[PATCH 3.16.y-ckt 209/254] zram: avoid kunmap_atomic() of a NULL pointer

[Luis Henriques]

3.16.7-ckt2 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Weijie Yang <weijie.yang at samsung.com>

commit c406515239376fc93a30d5d03192182160cbd3fb upstream.

zram could kunmap_atomic() a NULL pointer in a rare situation: a zram
page becomes a full-zeroed page after a partial write io.  The current
code doesn't handle this case and performs kunmap_atomic() on a NULL
pointer, which panics the kernel.

This patch fixes this issue.

Signed-off-by: Weijie Yang <weijie.yang at samsung.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky at gmail.com>
Cc: Dan Streetman <ddstreet at ieee.org>
Cc: Nitin Gupta <ngupta at vflare.org>
Cc: Weijie Yang <weijie.yang.kh at gmail.com>
Acked-by: Jerome Marchand <jmarchan at redhat.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 drivers/block/zram/zram_drv.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 36e54be402df..2b30485cbd26 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -459,7 +459,8 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
 	}
 
 	if (page_zero_filled(uncmem)) {
-		kunmap_atomic(user_mem);
+		if (user_mem)
+			kunmap_atomic(user_mem);
 		/* Free memory associated with this sector now. */
 		write_lock(&zram->meta->tb_lock);
 		zram_free_page(zram, index);
-- 
2.1.0