linux-kernel - [PATCH 3.2 089/200] mm/swap: fix race on swap

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Message-ID: <lsq.1396221815.648295261@decadent.org.uk>
Date:	Mon, 31 Mar 2014 00:23:35 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "Weijie Yang" <weijie.yang@...sung.com>,
	"Krzysztof Kozlowski" <k.kozlowski@...sung.com>,
	"Hugh Dickins" <hughd@...gle.com>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>
Subject: [PATCH 3.2 089/200] mm/swap: fix race on swap_info reuse between
 swapoff and swapon

3.2.56-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Weijie Yang <weijie.yang@...sung.com>

commit f893ab41e4dae2fe8991faf5d86d029068d1ef3a upstream.

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 clears the SWP_USED flag after all its resources are freed,
so that swapon can reuse this swap_info by alloc_swap_info() safely.

[akpm@...ux-foundation.org: tidy up code comment]
Signed-off-by: Weijie Yang <weijie.yang@...sung.com>
Acked-by: Hugh Dickins <hughd@...gle.com>
Cc: Krzysztof Kozlowski <k.kozlowski@...sung.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 mm/swapfile.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1649,7 +1649,6 @@ SYSCALL_DEFINE1(swapoff, const char __us
 	p->max = 0;
 	swap_map = p->swap_map;
 	p->swap_map = NULL;
-	p->flags = 0;
 	spin_unlock(&swap_lock);
 	mutex_unlock(&swapon_mutex);
 	vfree(swap_map);
@@ -1667,6 +1666,16 @@ SYSCALL_DEFINE1(swapoff, const char __us
 		mutex_unlock(&inode->i_mutex);
 	}
 	filp_close(swap_file, NULL);
+
+	/*
+	 * Clear the SWP_USED flag after all resources are 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);

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