usb: dwc2: host: Fix use after free w/ simultaneous irqs

While plugging / unplugging on a DWC2 host port with "slub_debug=FZPUA"
enabled, I found a crash that was quite obviously a use after free.

It appears that in some cases when we handle the various sub-cases of
HCINT we may end up freeing the QTD.  If there is more than one bit set
in HCINT we may then end up continuing to use the QTD, which is bad.
Let's be paranoid and check for this after each sub-case.  This should
be safe since we officially have the "hsotg->lock" (it was grabbed in
dwc2_handle_hcd_intr).

The specific crash I found was:
 Unable to handle kernel paging request at virtual address 6b6b6b9f

At the time of the crash, the kernel reported:
 (dwc2_hc_nak_intr+0x5c/0x198)
 (dwc2_handle_hcd_intr+0xa84/0xbf8)
 (_dwc2_hcd_irq+0x1c/0x20)
 (usb_hcd_irq+0x34/0x48)

Popping into kgdb found that "*qtd" was filled with "0x6b", AKA qtd had
been freed and filled with slub_debug poison.

kgdb gave a little better stack crawl:
 0 dwc2_hc_nak_intr (hsotg=hsotg@entry=0xec42e058,
     chan=chan@entry=0xec546dc0, chnum=chnum@entry=4,
     qtd=qtd@entry=0xec679600) at drivers/usb/dwc2/hcd_intr.c:1237
 1 dwc2_hc_n_intr (chnum=4, hsotg=0xec42e058) at
     drivers/usb/dwc2/hcd_intr.c:2041
 2 dwc2_hc_intr (hsotg=0xec42e058) at drivers/usb/dwc2/hcd_intr.c:2078
 3 dwc2_handle_hcd_intr (hsotg=0xec42e058) at
     drivers/usb/dwc2/hcd_intr.c:2128
 4 _dwc2_hcd_irq (hcd=<optimized out>) at drivers/usb/dwc2/hcd.c:2837
 5 usb_hcd_irq (irq=<optimized out>, __hcd=<optimized out>) at
     drivers/usb/core/hcd.c:2353

Popping up to frame #1 (dwc2_hc_n_intr) found:
 (gdb) print /x hcint
 $12 = 0x12

AKA:
 #define HCINTMSK_CHHLTD  (1 << 1)
 #define HCINTMSK_NAK     (1 << 4)

Further debugging found that by simulating receiving those two
interrupts at the same time it was trivial to replicate the
use-after-free.  See <http://crosreview.com/305712> for a patch and
instructions.  This lead to getting the following stack crawl of the
actual free:
 0  arch_kgdb_breakpoint () at arch/arm/include/asm/outercache.h:103
 1  kgdb_breakpoint () at kernel/debug/debug_core.c:1054
 2  dwc2_hcd_qtd_unlink_and_free (hsotg=<optimized out>, qh=<optimized
      out>, qtd=0xe4479a00) at drivers/usb/dwc2/hcd.h:488
 3  dwc2_deactivate_qh (free_qtd=<optimized out>, qh=0xe5efa280,
      hsotg=0xed424618) at drivers/usb/dwc2/hcd_intr.c:671
 4  dwc2_release_channel (hsotg=hsotg@entry=0xed424618,
      chan=chan@entry=0xed5be000, qtd=<optimized out>,
      halt_status=<optimized out>) at drivers/usb/dwc2/hcd_intr.c:742
 5  dwc2_halt_channel (hsotg=0xed424618, chan=0xed5be000, qtd=<optimized
      out>, halt_status=<optimized out>) at
      drivers/usb/dwc2/hcd_intr.c:804
 6  dwc2_complete_non_periodic_xfer (chnum=<optimized out>,
      halt_status=<optimized out>, qtd=<optimized out>, chan=<optimized
      out>, hsotg=<optimized out>) at drivers/usb/dwc2/hcd_intr.c:889
 7  dwc2_hc_xfercomp_intr (hsotg=hsotg@entry=0xed424618,
      chan=chan@entry=0xed5be000, chnum=chnum@entry=6,
      qtd=qtd@entry=0xe4479a00) at drivers/usb/dwc2/hcd_intr.c:1065
 8  dwc2_hc_chhltd_intr_dma (qtd=0xe4479a00, chnum=6, chan=0xed5be000,
      hsotg=0xed424618) at drivers/usb/dwc2/hcd_intr.c:1823
 9  dwc2_hc_chhltd_intr (qtd=0xe4479a00, chnum=6, chan=0xed5be000,
      hsotg=0xed424618) at drivers/usb/dwc2/hcd_intr.c:1944
 10 dwc2_hc_n_intr (chnum=6, hsotg=0xed424618) at
      drivers/usb/dwc2/hcd_intr.c:2052
 11 dwc2_hc_intr (hsotg=0xed424618) at drivers/usb/dwc2/hcd_intr.c:2097
 12 dwc2_handle_hcd_intr (hsotg=0xed424618) at
      drivers/usb/dwc2/hcd_intr.c:2147
 13 _dwc2_hcd_irq (hcd=<optimized out>) at drivers/usb/dwc2/hcd.c:2837
 14 usb_hcd_irq (irq=<optimized out>, __hcd=<optimized out>) at
      drivers/usb/core/hcd.c:2353

Though we could add specific code to handle this case, adding the
general purpose code to check for all cases where qtd might be freed
seemed safer.
Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/dwc2/hcd_intr.c

@@ -1949,6 +1949,24 @@ static void dwc2_hc_chhltd_intr(struct dwc2_hsotg *hsotg,
 	}
 }
 
+/*
+ * Check if the given qtd is still the top of the list (and thus valid).
+ *
+ * If dwc2_hcd_qtd_unlink_and_free() has been called since we grabbed
+ * the qtd from the top of the list, this will return false (otherwise true).
+ */
+static bool dwc2_check_qtd_still_ok(struct dwc2_qtd *qtd, struct dwc2_qh *qh)
+{
+	struct dwc2_qtd *cur_head;
+
+	if (qh == NULL)
+		return false;
+
+	cur_head = list_first_entry(&qh->qtd_list, struct dwc2_qtd,
+				    qtd_list_entry);
+	return (cur_head == qtd);
+}
+
 /* Handles interrupt for a specific Host Channel */
 static void dwc2_hc_n_intr(struct dwc2_hsotg *hsotg, int chnum)
 {
@@ -2031,27 +2049,59 @@ static void dwc2_hc_n_intr(struct dwc2_hsotg *hsotg, int chnum)
 		 */
 		hcint &= ~HCINTMSK_NYET;
 	}
-	if (hcint & HCINTMSK_CHHLTD)
+
+	if (hcint & HCINTMSK_CHHLTD) {
 		dwc2_hc_chhltd_intr(hsotg, chan, chnum, qtd);
-	if (hcint & HCINTMSK_AHBERR)
+		if (!dwc2_check_qtd_still_ok(qtd, chan->qh))
+			goto exit;
+	}
+	if (hcint & HCINTMSK_AHBERR) {
 		dwc2_hc_ahberr_intr(hsotg, chan, chnum, qtd);
-	if (hcint & HCINTMSK_STALL)
+		if (!dwc2_check_qtd_still_ok(qtd, chan->qh))
+			goto exit;
+	}
+	if (hcint & HCINTMSK_STALL) {
 		dwc2_hc_stall_intr(hsotg, chan, chnum, qtd);
-	if (hcint & HCINTMSK_NAK)
+		if (!dwc2_check_qtd_still_ok(qtd, chan->qh))
+			goto exit;
+	}
+	if (hcint & HCINTMSK_NAK) {
 		dwc2_hc_nak_intr(hsotg, chan, chnum, qtd);
-	if (hcint & HCINTMSK_ACK)
+		if (!dwc2_check_qtd_still_ok(qtd, chan->qh))
+			goto exit;
+	}
+	if (hcint & HCINTMSK_ACK) {
 		dwc2_hc_ack_intr(hsotg, chan, chnum, qtd);
-	if (hcint & HCINTMSK_NYET)
+		if (!dwc2_check_qtd_still_ok(qtd, chan->qh))
+			goto exit;
+	}
+	if (hcint & HCINTMSK_NYET) {
 		dwc2_hc_nyet_intr(hsotg, chan, chnum, qtd);
-	if (hcint & HCINTMSK_XACTERR)
+		if (!dwc2_check_qtd_still_ok(qtd, chan->qh))
+			goto exit;
+	}
+	if (hcint & HCINTMSK_XACTERR) {
 		dwc2_hc_xacterr_intr(hsotg, chan, chnum, qtd);
-	if (hcint & HCINTMSK_BBLERR)
+		if (!dwc2_check_qtd_still_ok(qtd, chan->qh))
+			goto exit;
+	}
+	if (hcint & HCINTMSK_BBLERR) {
 		dwc2_hc_babble_intr(hsotg, chan, chnum, qtd);
-	if (hcint & HCINTMSK_FRMOVRUN)
+		if (!dwc2_check_qtd_still_ok(qtd, chan->qh))
+			goto exit;
+	}
+	if (hcint & HCINTMSK_FRMOVRUN) {
 		dwc2_hc_frmovrun_intr(hsotg, chan, chnum, qtd);
-	if (hcint & HCINTMSK_DATATGLERR)
+		if (!dwc2_check_qtd_still_ok(qtd, chan->qh))
+			goto exit;
+	}
+	if (hcint & HCINTMSK_DATATGLERR) {
 		dwc2_hc_datatglerr_intr(hsotg, chan, chnum, qtd);
+		if (!dwc2_check_qtd_still_ok(qtd, chan->qh))
+			goto exit;
+	}
 
+exit:
 	chan->hcint = 0;
 }