powerpc/pseries/iommu: Don't unset window if it was never set

On pSeries, when user attempts to use the same vfio container used by
different iommu group, the spapr_tce_set_window() returns -EPERM
and the subsequent cleanup leads to the below crash.

   Kernel attempted to read user page (308) - exploit attempt?
   BUG: Kernel NULL pointer dereference on read at 0x00000308
   Faulting instruction address: 0xc0000000001ce358
   Oops: Kernel access of bad area, sig: 11 [#1]
   NIP:  c0000000001ce358 LR: c0000000001ce05c CTR: c00000000005add0
   <snip>
   NIP [c0000000001ce358] spapr_tce_unset_window+0x3b8/0x510
   LR [c0000000001ce05c] spapr_tce_unset_window+0xbc/0x510
   Call Trace:
     spapr_tce_unset_window+0xbc/0x510 (unreliable)
     tce_iommu_attach_group+0x24c/0x340 [vfio_iommu_spapr_tce]
     vfio_container_attach_group+0xec/0x240 [vfio]
     vfio_group_fops_unl_ioctl+0x548/0xb00 [vfio]
     sys_ioctl+0x754/0x1580
     system_call_exception+0x13c/0x330
     system_call_vectored_common+0x15c/0x2ec
   <snip>
   --- interrupt: 3000

Fix this by having null check for the tbl passed to the
spapr_tce_unset_window().

Fixes: f431a8c ("powerpc/iommu: Reimplement the iommu_table_group_ops for pSeries")
Cc: [email protected]
Reported-by: Vaishnavi Bhat <[email protected]>
Signed-off-by: Shivaprasad G Bhat <[email protected]>
Signed-off-by: Madhavan Srinivasan <[email protected]>
Link: https://patch.msgid.link/[email protected]

arch/powerpc/platforms/pseries/iommu.c

@@ -2208,6 +2208,9 @@ static long spapr_tce_unset_window(struct iommu_table_group *table_group, int nu
 	const char *win_name;
 	int ret = -ENODEV;
 
+	if (!tbl) /* The table was never created OR window was never opened */
+		return 0;
+
 	mutex_lock(&dma_win_init_mutex);
 
 	if ((num == 0) && is_default_window_table(table_group, tbl))