gru: fix potential use-after-free when purging GRU tlbs

Fix potential SGI GRU bug that could cause a use-after-free.  If one
thread in a task is flushing the GRU and another thread destroys the GRU
context, there is the potential to access a table after it has been freed.

Copy the gms pointer to a local variable before unlocking the gts table.
Note that no refcnt is needed for the gms - the reference is held
indirectly by the task's mm_struct.
Signed-off-by: Jack Steiner <steiner@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/misc/sgi-gru/grufault.c

@@ -702,6 +702,7 @@ int gru_user_flush_tlb(unsigned long arg)
 {
 	struct gru_thread_state *gts;
 	struct gru_flush_tlb_req req;
+	struct gru_mm_struct *gms;
 
 	STAT(user_flush_tlb);
 	if (copy_from_user(&req, (void __user *)arg, sizeof(req)))
@@ -714,8 +715,9 @@ int gru_user_flush_tlb(unsigned long arg)
 	if (!gts)
 		return -EINVAL;
 
-	gru_flush_tlb_range(gts->ts_gms, req.vaddr, req.len);
+	gms = gts->ts_gms;
 	gru_unlock_gts(gts);
+	gru_flush_tlb_range(gms, req.vaddr, req.len);
 
 	return 0;
 }