[PATCH] zram: fix null dereference of handle

[Minchan Kim <minchan@kernel.org>]

For the testing, I found handle passed to zs_map_object in __zram_bvec_read
is NULL so that kernel goes the oops by pin_object.

The reason is there is no routine to check the slot's freeing
after getting the slot's lock. This patch fixes it.

Fixes: 1f7319c74275 ("zram: partial IO refactoring")
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
---
 drivers/block/zram/zram_drv.c | 34 ++++++++++------------------------
 1 file changed, 10 insertions(+), 24 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 91a72df41ab0..23172641fc01 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -758,27 +758,6 @@ static void zram_slot_unlock(struct zram *zram, u32 index)
 	bit_spin_unlock(ZRAM_ACCESS, &zram->table[index].value);
 }
 
-static bool zram_same_page_read(struct zram *zram, u32 index,
-				struct page *page,
-				unsigned int offset, unsigned int len)
-{
-	zram_slot_lock(zram, index);
-	if (unlikely(!zram_get_handle(zram, index) ||
-			zram_test_flag(zram, index, ZRAM_SAME))) {
-		void *mem;
-
-		zram_slot_unlock(zram, index);
-		mem = kmap_atomic(page);
-		zram_fill_page(mem + offset, len,
-					zram_get_element(zram, index));
-		kunmap_atomic(mem);
-		return true;
-	}
-	zram_slot_unlock(zram, index);
-
-	return false;
-}
-
 static void zram_meta_free(struct zram *zram, u64 disksize)
 {
 	size_t num_pages = disksize >> PAGE_SHIFT;
@@ -876,11 +855,18 @@ static int __zram_bvec_read(struct zram *zram, struct page *page, u32 index,
 		zram_slot_unlock(zram, index);
 	}
 
-	if (zram_same_page_read(zram, index, page, 0, PAGE_SIZE))
-		return 0;
-
 	zram_slot_lock(zram, index);
 	handle = zram_get_handle(zram, index);
+	if (unlikely(!handle || zram_test_flag(zram, index, ZRAM_SAME))) {
+		void *mem;
+
+		mem = kmap_atomic(page);
+		zram_fill_page(mem, PAGE_SIZE, zram_get_element(zram, index));
+		kunmap_atomic(mem);
+		zram_slot_unlock(zram, index);
+		return 0;
+	}
+
 	size = zram_get_obj_size(zram, index);
 
 	src = zs_map_object(zram->mem_pool, handle, ZS_MM_RO);
-- 
2.7.4