IB/qib: Fix potential buffer overrun in sending diag packet routine

Guard against a potential buffer overrun.  Right now the qib driver is
protected by the fact that the data structure in question is only 16
bits.  Should that ever change the problem will be exposed. There is a
similar defect in the ipath driver and this brings the two code paths
into sync.

Reported-by: Nico Golde <nico@ngolde.de>
Reported-by: Fabian Yamaguchi <fabs@goesec.de>
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/hw/qib/qib_diag.c

@@ -546,7 +546,7 @@ static ssize_t qib_diagpkt_write(struct file *fp,
 				 size_t count, loff_t *off)
 {
 	u32 __iomem *piobuf;
-	u32 plen, clen, pbufn;
+	u32 plen, pbufn, maxlen_reserve;
 	struct qib_diag_xpkt dp;
 	u32 *tmpbuf = NULL;
 	struct qib_devdata *dd;
@@ -590,15 +590,20 @@ static ssize_t qib_diagpkt_write(struct file *fp,
 	}
 	ppd = &dd->pport[dp.port - 1];
 
-	/* need total length before first word written */
-	/* +1 word is for the qword padding */
-	plen = sizeof(u32) + dp.len;
-	clen = dp.len >> 2;
-
-	if ((plen + 4) > ppd->ibmaxlen) {
+	/*
+	 * need total length before first word written, plus 2 Dwords. One Dword
+	 * is for padding so we get the full user data when not aligned on
+	 * a word boundary. The other Dword is to make sure we have room for the
+	 * ICRC which gets tacked on later.
+	 */
+	maxlen_reserve = 2 * sizeof(u32);
+	if (dp.len > ppd->ibmaxlen - maxlen_reserve) {
 		ret = -EINVAL;
-		goto bail;      /* before writing pbc */
+		goto bail;
 	}
+
+	plen = sizeof(u32) + dp.len;
+
 	tmpbuf = vmalloc(plen);
 	if (!tmpbuf) {
 		qib_devinfo(dd->pcidev,
@@ -638,11 +643,11 @@ static ssize_t qib_diagpkt_write(struct file *fp,
 	 */
 	if (dd->flags & QIB_PIO_FLUSH_WC) {
 		qib_flush_wc();
-		qib_pio_copy(piobuf + 2, tmpbuf, clen - 1);
+		qib_pio_copy(piobuf + 2, tmpbuf, plen - 1);
 		qib_flush_wc();
-		__raw_writel(tmpbuf[clen - 1], piobuf + clen + 1);
+		__raw_writel(tmpbuf[plen - 1], piobuf + plen + 1);
 	} else
-		qib_pio_copy(piobuf + 2, tmpbuf, clen);
+		qib_pio_copy(piobuf + 2, tmpbuf, plen);
 
 	if (dd->flags & QIB_USE_SPCL_TRIG) {
 		u32 spcl_off = (pbufn >= dd->piobcnt2k) ? 2047 : 1023;