iommu/vt-d: fix access after free issue in function free_dmar_iommu()

Function free_dmar_iommu() may access domain->iommu_lock by
	spin_unlock_irqrestore(&domain->iommu_lock, flags);
after freeing corresponding domain structure.

Sample stack dump:
[    8.912818] =========================
[    8.917072] [ BUG: held lock freed! ]
[    8.921335] 3.13.0-rc1-gerry+ #12 Not tainted
[    8.926375] -------------------------
[    8.930629] swapper/0/1 is freeing memory ffff880c23b56040-ffff880c23b5613f, with a lock still held there!
[    8.941675]  (&(&domain->iommu_lock)->rlock){......}, at: [<ffffffff81dc775c>] init_dmars+0x72c/0x95b
[    8.952582] 1 lock held by swapper/0/1:
[    8.957031]  #0:  (&(&domain->iommu_lock)->rlock){......}, at: [<ffffffff81dc775c>] init_dmars+0x72c/0x95b
[    8.968487]
[    8.968487] stack backtrace:
[    8.973602] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.13.0-rc1-gerry+ #12
[    8.981556] Hardware name: Intel Corporation LH Pass ........../SVRBD-ROW_T, BIOS SE5C600.86B.99.99.x059.091020121352 09/10/2012
[    8.994742]  ffff880c23b56040 ffff88042dd33c98 ffffffff815617fd ffff88042dd38b28
[    9.003566]  ffff88042dd33cd0 ffffffff810a977a ffff880c23b56040 0000000000000086
[    9.012403]  ffff88102c4923c0 ffff88042ddb4800 ffffffff81b1e8c0 ffff88042dd33d28
[    9.021240] Call Trace:
[    9.024138]  [<ffffffff815617fd>] dump_stack+0x4d/0x66
[    9.030057]  [<ffffffff810a977a>] debug_check_no_locks_freed+0x15a/0x160
[    9.037723]  [<ffffffff811aa1c2>] kmem_cache_free+0x62/0x5b0
[    9.044225]  [<ffffffff81465e27>] domain_exit+0x197/0x1c0
[    9.050418]  [<ffffffff81dc7788>] init_dmars+0x758/0x95b
[    9.056527]  [<ffffffff81dc7dfa>] intel_iommu_init+0x351/0x438
[    9.063207]  [<ffffffff81d8a711>] ? iommu_setup+0x27d/0x27d
[    9.069601]  [<ffffffff81d8a739>] pci_iommu_init+0x28/0x52
[    9.075910]  [<ffffffff81000342>] do_one_initcall+0x122/0x180
[    9.082509]  [<ffffffff81077738>] ? parse_args+0x1e8/0x320
[    9.088815]  [<ffffffff81d850e8>] kernel_init_freeable+0x1e1/0x26c
[    9.095895]  [<ffffffff81d84833>] ? do_early_param+0x88/0x88
[    9.102396]  [<ffffffff8154f580>] ? rest_init+0xd0/0xd0
[    9.108410]  [<ffffffff8154f58e>] kernel_init+0xe/0x130
[    9.114423]  [<ffffffff81574a2c>] ret_from_fork+0x7c/0xb0
[    9.120612]  [<ffffffff8154f580>] ? rest_init+0xd0/0xd0

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>

drivers/iommu/intel-iommu.c

@@ -1268,7 +1268,7 @@ static void vm_domain_exit(struct dmar_domain *domain);
 static void free_dmar_iommu(struct intel_iommu *iommu)
 {
 	struct dmar_domain *domain;
-	int i;
+	int i, count;
 	unsigned long flags;
 
 	if ((iommu->domains) && (iommu->domain_ids)) {
@@ -1277,13 +1277,14 @@ static void free_dmar_iommu(struct intel_iommu *iommu)
 			clear_bit(i, iommu->domain_ids);
 
 			spin_lock_irqsave(&domain->iommu_lock, flags);
-			if (--domain->iommu_count == 0) {
+			count = --domain->iommu_count;
+			spin_unlock_irqrestore(&domain->iommu_lock, flags);
+			if (count == 0) {
 				if (domain->flags & DOMAIN_FLAG_VIRTUAL_MACHINE)
 					vm_domain_exit(domain);
 				else
 					domain_exit(domain);
 			}
-			spin_unlock_irqrestore(&domain->iommu_lock, flags);
 		}
 	}