fix use after free in amd create gatt pages

Coverity spotted a "use after free" bug in
drivers/char/agp/amd-k7-agp.c::amd_create_gatt_pages().

The problem is this:
	If "entry = kzalloc(sizeof(struct amd_page_map), GFP_KERNEL);"
fails, then there's a loop in the function to free all entries
allocated so far and break out of the allocation loop. That in itself
is pretty sane, but then the (now freed) 'tables' is assigned to
amd_irongate_private.gatt_pages and 'retval' is set to -ENOMEM which
causes amd_free_gatt_pages(); to be called at the end of the function.
The problem with this is that amd_free_gatt_pages() will then loop
'amd_irongate_private.num_tables' times and try to free each entry in
tables[] - this is bad since tables has already been freed and
furthermore it will call kfree(tables) at the end - a double free.

This patch removes the freeing loop in amd_create_gatt_pages() and
instead relies entirely on the call to amd_free_gatt_pages() to free
everything we allocated in case of an error. It also sets
amd_irongate_private.num_tables to the actual number of entries
allocated instead of just using the value passed in from the caller -
this ensures that amd_free_gatt_pages() will only attempt to free
stuff that was actually allocated.

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/char/agp/amd-k7-agp.c

@@ -100,21 +100,16 @@ static int amd_create_gatt_pages(int nr_tables)
 
 	for (i = 0; i < nr_tables; i++) {
 		entry = kzalloc(sizeof(struct amd_page_map), GFP_KERNEL);
+		tables[i] = entry;
 		if (entry == NULL) {
-			while (i > 0) {
-				kfree(tables[i-1]);
-				i--;
-			}
-			kfree(tables);
 			retval = -ENOMEM;
 			break;
 		}
-		tables[i] = entry;
 		retval = amd_create_page_map(entry);
 		if (retval != 0)
 			break;
 	}
-	amd_irongate_private.num_tables = nr_tables;
+	amd_irongate_private.num_tables = i;
 	amd_irongate_private.gatt_pages = tables;
 
 	if (retval != 0)