linux-kernel - [PATCH] mm/swap: fix race on swap_info reuse between swapoff and swapon

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Message-id: <000001cf0cfd$6d251640$476f42c0$%yang@samsung.com>
Date:	Thu, 09 Jan 2014 13:39:55 +0800
From:	Weijie Yang <weijie.yang@...sung.com>
To:	'linux-kernel' <linux-kernel@...r.kernel.org>,
	'Linux-MM' <linux-mm@...ck.org>
Cc:	'Andrew Morton' <akpm@...ux-foundation.org>, hughd@...gle.com,
	'Minchan Kim' <minchan@...nel.org>, shli@...ionio.com,
	'Bob Liu' <bob.liu@...cle.com>, k.kozlowski@...sung.com,
	stable@...r.kernel.org, weijie.yang.kh@...il.com
Subject: [PATCH] mm/swap: fix race on swap_info reuse between swapoff and swapon

swapoff clear swap_info's SWP_USED flag prematurely and free its resources
after that. A concurrent swapon will reuse this swap_info while its previous
resources are not cleared completely.

These late freed resources are:
- p->percpu_cluster
- swap_cgroup_ctrl[type]
- block_device setting
- inode->i_flags &= ~S_SWAPFILE

This patch clear SWP_USED flag after all its resources freed, so that swapon
can reuse this swap_info by alloc_swap_info() safely.

Signed-off-by: Weijie Yang <weijie.yang@...sung.com>
---
 mm/swapfile.c |   11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 612a7c9..89071c3
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1922,7 +1922,6 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	p->swap_map = NULL;
 	cluster_info = p->cluster_info;
 	p->cluster_info = NULL;
-	p->flags = 0;
 	frontswap_map = frontswap_map_get(p);
 	spin_unlock(&p->lock);
 	spin_unlock(&swap_lock);
@@ -1948,6 +1947,16 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 		mutex_unlock(&inode->i_mutex);
 	}
 	filp_close(swap_file, NULL);
+
+	/*
+	* clear SWP_USED flag after all resources freed
+	* so that swapon can reuse this swap_info in alloc_swap_info() safely
+	* it is ok to not hold p->lock after we cleared its SWP_WRITEOK
+	*/
+	spin_lock(&swap_lock);
+	p->flags = 0;
+	spin_unlock(&swap_lock);
+
 	err = 0;
 	atomic_inc(&proc_poll_event);
 	wake_up_interruptible(&proc_poll_wait);
-- 
1.7.10.4


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