netdev - [PATCH net-next 3/3] mctp i2c: Fix hard head TX bounds length check

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Message-Id: <20220225053938.643605-4-matt@codeconstruct.com.au>
Date:   Fri, 25 Feb 2022 13:39:38 +0800
From:   Matt Johnston <matt@...econstruct.com.au>
To:     netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jeremy Kerr <jk@...econstruct.com.au>
Subject: [PATCH net-next 3/3] mctp i2c: Fix hard head TX bounds length check

We should be testing the length before fitting into the u8 byte_count.
This is just a sanity check, the MCTP stack should have limited to MTU
which is checked, and we check consistency later in mctp_i2c_xmit().

Found by Smatch
mctp_i2c_header_create() warn: impossible condition
    '(hdr->byte_count > 255) => (0-255 > 255)'

Signed-off-by: Matt Johnston <matt@...econstruct.com.au>
---
 drivers/net/mctp/mctp-i2c.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/mctp/mctp-i2c.c b/drivers/net/mctp/mctp-i2c.c
index 470682c88d7e..baf7afac7857 100644
--- a/drivers/net/mctp/mctp-i2c.c
+++ b/drivers/net/mctp/mctp-i2c.c
@@ -537,6 +537,9 @@ static int mctp_i2c_header_create(struct sk_buff *skb, struct net_device *dev,
 	struct mctp_hdr *mhdr;
 	u8 lldst, llsrc;
 
+	if (len > MCTP_I2C_MAXMTU)
+		return -EMSGSIZE;
+
 	lldst = *((u8 *)daddr);
 	llsrc = *((u8 *)saddr);
 
@@ -547,8 +550,6 @@ static int mctp_i2c_header_create(struct sk_buff *skb, struct net_device *dev,
 	hdr->dest_slave = (lldst << 1) & 0xff;
 	hdr->command = MCTP_I2C_COMMANDCODE;
 	hdr->byte_count = len + 1;
-	if (hdr->byte_count > MCTP_I2C_MAXBLOCK)
-		return -EMSGSIZE;
 	hdr->source_slave = ((llsrc << 1) & 0xff) | 0x01;
 	mhdr->ver = 0x01;
 
-- 
2.32.0