[PATCH] isdn: hisax: hfc_pci: Fix a possible concurrency use-after-free bug in HFCPCI_l1hw()

[Jia-Ju Bai <baijiaju1990@gmail.com>]

In drivers/isdn/hisax/hfc_pci.c, the functions hfcpci_interrupt() and
HFCPCI_l1hw() may be concurrently executed.

HFCPCI_l1hw()
  line 1173: if (!cs->tx_skb)

hfcpci_interrupt()
  line 942: spin_lock_irqsave();
  line 1066: dev_kfree_skb_irq(cs->tx_skb);

Thus, a possible concurrency use-after-free bug may occur 
in HFCPCI_l1hw().

To fix these bugs, the calls to spin_lock_irqsave() and
spin_unlock_irqrestore() are added in HFCPCI_l1hw(), to protect the
access to cs->tx_skb.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
---
 drivers/isdn/hisax/hfc_pci.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/isdn/hisax/hfc_pci.c b/drivers/isdn/hisax/hfc_pci.c
index ea0e4c6de3fb..0109e0e8bcb6 100644
--- a/drivers/isdn/hisax/hfc_pci.c
+++ b/drivers/isdn/hisax/hfc_pci.c
@@ -1170,11 +1170,13 @@ HFCPCI_l1hw(struct PStack *st, int pr, void *arg)
 		if (cs->debug & L1_DEB_LAPD)
 			debugl1(cs, "-> PH_REQUEST_PULL");
 #endif
+		spin_lock_irqsave(&cs->lock, flags);
 		if (!cs->tx_skb) {
 			test_and_clear_bit(FLG_L1_PULL_REQ, &st->l1.Flags);
 			st->l1.l1l2(st, PH_PULL | CONFIRM, NULL);
 		} else
 			test_and_set_bit(FLG_L1_PULL_REQ, &st->l1.Flags);
+		spin_unlock_irqrestore(&cs->lock, flags);
 		break;
 	case (HW_RESET | REQUEST):
 		spin_lock_irqsave(&cs->lock, flags);
-- 
2.17.0