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

From: Ben Wei
Date: Mon Sep 02 2019 - 19:44:01 EST

> > 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@xxxxxx>
> 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;
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;
- 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