zram: avoid NULL pointer access in concurrent situation

There is a rare NULL pointer bug in mem_used_total_show() and
mem_used_max_store() in concurrent situation, like this:

zram is not initialized, process A is a mem_used_total reader which runs
periodically, while process B try to init zram.

	process A 				process B
  access meta, get a NULL value
						init zram, done
  init_done() is true
  access meta->mem_pool, get a NULL pointer BUG

This patch fixes this issue.

Signed-off-by: Weijie Yang <weijie.yang@samsung.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 4 deletions

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 0e63e8aa8279..2ad0b5bce44b 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -99,11 +99,12 @@ static ssize_t mem_used_total_show(struct device *dev,
 {
         u64 val = 0;
         struct zram *zram = dev_to_zram(dev);
-        struct zram_meta *meta = zram->meta;
 
         down_read(&zram->init_lock);
-        if (init_done(zram))
+        if (init_done(zram)) {
+                struct zram_meta *meta = zram->meta;
                 val = zs_get_total_pages(meta->mem_pool);
+        }
         up_read(&zram->init_lock);
 
         return scnprintf(buf, PAGE_SIZE, "%llu\n", val << PAGE_SHIFT);
@@ -173,16 +174,17 @@ static ssize_t mem_used_max_store(struct device *dev,
         int err;
         unsigned long val;
         struct zram *zram = dev_to_zram(dev);
-        struct zram_meta *meta = zram->meta;
 
         err = kstrtoul(buf, 10, &val);
         if (err || val != 0)
                 return -EINVAL;
 
         down_read(&zram->init_lock);
-        if (init_done(zram))
+        if (init_done(zram)) {
+                struct zram_meta *meta = zram->meta;
                 atomic_long_set(&zram->stats.max_used_pages,
                                 zs_get_total_pages(meta->mem_pool));
+        }
         up_read(&zram->init_lock);
 
         return len;