----
> From: Doron Roberts-Kedes [mailto:doronrk@...com]
> Sent: Tuesday, July 24, 2018 3:50 AM
> To: David S . Miller <davem@...emloft.net>
> Cc: Dave Watson <davejwatson@...com>; Vakul Garg
> <vakul.garg@....com>; Matt Mullins <mmullins@...com>;
> netdev@...r.kernel.org; Doron Roberts-Kedes <doronrk@...com>
> Subject: [PATCH net-next] tls: Fix improper revert in zerocopy_from_iter
> 
> The current code is problematic because the iov_iter is reverted and never
> advanced in the non-error case. This patch skips the revert in the non-error
> case. This patch also fixes the amount by which the iov_iter is reverted.
> Currently, iov_iter is reverted by size, which can be greater than the amount
> by which the iter was actually advanced.
> Instead, mimic the tx path which reverts by the difference before and after
> zerocopy_from_iter.
> 
> Fixes: 4718799817c5 ("tls: Fix zerocopy_from_iter iov handling")
> Signed-off-by: Doron Roberts-Kedes <doronrk@...com>
> ---
>  net/tls/tls_sw.c | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/net/tls/tls_sw.c b/net/tls/tls_sw.c index
> 490f2bcc6313..2ea000baebf8 100644
> --- a/net/tls/tls_sw.c
> +++ b/net/tls/tls_sw.c
> @@ -276,7 +276,7 @@ static int zerocopy_from_iter(struct sock *sk, struct
> iov_iter *from,
>  			      int length, int *pages_used,
>  			      unsigned int *size_used,
>  			      struct scatterlist *to, int to_max_pages,
> -			      bool charge, bool revert)
> +			      bool charge)
>  {
>  	struct page *pages[MAX_SKB_FRAGS];
> 
> @@ -327,8 +327,6 @@ static int zerocopy_from_iter(struct sock *sk, struct
> iov_iter *from,
>  out:
>  	*size_used = size;
>  	*pages_used = num_elem;
> -	if (revert)
> -		iov_iter_revert(from, size);
> 
>  	return rc;
>  }
> @@ -431,7 +429,7 @@ int tls_sw_sendmsg(struct sock *sk, struct msghdr
> *msg, size_t size)
>  				&ctx->sg_plaintext_size,
>  				ctx->sg_plaintext_data,
>  				ARRAY_SIZE(ctx->sg_plaintext_data),
> -				true, false);
> +				true);
>  			if (ret)
>  				goto fallback_to_reg_send;
> 
> @@ -811,6 +809,7 @@ int tls_sw_recvmsg(struct sock *sk,
>  			    likely(!(flags & MSG_PEEK)))  {
>  				struct scatterlist sgin[MAX_SKB_FRAGS + 1];
>  				int pages = 0;
> +				int orig_chunk = chunk;
> 
>  				zc = true;
>  				sg_init_table(sgin, MAX_SKB_FRAGS + 1);
> @@ -820,9 +819,11 @@ int tls_sw_recvmsg(struct sock *sk,
>  				err = zerocopy_from_iter(sk, &msg-
> >msg_iter,
>  							 to_copy, &pages,
>  							 &chunk, &sgin[1],
> -							 MAX_SKB_FRAGS,
> 	false, true);
> -				if (err < 0)
> +							 MAX_SKB_FRAGS,
> 	false);
> +				if (err < 0) {
> +					iov_iter_revert(&msg->msg_iter,
> chunk - orig_chunk);
>  					goto fallback_to_reg_recv;
> +				}

This assumes that msg_iter gets advanced even if zerocopy_from_iter() fails.
It is easier from code readability perspective if functions upon failure do not leave any side effects for the caller to clean-up.
I suggest that iov_iter_revert() should be called from zerocopy_from_iter() itself if it is going to fail. 

 

> 
>  				err = decrypt_skb(sk, skb, sgin);
>  				for (; pages > 0; pages--)
> --
> 2.17.1

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