tg3: fix length overflow in VPD firmware parsing

commit 715230a4 upstream.

Commit 184b8904 ("tg3: Use VPD fw version
when present") introduced VPD parsing that contained a potential length
overflow.

Limit the hardware's reported firmware string length (max 255 bytes) to
stay inside the driver's firmware string length (32 bytes). On overflow,
truncate the formatted firmware string instead of potentially overwriting
portions of the tg3 struct.

http://cansecwest.com/slides/2013/PrivateCore%20CSW%202013.pdfSigned-off-by: Kees Cook <keescook@chromium.org>
Reported-by: Oded Horovitz <oded@privatecore.com>
Reported-by: Brad Spengler <spender@grsecurity.net>
Cc: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/net/ethernet/broadcom/tg3.c

@@ -13433,8 +13433,11 @@ static void __devinit tg3_read_vpd(struct tg3 *tp)
 		if (j + len > block_end)
 			goto partno;
 
-		memcpy(tp->fw_ver, &vpd_data[j], len);
-		strncat(tp->fw_ver, " bc ", vpdlen - len - 1);
+		if (len >= sizeof(tp->fw_ver))
+			len = sizeof(tp->fw_ver) - 1;
+		memset(tp->fw_ver, 0, sizeof(tp->fw_ver));
+		snprintf(tp->fw_ver, sizeof(tp->fw_ver), "%.*s bc ", len,
+			 &vpd_data[j]);
 	}
 
 partno: