linux-kernel - [PATCH] zram: avoid NULL pointer access in concurrent situation

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Message-id: <000001cff18a$52d70d80$f8852880$%yang@samsung.com>
Date:	Mon, 27 Oct 2014 10:03:19 +0800
From:	Weijie Yang <weijie.yang@...sung.com>
To:	'Andrew Morton' <akpm@...ux-foundation.org>
Cc:	'Minchan Kim' <minchan@...nel.org>,
	'Sergey Senozhatsky' <sergey.senozhatsky@...il.com>,
	'Dan Streetman' <ddstreet@...e.org>,
	'Nitin Gupta' <ngupta@...are.org>,
	'linux-kernel' <linux-kernel@...r.kernel.org>,
	'Linux-MM' <linux-mm@...ck.org>,
	'Weijie Yang' <weijie.yang.kh@...il.com>
Subject: [PATCH] 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@...sung.com>
Acked-by: Minchan Kim <minchan@...nel.org>
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
---
 drivers/block/zram/zram_drv.c |   10 ++++++----
 1 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 0e63e8a..2ad0b5b 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;
-- 
1.7.0.4


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