netdev - RE: [PATCH net-next] net/ncsi: support unaligned payload size in NC-SI cmd handler

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Message-ID: <CH2PR15MB36869BE1AA44813CE293891BA3BE0@CH2PR15MB3686.namprd15.prod.outlook.com>
Date:   Mon, 2 Sep 2019 23:43:42 +0000
From:   Ben Wei <benwei@...com>
To:     David Miller <davem@...emloft.net>
CC:     "sam@...dozajonas.com" <sam@...dozajonas.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "openbmc@...ts.ozlabs.org" <openbmc@...ts.ozlabs.org>
Subject: RE: [PATCH net-next] net/ncsi: support unaligned payload size in
 NC-SI cmd handler

> > Update NC-SI command handler (both standard and OEM) to take into
> > account of payload paddings in allocating skb (in case of payload
> > size is not 32-bit aligned).
> > 
> > The checksum field follows payload field, without taking payload
> > padding into account can cause checksum being truncated, leading to
> > dropped packets.
> > 
> > Signed-off-by: Ben Wei <benwei@...com>
>
> If you have to align and add padding, I do not see where you are
> clearing out that padding memory to make sure it is initialized.
>
> You do comparisons with 'payload' but make adjustments to 'len'.
>
> The logic is very confusing.

Yes let me clarify a bit. 

In the code 'payload' is the exact NC-SI payload length, which goes into NC-SI packet header
and needs to be actual unpadded payload length.

'len' is used to allocate total NC-SI packet buffer (include padding). 

The original calculation of 'len' was done by summing up NCSI header + payload + checksum,
without taking into account of possible padding, e.g.

        len += sizeof(struct ncsi_cmd_pkt_hdr) + 4;  /* 4 is the checksum size */
       if (nca->payload < 26)
                len += 26;
        else
               len += nca->payload;
        /* Allocate skb */
        skb = alloc_skb(len, GFP_ATOMIC);

This works today for all standard NC-SI commands (in spec v1.1) because all standard commands
have payload size < 26, and packet size is then set to minimum of 46 (16 hdr + 26 payload + 4 cksum) bytes.

And mem clearing is done in each of the standard cmd handlers, e.g. 
ncsi_cmd_handler_sp, ncsi_cmd_handler_ae.

The problem occurs if payload >= 26 and is unaligned.  This could happen on some OEM commands,
and I see this happening when we carry PLDM traffic over NC-SI packet. 
(PLDM header being 3 bytes and payload size can be large) 

The skb allocated would be too small, and later when checksum is calculated and written:

	pchecksum = (__be32 *)((void *)h + sizeof(struct ncsi_pkt_hdr) +
		    ALIGN(nca->payload, 4));
	*pchecksum = htonl(checksum);

Part of the checksum would fall outside of our allocated buffer.

PLDM over NC-SI and OEM NC-SI commands are currently handled in

@@ -213,17 +213,22 @@ static int ncsi_cmd_handler_oem(struct sk_buff *skb,

So here I ensure the skb allocation takes padding into account, and do the initial mem clearing
to set the padding bytes

+       unsigned short payload = ALIGN(nca->payload, 4);

        len = sizeof(struct ncsi_cmd_pkt_hdr) + 4;
-       if (nca->payload < 26)
+       if (payload < 26)
                len += 26;
        else
-               len += nca->payload;
+               len += payload;

        cmd = skb_put_zero(skb, len);
        memcpy(&cmd->mfr_id, nca->data, nca->payload);

So in this patch I updated both standard command handler (in case future spec updates adds
commands with payload >= 26) and OEM/generic command handler to support unaligned payload
size.  

Regards,
-Ben